Sig Fig Calculator
Type a number exactly as written — trailing zeros matter — and the calculator counts its significant figures, lists which digits count, and optionally rounds it to the sig figs you need.
Example: with Number (as written, e.g. 0.00420) 0.00420 · Round it to (sig figs, optional) 2 → Significant figures: 3 sig figs.
Computed by the calculator below using its default values. Change any input to see your own numbers.
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Check it outThe sig fig rules, with examples
Significant figures are the digits that carry real measurement information. Four rules cover every case:
- Non-zero digits always count. 523 has 3 sig figs.
- Zeros between non-zero digits count. 4,005 has 4.
- Leading zeros never count — they only set the scale. 0.00420 has 3 sig figs (4, 2, and the final 0).
- Trailing zeros count only when there is a decimal point. 12.30 has 4 sig figs, but 1200 as written has just 2 — write 1200. or 1.200 × 10³ to claim all four.
Because the trailing-zero rule depends on how the number is written, type it exactly as it appears — 0.00420 and 0.0042 are the same value but carry 3 and 2 sig figs respectively. Scientific notation removes the ambiguity: every digit of the mantissa in 3.40 × 10⁵ is significant.
How it’s calculated
The calculator reads the number as typed. It strips any sign, then counts digits: leading zeros are skipped; if the number contains a decimal point, every remaining digit (including trailing zeros) is significant; without a decimal point, trailing zeros are treated as placeholders and excluded. In scientific notation only the mantissa is counted. Rounding to n sig figs keeps the first n significant digits and rounds by the next digit (5 rounds up), preserving significant trailing zeros — e.g. 0.050 rounded to 3 sig figs displays as 0.0500.
Results update as you type and are estimates, not professional advice — verify important decisions with a qualified professional.
Common mistakes
- Counting leading zeros — 0.005 has 1 sig fig, not 4; those zeros only position the decimal point.
- Assuming trailing zeros in a whole number count: 1200 as written has 2 sig figs. Add a decimal point (1200.) or use 1.200 × 10³ to make all four significant.
- Confusing sig figs with decimal places — 0.00420 has 5 decimal places but only 3 significant figures.
- Rounding in stages: 1.49 to 1 sig fig is 1, because the next digit is 4 — not 1.49 → 1.5 → 2.
Frequently asked questions
How many sig figs does 0.00420 have?
Three: 4, 2, and the trailing 0. The leading zeros never count, and the final zero counts because the number has a decimal point.
How many significant figures does 1200 have?
As written, two — the trailing zeros are placeholders. Written as 1200. it has four, and 1.2 × 10³ vs 1.200 × 10³ lets you claim exactly two or exactly four.
Do zeros ever count as significant figures?
Yes: zeros between non-zero digits always count (4,005 has 4 sig figs), and trailing zeros count when a decimal point is present (12.30 has 4). Only leading zeros never count.
How do I round to 3 significant figures?
Keep the first three significant digits and look at the next one: 5 or more rounds up. 1234.5 becomes 1230, and 0.0046728 becomes 0.00467.
How many sig figs should my answer have?
For multiplication and division, match the input with the fewest sig figs; for addition and subtraction, match the least precise decimal place. A 3-sig-fig measurement times a 2-sig-fig one deserves a 2-sig-fig answer.