CompelKit
Precision calculators for your money, your health, and your craft.
Get an instant, accurate answer — no signup, no clutter, no data mining.
Mortgage Calculator
Monthly payments, total interest & amortization breakdown.
CompelKit is a free collection of everyday calculators built for people who just want a quick, accurate answer without creating an account or digging through a cluttered spreadsheet. Whether you're budgeting for a new home, comparing tax structures as a freelancer, or trimming an audio clip for a podcast, each tool on this site is designed to be simple, fast, and precise.
Our Finance tools cover mortgages, property and sales tax, retirement planning, compound interest, and loan payoff strategies — all with exportable reports so you can save or share your results. Our Lifestyle tools help with everyday tasks like splitting a bill, counting calories, or converting units. And our Creative tools give podcasters, voiceover artists, and audio editors a way to time scripts, optimize bitrates, and edit audio directly in the browser.
Featured Guides
All guidesThe Freelancer's Tax Survival Guide: LLC vs. Sole Proprietorship
A practical, in-depth breakdown of how sole proprietorships and S-Corp LLCs are taxed differently — and exactly when switching structures actually saves you money.
FinanceCompound Interest: The Quiet Multiplier of Wealth
Why time — not timing — is the single biggest variable in long-term investing, and how to use compounding math to your advantage no matter your starting point.
CreativeAudio Bitrate Explained: Finding the Sweet Spot for Quality and Size
A practical, no-nonsense guide to what bitrate actually measures, how it affects perceived audio quality, and exactly which settings to use for podcasts, music, and archival masters.
Favourites
Finance
Mortgage Calculator
Monthly payments, total interest & amortization breakdown.
Sales Tax & Margin
Reverse sales tax, profit margins & markup calculations.
Property Tax & Affordability
Estimate property taxes & how much home you can afford.
Freelance Tax & LLC Savings
Compare sole proprietor vs LLC tax obligations.
Budget Planner
Plan and track home & business monthly budgets with exports.
Currency Converter
Real-time exchange rates & multi-currency comparisons.
Retirement Calculator
Plan retirement with income strategies, withdrawal projections & sustainability analysis.
Compound Interest
Grow any investment with compound interest & regular contributions.
Loan Payoff Calculator
See payoff timelines & interest saved with extra payments.
Hourly to Salary
Convert hourly wages to annual salary and back, instantly.
Lifestyle
Calorie Counter
Calculate your daily calorie target & log meals with macros.
Tip & Bill Splitter
Split any bill with custom tip percentages across your party.
Unit Converter
Convert length, weight, temperature, volume, speed, area & data.
Square Footage Calculator
Calculate total square footage across rooms & estimate material cost.
TDEE Calculator
Calculate your Total Daily Energy Expenditure & BMR.
Drywall Calculator
Visual panel, joint compound & screw estimator for any room.
Brick Calculator
Estimate bricks & mortar bags for any wall, visually.
Gravel Calculator
Estimate volume, weight & cost for gravel, stone, or sand fill.
Lumber Calculator
Calculate board footage for any lumber order.
Decking Calculator
Estimate deck boards & fasteners, visually.
Framing Calculator
Estimate wall studs & plate lumber, visually.
Tile Calculator
Estimate tiles needed for any room, visually.
Creative
Script Timer
Estimate read time for TTS & voiceover scripts.
Audio Bitrate Optimizer
Calculate file sizes & optimal bitrates for audio.
Audio Editor
Trim, adjust volume & export audio to MP3, WAV, OGG, AAC or FLAC.
Batch Processor
Process multiple audio files at once with the same effect & download as ZIP.
Mortgage Calculator
Calculate monthly payments, total interest, and loan breakdown.
Auto-fill Tax & Insurance by Zip Code
Payment Breakdown
Annual Payment Split
Remaining Loan Balance Over Time
Cumulative Interest Paid vs. Principal Recovered
Early in the loan, most payments go to interest — see when the balance shifts
💡 Principal recovered exceeds cumulative interest at Year 0 — before this point, most of your payments fund interest.
