Loading page
About this tool
Count significant figures with the reason for every digit, round to any number of sig figs, and add, subtract, multiply or divide using sig-fig rules.
This significant figures calculator reads your number exactly as you typed it, as text, so trailing zeros such as the last two in 2.500 are never lost to rounding the way they would be if the number were converted to a float. It marks every digit as significant or not and gives the rule behind each decision: non-zero digits, zeros between them, leading zeros, and trailing zeros with or without a decimal point. It also rounds to any number of significant figures, does sig-fig addition, subtraction, multiplication and division with the correct rule for each, and shows results in standard, scientific and e-notation. Everything runs in your browser.
Open Significant Figures Calculator on AltFTool — it loads instantly in your browser.
In the Count tab, type a number exactly as written — for example 0.004500 or 6.022 × 10^23 — and read the Significant figures total with each digit tile marked.
In the Round tab, enter the number, set Significant figures (1 to 100) and choose Round half up or Round half to even to get the Standard form, Scientific notation and E-notation.
In the Calculate tab, enter Value A and Value B, pick + − × or ÷, and tick A is an exact number for counted values; the rule used and the unrounded result appear with the answer.
In the Batch tab, paste one number per line to count up to 500 values at once, then use Download CSV.
Each digit is drawn as a tile — highlighted when it counts, struck through when it is only a placeholder — with the rule that applies to it listed underneath.
1200 is reported as 2 significant figures with a warning that it could be up to 4, and the tool shows how to write it unambiguously as 1200. or 1.200 × 10³.
Addition and subtraction round to the least precise decimal place; multiplication and division keep the fewest significant figures. Exact numbers can be excluded.
Values are combined as big integers before rounding, so 0.1 + 0.2 is exactly 0.3 and no floating-point error leaks into the last digit.
| Rule | Example | Significant figures |
|---|---|---|
| Non-zero digits are always significant | 4.562 | 4 |
| Zeros between non-zero digits are significant | 1002 | 4 |
| Leading zeros are never significant | 0.0045 | 2 |
| Trailing zeros after a decimal point are significant | 2.500 | 4 |
| Trailing zeros in a whole number with no decimal point are ambiguous (usually not counted) | 1200 | 2 (up to 4) |
| A trailing decimal point makes those zeros significant | 1200. | 4 |
| In scientific notation, count only the digits in the coefficient | 1.20 × 10³ | 3 |
| Exact numbers (counts, defined values) have unlimited significant figures | 12 eggs | ∞ |
For addition and subtraction, round the answer to the least precise decimal place among the values. For multiplication and division, round to the smallest number of significant figures among the values.
Significant figures are the digits in a number that carry meaning about its precision: every digit you actually measured or know, including the last, estimated one. In 12.30 cm there are four, because the ruler was read to a hundredth of a centimetre. Leading zeros, such as those in 0.0045, only place the decimal point and are not significant.
Start at the first non-zero digit and count to the right. Count every non-zero digit and every zero between non-zero digits. Count trailing zeros only if the number has a decimal point. Never count leading zeros. So 0.04050 has four significant figures: 4, 0, 5 and the final 0, which is after the decimal point.
Trailing zeros are significant when the number contains a decimal point, as in 2.500 (four) or 1200. (four). In a whole number written without a decimal point, such as 1200, they are ambiguous: they might be measured or just placeholders. Most courses count them as not significant, which gives two. Scientific notation, 1.200 × 10³, removes the doubt.
Written as 100 with no decimal point, it has one significant figure by the usual convention, because the two zeros could simply be placeholders. Written as 100. with a trailing decimal point it has three. In scientific notation you can show any precision: 1 × 10² is one, 1.0 × 10² is two, and 1.00 × 10² is three.
Find the first non-zero digit, count across to the number of figures you need, then look at the next digit: if it is 5 or more, round the last kept digit up; otherwise leave it. Replace dropped digits to the left of the decimal point with zeros. For example 123,456 to 3 significant figures is 123,000, and 0.004567 to 2 is 0.0046.
The result should have as many significant figures as the value with the fewest. For 4.56 × 1.4 the exact product is 6.384, but 1.4 has only two significant figures, so the answer is 6.4. The same applies to division: 10.0 ÷ 3.00 = 3.33, with three significant figures because both values have three.
For addition and subtraction it is decimal places that matter, not the count of significant figures. Round the answer to the last decimal place that every value shares. 12.11 + 18.0 = 30.11 exactly, but 18.0 is only known to the tenths place, so the answer is 30.1. A sum can therefore have more or fewer significant figures than its parts.
No. Counted quantities such as 12 students and defined values such as 100 cm in 1 m are exact, so they have unlimited significant figures and never limit the precision of a result. In the Calculate tab, tick the exact number box for that value and the rule is applied using only the measured value.
Zero is a special case and textbooks differ. This calculator treats the zeros after the decimal point in a zero value as showing precision, so 0.0 has one and 0.000 has three, while a plain 0 is counted as one. If your course uses a different convention for zero, follow your course.
Ordinary calculators store numbers in binary floating point, where 2.5 and 2.500 are the same value, so the zeros that show precision disappear. This tool reads your input as text and counts digits as written, and its arithmetic uses exact decimal integers, so 2.500 keeps four significant figures and 0.1 + 0.2 gives exactly 0.3.
Add the Significant Figures Calculator widget to your blog or website — free, responsive, no signup. Just keep the “Widget by AltFTool” credit link visible.
<iframe src="https://www.altftool.com/embed/widget/significant-figures-calculator"
title="Significant Figures Calculator — free AltFTool widget"
width="100%" height="640" style="border:0;border-radius:12px;overflow:hidden"
loading="lazy" referrerpolicy="no-referrer-when-downgrade" allow="clipboard-write"></iframe>
<p style="font-size:12px;margin:4px 0 0">Widget by <a href="https://www.altftool.com/tools/all/significant-figures-calculator?utm_source=embed&utm_medium=widget" rel="nofollow">AltFTool — free online tools</a></p>