Free Online Calculators
Ten focused calculators for finance, health, and everyday math — every one runs locally in your browser, with the formula and its limitations documented in plain English. No accounts, no tracking, no fluff.
All calculators
Mortgage Calculator
Monthly payment, total interest, and full amortization — including property tax, insurance, and PMI.
Auto Loan Calculator
Monthly car payment with sales tax, trade-in, and total interest over the term.
Compound Interest Calculator
Investment growth with regular contributions across any time horizon.
BMI Calculator
Body mass index in metric or imperial, with WHO category and healthy weight range.
TDEE Calculator
Daily calorie needs using the Mifflin-St Jeor equation across five activity levels.
Body Fat Calculator
Body fat percentage via the US Navy method — needs only a tape measure.
Pregnancy Due Date Calculator
Estimate due date from LMP, conception date, or IVF transfer.
Age Calculator
Exact age in years, months, weeks, and days between any two dates.
Percentage Calculator
Percent of, percent change, and reverse percent — every common variant.
Tip Calculator
Tip amount and bill split for any party size or service quality.
Which calculator should I use?
The 10 calculators here cluster into three buckets — pick by the question you're trying to answer.
Finance: "Can I afford this?"
- Mortgage — buying a home. Pulls in tax, insurance, and PMI so you see the real monthly cost (PITI), not just principal and interest.
- Auto loan — buying a car. Handles sales tax and trade-in value before computing the loan.
- Compound interest — saving or investing. Models monthly contributions and any compounding frequency.
- Percentage and tip — quick everyday math (discounts, gratuity, splitting bills).
Health & body composition: "Where am I now?"
- BMI — fastest screening number, but only as good as height + weight. Read the BMI vs body fat guide if you're unsure whether BMI is the right metric for you.
- Body fat % — uses the Navy tape-measure method. ±3.5% accuracy, plenty for tracking trends.
- TDEE — total daily calorie burn. Use this to set a calorie target for cutting, maintaining, or bulking.
- Pregnancy due date — three input methods (LMP, conception, IVF) with appropriate adjustments for each.
Time & dates: "How far apart are these?"
- Age calculator — exact age or interval between any two dates, including months and days.
Methodology and accuracy
Every calculator on this page uses a standard, peer-reviewed formula. The exact equations, their assumptions, and their typical error bands are documented on the methodology page. We update calculators when standards change — for instance, if the WHO revises BMI categories or the IRS publishes new tax tables.
Frequently asked questions
Are these calculators accurate?
For the math itself, yes — they implement the standard formula for each domain. Predictive equations (like BMI, TDEE, and body fat) have inherent error bands of 3–10% versus gold-standard measurements; that's the limit of the formula, not the calculator. Financial calculators are exact for the inputs you give them, but real loans include closing costs, points, and fee structures that aren't modelled here.
Are my inputs sent anywhere?
No. Every calculator runs entirely in your browser using JavaScript. There's no backend to send anything to. You can verify this by opening DevTools and watching the Network tab as you calculate.
Why doesn't my mortgage payment match my lender's quote?
Lenders include things this tool doesn't: discount points, origination fees, escrow shortages, HOA dues, and any rate buy-down or buy-up. The principal-and-interest portion should match exactly; the all-in cost will differ by a few dollars per month.
Can I rely on these for medical or financial decisions?
Use them as a first-pass estimate, not as a substitute for professional advice. For health questions, talk to a clinician; for loans or investments, talk to a licensed lender or financial advisor. The numbers here are accurate as estimators, but they don't know your specific situation.
Do you log anything I calculate?
No. We may use page-view analytics (to know which calculators are popular), but the inputs and results of any calculator never leave your browser.