Calculators that show their work
Finance, math, dates, unit conversions, home projects and fitness. Each answer comes with the formula, your own numbers worked through it, and the assumptions it rests on.
Common tasks
- Work out a monthly mortgage payment Mortgage Calculator
- See a loan’s amortization schedule Loan Payment Calculator
- Find how long a card balance takes to clear Credit Card Payoff Calculator
- Project savings with regular deposits Compound Interest Calculator
- Work out a percentage change Percentage Calculator
- Count the days between two dates Days Between Dates Calculator
- Split a bill and the tip Tip Calculator
- Work out a running pace or finish time Pace Calculator
Guides
- How investment fees compound: 0.1% vs. 1% over 30 years
What a yearly percentage fee takes from a long-term balance, why most of the damage lands in the last decade, and how to compare fund, advisory and flat account fees.
- How loan amortization works, with a worked schedule
The payment formula, how each payment splits between interest and principal, why the last payment differs slightly from the rest, when principal overtakes interest on a long loan, and what extra payments and negative amortization do to the schedule.
- How to compare loan offers: APR, fees and total cost
Match the offers on cash received and term, then compare APR and total cost. A worked $15,000 example shows how fees, longer terms and early payoff can change which offer is cheaper.
- Rental property cash flow: NOI, cap rate, cash-on-cash and DSCR
Follow one example duplex from scheduled rent to net operating income to cash flow, see why its cap rate, cash-on-cash return and DSCR disagree, and find out how little it takes to turn the cash flow negative.
- Scheduling across time zones and daylight saving time
Why a weekly meeting shifts by an hour for some attendees in March, April, September, October and November, what happens to times that are skipped or repeated, and which zone a recurring invite should follow.
- Solving triangles: which rule to use, and the SSA ambiguous case
A table for choosing the law of sines or the law of cosines, a worked SSA example with two valid answers, and the step-order and rounding traps that make hand-solved triangles go wrong.
How these calculators work
Every calculation runs in your browser. The numbers you enter are not sent to this site or to anyone else, and there is nothing to sign up for. Your inputs leave your browser only when you copy, print, download or share them, and a share link is made only when you ask for it.
Each calculator opens with example values already worked through, so you can see what kind of answer it gives before typing anything. Change a value and the result updates as you type. If an input can’t be used, the result is removed and the field explains what it needs, so an old answer is never left on screen beside new numbers.
Where a result comes from a formula, the steps sit below it: the formula, then the same formula with your numbers in it. The assumptions sit next to the answer, such as whether payments fall at the start or end of a period, how a month is counted or which published equation an estimate comes from. Interest rates, prices, tax rates and fees are always inputs you set; nothing is fetched and presented as a current figure.
Results can be copied as text or printed, tables download as CSV, and most calculators can save up to three scenarios side by side, for example the same loan over 15 and 30 years.
Choosing the right calculator
Some calculators look as if they answer the same question. These are worth telling apart:
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Comparing loan offers. The payment depends on the amount, the rate, the term and how often you pay. Fees change what the loan really costs, so work out the payment first, then fold the fees into an APR to compare fixed-rate offers over the same term. APR assumes you keep the loan to the end, so repaying early changes the comparison.
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Buying a home. Affordability works back from your income to a price; the mortgage payment works forward from a price to a monthly bill that includes tax, insurance and PMI. Most buyers need both, in that order.
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Growing savings. If you know what you will put in, project the balance. If you know the target, solve for the deposit instead. To compare accounts, put every rate on the same footing as an APY first. Where interest is paid on the original amount only, the simpler formula applies.
Compound Interest Calculator Savings Goal Calculator APY Calculator Simple Interest Calculator
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Clearing card debt. With one balance, find how long a given payment takes, or the payment that clears it by a target date. With several, compare paying the smallest balance first against paying the highest rate first.
Credit Card Payoff Calculator Debt Snowball vs. Avalanche Calculator
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Buying materials for a project. Enter each measurement in the unit its field asks for: a slab’s thickness is usually in inches, its length and width in feet. The waste allowance is yours to set, and the result is rounded up to whole cans, boxes or bags, since stores don’t sell fractions of them.
Paint Calculator Flooring and Tile Calculator Concrete Calculator
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Counting days. For an age, count years, months and days from the date of birth; a February 29 birthday needs a rule for years without that date. Count calendar days for notice periods and deadlines stated in days, and business days when weekends and the holidays you enter don’t count. To land on a date rather than count up to one, add or subtract instead.
Age Calculator Days Between Dates Calculator Business Days Calculator Add or Subtract Dates Calculator
How the math is checked
Every formula comes from the sources listed on its page. Each calculator’s automated tests compare its results with expected values worked out independently (by hand, from published worked examples or with separate high-precision software), never produced by the calculator’s own code, and each formula-based result shows the work behind it. Spotted a mistake? Corrections are welcome.