Calculators that show their working
8 free calculators for mortgages, savings, health, math, conversions and business — each one with the formula, the assumptions and a full breakdown of how the answer was reached.
Most used
Mortgage Payment
Work out your monthly mortgage payment including taxes, insurance and PMI, then see exactly how much interest you will pay over the life of the loan.
Compound Interest
See what your savings grow into once interest starts earning interest, with regular contributions and any compounding frequency.
BMI
Work out your body mass index in metric or imperial units, see which WHO category it falls in, and find the weight range that would put you in the healthy band.
Percentage
Answer all four of the common percentage questions — what is X% of Y, X is what percent of Y, percentage increase or decrease, and the original value before a change.
Date Difference
Count the days, weeks, months and years between two dates, including an exact working-day count that skips weekends.
Profit Margin
Turn cost and revenue into gross margin, markup and profit per unit — and work backwards from a target margin to the price you need to charge.
Browse by category
Finance
Mortgages, loans, savings, investments, tax and retirement. Work out payments, interest and timelines before you commit to anything.
All 2 finance calculatorsHealth & Fitness
BMI, body fat, calories, macros, pregnancy and pace. Evidence-based formulas with the assumptions spelled out.
All 1 health & fitness calculatorsMath
Percentages, fractions, ratios, algebra, geometry and statistics, each shown with the working so you can check it.
All 1 math calculatorsUnit Converters
Length, weight, volume, temperature, area, speed, data and cooking measures, converted exactly.
All 1 unit converters calculatorsDate & Time
Date differences, deadlines, working days, age, timezones and durations, with leap years and holidays handled properly.
All 1 date & time calculatorsBusiness
Margin, markup, break-even, VAT, payroll, invoicing and ROI for people running the numbers on a real business.
All 1 business calculatorsEveryday
Tips, fuel costs, split bills, discounts, paint and materials — the small sums that come up constantly.
All 1 everyday calculatorsWhy these calculators are different
The formula, not just the answer
Every calculator shows the equation it used, what each symbol means and why it works, so you can check the result instead of trusting it.
Instant, and genuinely fast
Results update as you type, with no page reloads and no waiting. Pages are prerendered static HTML, so they load immediately on any connection.
Free to use and to embed
No sign-up, no paywall and no limits. Every calculator can be embedded on your own site, and there is a public JSON API for each one.