Calculator Hub

Six calculators, one page

The percentage problems, untangled

Most percentage confusion comes from three questions looking alike while needing different math. "What is 15% of 80?" multiplies (80 × 0.15 = 12). "12 is what percent of 80?" divides (12 ÷ 80 = 15%). And "the price went from 80 to 92 — what's the increase?" compares the change to the original value (12 ÷ 80 = +15%). The percentage tab lays these out as separate inputs so you pick the question, not the formula — worth remembering the asymmetry, too: a 50% drop needs a 100% rise to get back where you started.

What the loan calculator really tells you

The monthly payment uses the standard amortization formula from principal, annual rate and term — but the number worth staring at is total interest. A 30-year term makes payments comfortable while quietly doubling what the borrowed money costs; running the same loan at 15 and 30 years here takes ten seconds and shows the trade-off in currency, not abstractions. Rates compound the effect: a single percentage point on a long mortgage often exceeds a year's salary in total interest. Use the calculator to compare scenarios before talking to a lender, so their numbers land on prepared ground.

Dates, ages and the BMI category

The date tab counts the exact days between two dates — deadline math, visa windows, subscription periods — and the age calculator handles the leap-year edge cases people get wrong by hand. BMI divides weight by height squared and maps the result to the WHO categories, in metric or imperial. It's a population-level screening index, not a diagnosis: athletes with high muscle mass score "overweight" while carrying little fat, so treat the category as a conversation starter with a professional, not a verdict.

Step-by-step: comparing two loan offers

Open the Loan / Mortgage tab from the row of tabs under the heading — each tab also has its own URL hash (for example #loan or #bmi), so you can bookmark or share the one you use most. Enter the principal, the annual interest rate as quoted (6.5, not 0.065) and the term in years; the monthly payment, total interest and total cost update on every keystroke, with no button to press. To compare offers, leave the amount alone and change one variable at a time: a 15-year versus 30-year term, or 6.5% versus 5.9%. Write down the total-interest figure for each before moving on — that single number is the real price difference between the offers. The other tabs follow the same pattern: Percentage shows three small cards for its three question types, Tip and Discount take a bill or price plus a percentage, BMI has a metric/imperial switch, and Age & Date uses native date pickers.

Real-world use cases

• Stacked discounts. "30% off, then an extra 20% at checkout" is not 50% off. Run the first discount, feed the result into the second: a 100 item becomes 70, then 56 — a 44% reduction. The Discount tab makes the two-step arithmetic visible.
• Splitting a dinner bill fairly. Enter the bill, 18% tip and the number of people; the per-person figure already includes the tip, so nobody has to do mental math at the table.
• Visa and contract day counts. The days-between calculator answers "how many days have I been in the Schengen area" or "how long until the lease ends" exactly, including leap days.
• Tracking weight in imperial, reporting in metric. Switch BMI to Imperial (lb / ft-in), enter pounds and feet-plus-inches, and the category line is the same WHO scale a clinic would quote.
• Sanity-checking a "percent change" claim. A headline says a figure rose from 80 to 100: the increase/decrease card confirms +25%, and reversing the numbers shows the drop back would be only −20%.

How the calculators work under the hood

The loan payment is the standard amortization formula: with monthly rate r (annual rate ÷ 12 ÷ 100) and n monthly payments, payment = P × r ÷ (1 − (1 + r)⁻ⁿ); total interest is simply payment × n − P. At 0% the formula divides by zero, so the calculator falls back to P ÷ n. Percentage change always divides by the from value, which is why growing then shrinking by the same percentage does not return to the start. BMI is kg ÷ m², with imperial inputs converted first (pounds × 0.4536, feet and inches to metres) and categories at the WHO cut-offs of 18.5, 25 and 30. Date differences are computed from the two calendar dates as whole days, so daylight-saving shifts never produce an off-by-one, and the age calculator subtracts years then checks whether this year's birthday has passed — the rule people most often get wrong by hand. All results are formatted with your browser's locale, so thousands separators and decimal marks match what you are used to seeing.

Further reading

If your loan comparisons end up in a spreadsheet, our guide to CSV, JSON and XLSX conversion pitfalls covers the number-formatting traps; for another everyday estimate demystified, see LLM tokens explained. Or browse all guides.

Frequently asked questions