Calculator Tools
Significant Figures Calculator
Count significant figures in any number, round to N sig figs, and apply the textbook add, subtract, multiply, and divide rules in your browser.
Significant figures calculator
Calculator mode
Type any number to see how many significant figures it has and which digits count.
Decimals, integers, scientific notation (1.23e-4), and underscores or thousands commas are accepted.
Worked examples
Tap any example to load it. Each is a standard textbook case for counting, rounding, or applying the sig fig rules.
How significant figures are counted
- All non-zero digits count. In 12.4 every digit is significant: 3 sig figs.
- Zeros between non-zero digits count. In 5005 every digit counts: 4 sig figs.
- Leading zeros never count. In 0.0035 the leading zeros mark the magnitude only: 2 sig figs.
- Trailing zeros after a decimal point count. In 12.30 the final zero is significant: 4 sig figs.
- Trailing zeros on a bare integer are ambiguous. 1500 is conventionally 2 sig figs; 1500. is 4 sig figs; 1.500 x 10^3 is 4 sig figs.
- Scientific notation is unambiguous. Every digit of the mantissa is significant; the exponent only sets the magnitude.
Arithmetic rules and common pitfalls
- Add or subtract: the result keeps the least number of decimal places of the inputs (place-value rule), not the least number of sig figs.
- Multiply or divide: the result keeps the least number of sig figs of the inputs.
- Round at the end, not in the middle. Carry one or two extra digits through any chain of calculations and round only the final answer to avoid compounding rounding errors.
- Exact numbers do not limit precision. Defined constants and counted quantities (12 eggs, 2 in the formula for the area of a triangle) are treated as having infinite sig figs.
- Logarithms keep decimal places equal to the input sig figs. A log of a 3 sig fig number is reported with 3 decimal places in the mantissa of the log.
How to use
- Pick a mode at the top: Count sig figs, Round to N sig figs, or Add, subtract, multiply, divide.
- Type any number into the input. Plain decimals, integers, and scientific notation (1.23e-4) are all accepted; thousands commas and underscores are stripped automatically.
- In Count mode, read the headline sig fig count and check the digit-by-digit breakdown (green digits count, grey digits do not, hover any digit for the reason).
- In Round mode, drag the slider or pick a preset to choose how many sig figs you want, then copy the standard or scientific form result.
- In Arithmetic mode, pick an operation, enter two numbers, and the tool applies the right rule automatically: least decimal places for add or subtract, least sig figs for multiply or divide.
About this tool
Significant Figures Calculator covers the three things students and lab workers actually need from a sig fig tool. Count mode parses any number you type and shows how many sig figs it has, with a digit-by-digit breakdown that flags leading zeros, internal zeros, trailing zeros after a decimal point, and ambiguous trailing zeros on bare integers. Round mode rewrites your number with exactly the number of sig figs you choose, from 1 up to 15, using half-away-from-zero rounding (the convention taught in most chemistry and physics classes), and always shows both the standard form and the unambiguous scientific form so you can pick whichever your assignment requires. Arithmetic mode adds, subtracts, multiplies, or divides two numbers and applies the correct textbook rule automatically: addition and subtraction round to the least number of decimal places (the place-value rule), while multiplication and division round to the least number of sig figs. The result panel always shows both the full-precision answer and the rule-respecting answer, plus a short sentence explaining which input set the precision and why. Built-in worked examples cover the canonical homework cases (0.00420, 1500 vs 1500., rounding 12345 to 3 sig figs, 12.11 + 18.0, 4.56 x 1.4, 98.7 / 7.3) so you can see the rules in action before applying them to your own numbers. Useful for general chemistry, general physics, analytical chemistry, intro engineering courses, lab reports, calibration work, and any place a measurement needs to be reported with honest precision. Everything runs locally in your browser; the numbers you enter are never uploaded.
Free to use. Works in your browser. No signup, no login.
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