Significant Figures Calculator & Rounder

Understanding the significant figures calculator — concept and principle

Significant figures (sig figs) are the digits in a number that carry real information about its precision. This calculator counts the significant figures in the number you type and rounds it to the number of significant figures you choose. It is used in lab reports and science homework, where a result should not claim more precision than its measurements.

Leading zeros never count; the trailing zero after the point does, so 0.004560 has 4.

How it is calculated

The count follows the standard rules, applied to the digits as typed (a sign and any exponent part are ignored):

All non-zero digits are significant.
Zeros between non-zero digits are significant (1,005 has 4).
Leading zeros are never significant; they only place the decimal point (0.0045 has 2).
Trailing zeros are significant when the number contains a decimal point (2.50 has 3). Without a decimal point they are treated as not significant (1,200 is counted as 2).

Rounding keeps the requested number of digits, counted from the first non-zero digit.

The principle

Every measurement has limited precision, and the last recorded digit is the first uncertain one. Writing a trailing zero after the decimal point is a statement that the zero was actually measured, whereas leading zeros only express size. Scientific notation removes the ambiguity: 1.20 × 10³ clearly has 3 significant figures, while 1,200 on its own could have 2, 3 or 4.

Worked example

Input: 0.004560, round to 2 significant figures.
The leading "0.00" is not significant; 4, 5, 6 and the trailing 0 after the decimal point are.
Significant figures in the input: 4.
Keep two significant digits, 4 and 5; the next digit is 6, which is 5 or more, so round 5 up to 6.
Rounded result: 0.0046.

How to read the result

The count tells you how precisely the number was stated: 4 sig figs means a relative precision of roughly one part in a thousand. In calculations, a product or quotient should be reported with as many sig figs as the least precise input, while a sum or difference should be rounded to the least precise decimal place.

Common mistakes

Counting leading zeros, so that 0.0046 is wrongly said to have 5 figures.
Removing a measured trailing zero (writing 2.5 for a reading of 2.50).
Applying sig-fig rules to exact numbers such as counted items or defined constants (12 eggs, 100 cm in 1 m), which have unlimited precision.

Keep in mind: Significant figures are only a rough guide to uncertainty. For formal measurement work, state an explicit uncertainty as described in the GUM, Binary floating point can also round a halfway case such as 2.675 differently from hand calculation.

Frequently asked questions

How many significant figures does 100 have?

It is ambiguous: 1 by the usual rule, but it could be 2 or 3. Write 1.0 × 10² or 1.00 × 10² to show the intended precision.

How is rounding to sig figs different from rounding to decimal places?

Decimal places count from the decimal point; sig figs count from the first non-zero digit. 0.004560 to 2 decimal places is 0.00, but to 2 sig figs it is 0.0046.

Sources: JCGM 100:2008, Evaluation of measurement data — Guide to the expression of uncertainty in measurement (GUM) · ISO 80000-1:2009, Quantities and units — Part 1: General, Annex B (rounding of numbers)

Checked 8 Nov 2026. Standard formulas, computed in your browser with double-precision numbers.

Your numbers