Significant Figures Calculator

Enter a number to count its significant figures, with each significant digit highlighted, or round it to any number of significant figures in standard and scientific notation. A third tab adds, subtracts, multiplies or divides two measurements and applies the sig-fig rules.

Free, no signupNo daily limitRuns in your browser - nothing you enter is uploadedUpdated 7 October 2026

Number

Significant digits are highlighted.

The rules, with examples

Rule	Example	Sig figs
Non-zero digits always count	4.53	3
Zeros between non-zero digits count	1007	4
Leading zeros never count	0.0045	2
Trailing zeros after a decimal point count	0.004500	4
Trailing zeros in a whole number are ambiguous	1200	2 to 4
…unless a decimal point or scientific notation settles it	1200. / 1.20 × 10³	4 / 3

Worked example: a lab calculation

A block measures 2.5 cm by 3.42 cm. What is its area?

Multiply2.5 × 3.42 = 8.55 (calculator value)
Fewest significant figures in the inputs2.5 has 2, 3.42 has 3 → keep 2
Round to 2 significant figures8.55 → 8.6 cm²

Adding instead follows decimal places: 12.11 + 18.0 = 30.11, and 18.0 has one decimal place, so the answer is 30.1.

Why significant figures matter

A measurement is only as precise as the instrument. Writing 8.55 cm² for a block measured with a ruler marked in millimetres claims a precision you do not have. Significant figures carry that precision through a calculation, so the answer is never more exact than the least exact measurement that went into it.

Two habits avoid most mistakes: keep every digit until the final step and round once at the end, and treat counted or defined numbers (3 trials, 100 cm per metre) as exact - they never limit the answer.

What this calculator does not do

Two numbers and one operation at a time in the calculation tab; for chains of operations, round only at the end.
Rounding is round-half-up; round-half-to-even is not offered.
Zero itself has no agreed number of significant figures, so the counter flags it instead of guessing.

Frequently asked questions

What are the rules for significant figures?

Non-zero digits always count. Zeros between non-zero digits count. Leading zeros (as in 0.0045) never count. Trailing zeros count if the number has a decimal point (2.50 has 3), but are ambiguous in a whole number without one (1200 could have 2, 3 or 4).

How many significant figures does 1200 have?

It is ambiguous: at least 2, at most 4. Write 1200. (with a point) or 1.200 × 10³ to show that all four digits are measured, or 1.2 × 10³ for two.

How do you round to significant figures?

Count from the first non-zero digit, keep that many digits, and round the last one up if the next digit is 5 or more. 0.012345 to 3 significant figures is 0.0123; 2.996 to 3 is 3.00.

How many significant figures should an answer have?

When multiplying or dividing, keep as many significant figures as the measurement with the fewest. When adding or subtracting, keep as many decimal places as the measurement with the fewest. 2.5 × 3.42 = 8.6, and 12.11 + 18.0 = 30.1.

Do exact numbers limit significant figures?

No. Counted values and defined constants (12 eggs, 100 cm in a metre) are exact and have unlimited significant figures, so they never limit the answer. Enter only measured values here.

Which rounding rule is used?

Round half up: a following 5 rounds up. Some science courses use round-half-to-even instead, which can differ when the dropped part is exactly 5.

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Step-by-step guide: How to count and round significant figures →

Built and checked by Taimur Hassan Siddiqui, who has more than ten years in computer-science education. The formulas on this page are covered by an automated test suite, and the worked example on this page is one of its cases. Last updated 7 October 2026.