Loan Term Comparison
15 vs 20 vs 30-year breakdown at 6.5% · $280,000 loan
| Term | Monthly Payment | Total Interest | Total Paid | vs 30yr Interest |
|---|---|---|---|---|
| 15-year | $2,439.10 | $159,038 | $439,038 | −$198,086 |
| 20-year | $2,087.60 | $221,025 | $501,025 | −$136,099 |
| 30-year | $1,769.79 | $357,125 | $637,125 | baseline |
15-Year Term
20-Year Term
30-Year Term
How This Calculator Works
A mortgage is an amortizing loan used to buy a home, repaid in fixed monthly installments across a set term (commonly 15 or 30 years). Each payment splits into two parts: principal, the borrowed amount you are paying back, and interest, the lender's fee calculated on the remaining balance. Because interest is charged on the outstanding balance, early payments are interest-heavy — a pattern called front-loaded amortization. Your down payment reduces the loan size; a 20% down payment also avoids Private Mortgage Insurance (PMI). Adding extra principal each month reduces the balance faster, which shrinks future interest and shortens the term. The PITI figure combines principal and interest with property tax and homeowners insurance to show your true monthly housing cost, which is what lenders use when qualifying you against a debt-to-income ratio.
The Mathematical Formula & Logic
The standard amortization formula for the fixed monthly principal-and-interest payment is M = P × [ r(1 + r)ⁿ ] / [ (1 + r)ⁿ − 1 ]. P is the loan principal (home price minus down payment). r is the monthly interest rate (annual rate ÷ 12, expressed as a decimal). n is the total number of monthly payments (loan term in years × 12). Total interest paid over the loan equals (M × n) − P. Each month the remaining balance updates as B_new = B_old × (1 + r) − M.
Worked Step-by-Step Example
Consider a $350,000 home with a 20% down payment on a 30-year fixed loan at 6.5% interest. The down payment is $70,000, so the loan principal P = $280,000. The monthly rate r = 0.065 ÷ 12 = 0.0054167, and n = 360 payments. Plugging into the formula gives a monthly P&I payment of about $1,771.66. Over 360 months you pay $637,798 total, of which $357,798 is interest. Adding a $200/month extra principal payment reduces total interest by roughly $97,000 and pays the loan off about 6 years early.
Frequently Asked Questions
Disclaimer: CompelKit tools and calculators are provided for educational and estimation purposes only and do not constitute certified legal, financial, or tax advice.
Read the full guide: How Mortgage Amortization Really Works →Audio Editor
Trim, adjust and export using the menu bar above
Drop an audio file here
or click to browse — MP3, WAV, OGG, FLAC, AAC, M4A
Tip: use File menu or drag a file to get started
How to use the Audio Editor
- Load audio via the drop zone above, File → Open, Load from URL, or record directly from your microphone.
- Drag the Trim Start / Trim End sliders to select the section you want to keep.
- Adjust Volume, or open the Effects menu for gain, fades, EQ, noise reduction, and more.
- Use Edit → Undo/Redo at any point to step back through your changes.
- Export your result from the Quick Export buttons or File → Export as MP3, WAV, OGG, AAC, or FLAC.
Everything runs locally in your browser — no files are uploaded to a server.
How This Calculator Works
The CompelKit audio editor runs entirely in your browser using ffmpeg.wasm, so your files are never uploaded to a server. You can load an audio file, trim it to a region, apply effects (fade, normalize, speed, volume, EQ), and export to MP3, WAV, OGG, AAC, or FLAC. Editing is non-destructive: trims and effect settings are stored as parameters and applied during export, so you can adjust and re-export without quality loss from repeated re-encoding of the source. Multitrack mixing lets you layer two clips and balance their levels before export.
The Mathematical Formula & Logic
Trim region is encoded as a start time and end time (in seconds).
Export pipeline = ffmpeg filter chain applied to the source: trim → effects (fade, volume, atempo) → encoder (libmp3lame, pcm_s16le, libvorbis, aac, flac). Output file size follows the bitrate formula: (Bitrate × Duration) / 8 / 1024 MB.
