How to calculate percentages, discounts, units, and currency
Percent-of math, sale prices, SI-style unit conversion, live FX, and a scientific keypad — pick the formula that matches the story.
Percentage modes are not a discount button
Percentage Calculator at /tools/percentage-calculator can answer “what is B% of A,” “A is what % of B,” increase/decrease by a percent, percent change, percent difference, and reverse percent. Example: 15% of 80 is 12. Discount Calculator at /tools/discount-calculator is the shop path: original price and percent off → savings and final. 80 off 25% is savings 20, final 60. Do not use percent-difference when you meant markdown; the formulas differ.
Units vs currency vs scientific
Unit Converter at /tools/unit-converter switches category (length, mass, temperature, volume, area, speed, data) then from/to units. Temperature is a special formula; length uses SI factors (1 in = 0.0254 m). Currency Converter at /tools/currency-converter fetches a public mid-market rate for the pair — it is not a bank quote, not cash, not a fee-inclusive tourist rate. Scientific Calculator at /tools/scientific-calculator evaluates expressions with sin, cos, tan, log, and powers in the tab. It is not a computer algebra system.
Steps
1) Choose the page that names your problem (percent, discount, unit, currency, or scientific). 2) Fill the fields; article calculators update as you type. Currency needs a network read for the rate. 3) Copy the number you need. 4) For margin vs markup, use Profit Margin Calculator instead. For tips, use Tip Calculator. For loans, use Loan Calculator.
Worked example
A shirt list 40, 30% off → Discount Calculator. “12 is what percent of 80” → Percentage Calculator is-what mode (15%). 5 km to miles → Unit Converter length. 100 USD to EUR → Currency Converter (rate dated by the feed). sin(π/2) → Scientific Calculator if you work in radians as the keypad documents. Mixing unit conversion into the percentage page will not convert miles.
Related calculators
Tip and profit margin have their own guide. BMI, GPA, age, and working days are not percent-of tools. Number Base Converter is integer bases, not money. Time Zone Converter is clocks, not FX.
Privacy
Amounts stay in the browser except the currency pair requested from the rate API (codes and not your identity). Related: /tools/discount-calculator, /tools/unit-converter, /tools/currency-converter, /tools/scientific-calculator, /tools/profit-margin-calculator.