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Significant Figures Calculator – Count and Round Sig Figs

Count the significant figures in any number and round it to the number of significant figures you need. Type numbers exactly as written — including trailing zeros and decimal points — because they change the count.

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How to use the significant figures calculator

  1. Type the number exactly as written, for example 0.00450, 1200, 1200. or 6.02e23.
  2. Choose how many significant figures to round to.

You’ll see how many significant figures the number has (with a warning when trailing zeros are ambiguous) and the number rounded to your chosen precision.

The rules for significant figures

  1. Non-zero digits are always significant. 457 has 3.
  2. Zeros between non-zero digits are significant. 4005 has 4.
  3. Leading zeros are never significant. 0.0032 has 2.
  4. Trailing zeros after a decimal point are significant. 3.200 has 4.
  5. Trailing zeros in a whole number are ambiguous. 3200 has 2, 3 or 4 — the calculator counts 2 and flags it.
  6. In scientific notation, every digit shown counts. 3.20 × 10³ has 3.

Worked examples

NumberSignificant figuresWhy
0.00450605leading zeros don’t count; the final zero does
12002 (ambiguous)trailing zeros without a decimal point
1200.4the decimal point makes the zeros significant
1.20e33every digit in the mantissa counts
40.074zeros between digits count

Rounding examples

  • 123,456 to 3 s.f. → 123,000
  • 0.0012345 to 2 s.f. → 0.0012
  • 2.71828 to 4 s.f. → 2.718
  • 9.996 to 3 s.f. → 10.0

Significant figures in calculations

Multiplying and dividing: the answer has as many significant figures as the least precise number. 4.56 × 1.4 = 6.384, which rounds to 6.4 (two significant figures, because 1.4 has two).

Adding and subtracting: the answer has as many decimal places as the number with the fewest decimal places. 12.11 + 18.0 + 1.013 = 31.123, which rounds to 31.1.

Round only at the end of a calculation; rounding at each step adds error.

Rounding fives

When the first dropped digit is exactly 5, the usual school rule is to round up: 2.45 to two significant figures is 2.5. Some scientific fields use “round half to even” instead to avoid a small upward bias, giving 2.4. Check which rule your course or lab uses. This calculator rounds the way your computer’s standard number formatting does, which matches the school rule for most values.

Why significant figures matter

Significant figures show how precise a measurement is. A length written as 2.50 m was measured to the nearest centimetre; 2.5 m only to the nearest 10 cm. Reporting a calculated answer with more digits than your measurements justify suggests a precision you don’t have. That’s why chemistry, physics and engineering courses ask for answers to a stated number of significant figures.

To write numbers in standard form, use the scientific notation converter. For calculations, try the scientific calculator.

Frequently asked questions

How many significant figures does 0.0045060 have?

Five: 4, 5, 0, 6 and the final 0. The leading zeros only show where the decimal point is, so they don’t count; the zero between 5 and 6 counts; and the trailing zero after the decimal point counts.

Are trailing zeros significant?

After a decimal point, yes: 2.50 has three significant figures. In a whole number without a decimal point, such as 1200, they are ambiguous — it could have 2, 3 or 4. Writing 1200. or 1.200 × 10³ makes it clear.

How do I round to 3 significant figures?

Count three digits from the first non-zero digit and round the rest. 123,456 becomes 123,000; 0.0045678 becomes 0.00457; 9.996 becomes 10.0.

Do leading zeros count?

No. In 0.0071, only 7 and 1 are significant — two significant figures.

How many significant figures should my answer have?

When multiplying or dividing, round to the fewest significant figures in the numbers you started with. When adding or subtracting, round to the fewest decimal places.

Are exact numbers limited by significant figures?

No. Counted numbers (12 eggs) and defined values (100 cm in 1 m) are exact and have unlimited significant figures.

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