Worked Step-by-Step Example
Trim a 3:00 clip to the first 2:30 by setting the end marker to 2 minutes 30 seconds, then export to MP3 at 128 kbps. The exported file is 2:30 long and about (128 × 150) / 8 / 1024 ≈ 2.34 MB — no upload, no server round-trip, the entire operation runs locally in your browser.
Frequently Asked Questions
Disclaimer: CompelKit tools and calculators are provided for educational and estimation purposes only and do not constitute certified legal, financial, or tax advice.
Read the full guide: A Browser-Based Audio Editor Explained →Currency Converter
Real-time exchange rates powered by Frankfurter (ECB data)
Could not load exchange rates. Check your connection and try again.
How This Calculator Works
Currency conversion multiplies an amount in one currency by the exchange rate to express it in another. Exchange rates fluctuate continuously based on supply and demand in foreign-exchange markets, central bank policy, interest rates, and trade flows. This tool pulls live mid-market rates so the result reflects the current wholesale rate; the rate a consumer actually receives at a bank or kiosk adds a margin (a spread), so the amount you convert at the airport is usually worse than the mid-market figure shown here. Use the comparison view to see several currencies at once and the historical chart to gauge whether a rate is favorable relative to the recent past.
The Mathematical Formula & Logic
Converted Amount = Amount × Rate (units of target currency per one unit of source currency). To invert: Rate_inverse = 1 ÷ Rate. A percentage change over time = ((Rate_now − Rate_then) ÷ Rate_then) × 100.
Worked Step-by-Step Example
Converting $1,000 USD to EUR at a rate of 0.92 euros per dollar gives 1,000 × 0.92 = €920. If the rate a week earlier was 0.90, the dollar strengthened by ((0.92 − 0.90) ÷ 0.90) × 100 ≈ 2.22%, meaning your $1,000 buys about €20 more than it did last week. A bank charging a 3% margin would give you only about €892 instead of the €920 mid-market figure.
Frequently Asked Questions
Disclaimer: CompelKit tools and calculators are provided for educational and estimation purposes only and do not constitute certified legal, financial, or tax advice.
Read the full guide: How Currency Exchange Rates Actually Work →Budget Planner
Plan and track your monthly income and expenses.
Expense Categories
| Category | Budgeted | Actual | Diff | |
|---|---|---|---|---|
| +$1,500.00 | ||||
| +$200.00 | ||||
| +$400.00 | ||||
| +$300.00 | ||||
| +$250.00 | ||||
| +$150.00 | ||||
| +$200.00 | ||||
| +$500.00 | ||||
| +$100.00 | ||||
| +$100.00 | ||||
| Totals | $3,700.00 | $0.00 |
💡 Home Budget Tips
- • Follow the 50/30/20 rule (needs/wants/savings)
- • Track your spending for the first month
- • Build an emergency fund first
- • Review subscriptions and cancel unused ones
- • Meal plan to reduce grocery costs
- • Automate your savings contributions
How This Calculator Works
A budget divides income into planned categories so you can see where money goes and adjust before overspending. This planner supports two templates: a personal (home) budget using the 50/30/20 framework (needs, wants, savings) and a business budget that separates cost of goods, operating expenses, and owner compensation with a tax reserve. Each category is editable, with subtotals for income and expenses and a net surplus or deficit that updates live. The business tab also estimates a quarterly tax reserve so owners set aside funds as they earn rather than scrambling at filing time.
The Mathematical Formula & Logic
Net = Total Income − Total Expenses. For the home budget, target allocation = Income × percentage (50% needs, 30% wants, 20% savings/debt). For the business budget: Net Operating Income = Revenue − COGS − Operating Expenses; Owner Draw = Net Operating Income − Tax Reserve; Tax Reserve = Taxable Income × estimated effective rate ÷ 4 per quarter.
