What are significant figures?

Significant figures are the digits in a measured value that carry meaningful precision — every reliably known digit, plus one estimated digit. They indicate how precisely a quantity was actually measured.

Do leading zeros count as significant?

No. Leading zeros (like the zeros in 0.0025) are never significant — they only indicate the position of the decimal point, not measurement precision.

Do trailing zeros count as significant?

Only if there's a decimal point. In 1500, the trailing zeros are ambiguous and conventionally not counted (2 sig figs). In 1500.0, the decimal point makes all 5 digits significant.

Why does 100 have only 1 significant figure by this calculator's rules?

Without a decimal point, trailing zeros in a whole number are considered not significant under the standard convention, since there's no way to know if they represent measured precision or just placeholders.

What's the rule for addition and subtraction?

The result is rounded to match the fewest decimal places among the values being added or subtracted — not the fewest significant figures. For example, 12.11 + 18.0 = 30.1 (rounded to 1 decimal place, matching 18.0).

What's the rule for multiplication and division?

The result is rounded to match the fewest significant figures among the values involved — regardless of decimal places. For example, 4.56 × 1.4 = 6.4 (rounded to 2 significant figures, matching 1.4).

Why do addition and multiplication use different rules?

Addition/subtraction precision is limited by the least precise decimal place (an absolute uncertainty), while multiplication/division precision is limited by the least precise relative uncertainty, which is what significant figures actually measure.

Where do significant figures matter most?

They're essential in any lab science (chemistry, physics), engineering measurements, and any context where reporting more digits than you actually measured would misleadingly imply greater precision than the data supports.

Significant Figures Calculator — Count, Round, and Calculate With Sig Figs

Our Significant Figures Calculator covers everything you need for working with measurement precision: count how many significant figures a number has (following the standard leading-zero, trailing-zero, and decimal-point rules), round any number to a chosen number of significant figures, or perform addition, subtraction, multiplication, and division between two measured values with the correct significant-figure rounding rule automatically applied to the result.

Quick Answer

Significant figures are the digits in a number that carry real measurement precision. This calculator counts them using the standard leading/trailing-zero rules, rounds any value to a chosen number of sig figs, and applies the correct sig-fig rounding rule when adding, subtracting, multiplying, or dividing two measurements.

Number (as measured)

Enter a value, then click Calculate.

How to Use the Significant Figures Calculator — Count & Round Sig Figs Online

1
Choose a mode: Count, Round, or Arithmetic.
2
In Count mode, type a number exactly as measured (including trailing zeros) to see its significant figure count.
3
In Round mode, enter a value and the number of significant figures to round to.
4
In Arithmetic mode, enter two measured values and an operation — the result is automatically rounded using the correct sig-fig rule for that operation.

Why Use Significant Figures Calculator — Count & Round Sig Figs Online?

Significant figures communicate how precisely a value was actually measured, and the rules for counting them are more subtle than most people remember: leading zeros never count, zeros between other digits always count, and trailing zeros only count if a decimal point is present. Getting this wrong in a lab report or engineering calculation can imply false precision or hide real precision. This calculator applies the exact standard rules for counting, rounding, and — critically — for arithmetic, where addition/subtraction and multiplication/division follow two entirely different rounding rules that are easy to mix up.

Frequently Asked Questions

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