Count or round significant figures. This free sig fig calculator counts the significant figures in any number (with correct trailing-zero and decimal rules) and rounds to a chosen number of sig figs.
Type the number exactly as written — trailing zeros and the decimal point matter. For example “120” has 2 significant figures, but “120.” has 3 and “120.0” has 4.
Rules used: all non-zero digits count; zeros between significant digits count; leading zeros don’t; trailing zeros count only when a decimal point is present.
How the Significant Figures Calculator Works
Significant figures indicate how precisely a measured or reported number is known. This calculator counts them in whatever number you type, then can round that number down to a chosen number of sig figs.
Formula: All non-zero digits are significant. Zeros sandwiched between significant digits are significant. Leading zeros (before the first non-zero digit) are never significant. Trailing zeros only count as significant when the number contains a decimal point. To round to N sig figs, the calculator keeps the first N significant digits (per these same rules) and rounds the next digit up or down accordingly, adjusting the decimal position or exponent as needed.
- Number — the value to analyze, typed exactly as written (trailing zeros and the decimal point both matter)
- Round to (optional) — the number of significant figures to round the value to
Example Scenarios
| Number | Round To | Result |
|---|---|---|
| 0.004520 | 3 | 4 sig figs; rounds to 0.00452 |
| 120 | 1 | 2 sig figs; rounds to 100 |
| 120. | 2 | 3 sig figs; rounds to 120 |
| 1002 | 2 | 4 sig figs; rounds to 1000 |
| 3.1400 | 3 | 5 sig figs; rounds to 3.14 |
| 0.0700 | 1 | 3 sig figs; rounds to 0.07 |
Significant Figures Calculator FAQ
Why does “120” have 2 significant figures but “120.” have 3?
Without a decimal point, the trailing zero in “120” is ambiguous — it might just be a placeholder, so it’s not counted as significant, giving 2 sig figs. Adding a decimal point (“120.”) signals that the zero was actually measured, so all three digits count.Do leading zeros ever count as significant figures?
No. Leading zeros — any zeros before the first non-zero digit, like the two zeros in “0.0045” — are never significant. They only serve to place the decimal point and don’t reflect measurement precision, so the sig fig count starts at the first non-zero digit.How do trailing zeros after a decimal point count?
Trailing zeros are always significant once a decimal point is present, since writing them intentionally signals precision. For example, “3.1400” has 5 significant figures — the two trailing zeros show the measurement was precise to that many decimal places.How do I round a number to a specific number of sig figs without changing its value?
Enter your number in the Number field and your target count in the “Round to” field. The calculator rounds based on the digit right after your Nth significant figure, following standard rounding rules, and returns the correctly rounded value at that precision.Why does the number of decimal places differ from the number of significant figures?
Decimal places count digits after the decimal point only, while significant figures count all meaningfully precise digits, including ones before the decimal point. A number like “1002” has zero decimal places but 4 significant figures, since the internal zeros count.How are significant figures used in science and lab reports?
They communicate the precision of a measurement so results aren’t reported with more (or less) certainty than the instrument actually provided. Calculations combining measured values are typically rounded to match the least precise input’s number of significant figures.Related Calculators
Once you’ve counted your sig figs, the rounding calculator covers other rounding rules like decimal places or nearest multiples, while the scientific notation converter is useful for expressing very large or small precise values compactly. For general arithmetic on the numbers involved, the scientific calculator is a good companion tool.