Worked Step-by-Step Example
On $5,000 monthly take-home, the 50/30/20 split allocates $2,500 to needs, $1,500 to wants, and $1,000 to savings or debt payoff. A business with $8,000 monthly revenue, $2,500 COGS, and $3,000 operating expenses has $2,500 in net operating income; setting aside 25% for taxes ($625/month) leaves $1,875 for owner draw and reinvestment — tracked across the full year to avoid a tax surprise.
Frequently Asked Questions
Disclaimer: CompelKit tools and calculators are provided for educational and estimation purposes only and do not constitute certified legal, financial, or tax advice.
Read the full guide: Budgeting Frameworks That Actually Work →Editorial Policy
Last updated: August 11, 2026
Our Mission
CompelKit exists to help people understand the "why" behind everyday calculations — mortgage amortization, compound interest, freelance tax, audio encoding, construction takeoffs, and similar practical problems — by pairing each free calculator with a plain-English guide. Our editorial policy defines how that content is researched, written, cited, reviewed, and updated so that visitors can rely on what they read here.
Who Writes Our Content
Every guide on this Site is written by the CompelKit editorial team — the same people who build and maintain each calculator. This keeps our articles grounded in the actual formulas, edge cases, and design decisions of the tool they accompany, rather than "boilerplate" content that rarely reflects how the calculator underneath actually works. Where a topic requires specialized credentials (advanced tax strategy, structural engineering, or medical nutrition), we either rely on authoritative external sources with citations or clearly state that the article is introductory and direct readers to a licensed professional.
Content Standards
Originality. All articles are written from scratch — no scraped, spun, or auto-generated content. Plagiarism of any kind is grounds for immediate rejection during internal review.
Evidence. Claims about tax law, lending rules, building codes, metabolic rates, or any other factual assertion must be sourced. Where applicable, sources are listed at the foot of the article using publicly available citations (e.g., IRS publications, CFPB bulletins, BLS data, building codes).
Practical utility. Each guide must explain a concept a reader can actually apply — either to better understand the relevant calculator or to use it correctly. We do not publish content for the sole purpose of adding keyword density.
Plain language. Our audience is not assumed to have a finance, audio, or construction background. Specialist jargon is defined on first use.
How We Cite Sources
Where a guide cites external sources, we list them in a Sources block at the foot of the article. Citations link to original, authoritative, publicly accessible pages (e.g., irs.gov, consumerfinance.gov, bls.gov, hud.gov) rather than secondary aggregators. Where the cited page is a regulatory or government source, we link to the official URL rather than a cached or summarized version.
Review Cadence
Annually. Each guide is re-read at least once a year for accuracy, currency of formulas, and consistency with the referenced calculator. The updated "Last reviewed" date is published on the article.
Trigger-based. Some guides require updates more frequently than annually — for example, federal-tax guides when IRS brackets change, retirement guides when significant legislation affects Social Security or withdrawal rules, and mortgage guides when refinance dynamics shift. We monitor for these changes and revise promptly.
Corrections Policy
When a factual error is identified by a reader, an external reviewer, or our own team, we investigate, fix the error, and add a visible note at the top of the affected guide if the correction is material (e.g., a wrong tax bracket, formula, or citation). Minor typographical and formatting fixes are made silently. We invite readers to report suspected errors via our Contact page — every report is reviewed.
No Paid Placements or Sponsored Reviews
CompelKit does not accept sponsored articles, paid reviews, or paid link insertions within our guides. Where advertising appears on the Site (via Google AdSense), it is clearly distinguished from editorial content and labeled as an advertisement by Google's policies. Where a guide links to a product, service, or external page, the link is chosen by CompelKit's editorial team based on its relevance to the topic — not because of a paid arrangement. If that ever changes, every paid link will be explicitly labeled as such at the point of the link.
AI, Automation, and Tooling
We use general-purpose research and language tools (including LLM-based drafting assistance) during the writing process, in much the same way a writer uses a spell-checker or a reference book. Final authority and accountability for every published guide sits with the CompelKit editorial team. Every article is reviewed by a human editor before publication and updated by a human editor on each review cycle. We do not publish unprompted LLM output as final content.
Reader Feedback
If you spot an issue with a guide or believe we should cover a topic we haven't yet, please reach out through the Contact page. We treat every correction notice as a quality signal and prioritize factual fixes.
Editorial Policy
Last updated: August 11, 2026
Our Mission
CompelKit exists to help people understand the "why" behind everyday calculations — mortgage amortization, compound interest, freelance tax, audio encoding, construction takeoffs, and similar practical problems — by pairing each free calculator with a plain-English guide. Our editorial policy defines how that content is researched, written, cited, reviewed, and updated so that visitors can rely on what they read here.
Who Writes Our Content
Every guide on this Site is written by the CompelKit editorial team — the same people who build and maintain each calculator. This keeps our articles grounded in the actual formulas, edge cases, and design decisions of the tool they accompany, rather than "boilerplate" content that rarely reflects how the calculator underneath actually works. Where a topic requires specialized credentials (advanced tax strategy, structural engineering, or medical nutrition), we either rely on authoritative external sources with citations or clearly state that the article is introductory and direct readers to a licensed professional.
Content Standards
Originality. All articles are written from scratch — no scraped, spun, or auto-generated content. Plagiarism of any kind is grounds for immediate rejection during internal review.
Evidence. Claims about tax law, lending rules, building codes, metabolic rates, or any other factual assertion must be sourced. Where applicable, sources are listed at the foot of the article using publicly available citations (e.g., IRS publications, CFPB bulletins, BLS data, building codes).
Practical utility. Each guide must explain a concept a reader can actually apply — either to better understand the relevant calculator or to use it correctly. We do not publish content for the sole purpose of adding keyword density.
Plain language. Our audience is not assumed to have a finance, audio, or construction background. Specialist jargon is defined on first use.
How We Cite Sources
Where a guide cites external sources, we list them in a Sources block at the foot of the article. Citations link to original, authoritative, publicly accessible pages (e.g., irs.gov, consumerfinance.gov, bls.gov, hud.gov) rather than secondary aggregators. Where the cited page is a regulatory or government source, we link to the official URL rather than a cached or summarized version.
Review Cadence
Annually. Each guide is re-read at least once a year for accuracy, currency of formulas, and consistency with the referenced calculator. The updated "Last reviewed" date is published on the article.
Trigger-based. Some guides require updates more frequently than annually — for example, federal-tax guides when IRS brackets change, retirement guides when significant legislation affects Social Security or withdrawal rules, and mortgage guides when refinance dynamics shift. We monitor for these changes and revise promptly.
Corrections Policy
When a factual error is identified by a reader, an external reviewer, or our own team, we investigate, fix the error, and add a visible note at the top of the affected guide if the correction is material (e.g., a wrong tax bracket, formula, or citation). Minor typographical and formatting fixes are made silently. We invite readers to report suspected errors via our Contact page — every report is reviewed.
No Paid Placements or Sponsored Reviews
CompelKit does not accept sponsored articles, paid reviews, or paid link insertions within our guides. Where advertising appears on the Site (via Google AdSense), it is clearly distinguished from editorial content and labeled as an advertisement by Google's policies. Where a guide links to a product, service, or external page, the link is chosen by CompelKit's editorial team based on its relevance to the topic — not because of a paid arrangement. If that ever changes, every paid link will be explicitly labeled as such at the point of the link.
AI, Automation, and Tooling
We use general-purpose research and language tools (including LLM-based drafting assistance) during the writing process, in much the same way a writer uses a spell-checker or a reference book. Final authority and accountability for every published guide sits with the CompelKit editorial team. Every article is reviewed by a human editor before publication and updated by a human editor on each review cycle. We do not publish unprompted LLM output as final content.
Reader Feedback
If you spot an issue with a guide or believe we should cover a topic we haven't yet, please reach out through the Contact page. We treat every correction notice as a quality signal and prioritize factual fixes.
