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Significant Figures Calculator
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This Significant Figures Calculator counts how many significant figures are in a number and, optionally, rounds it to a target number of significant figures — following standard rules where leading zeros aren't significant but trailing zeros after a decimal point are. Enter any number, and see its significant figure count instantly; enter a target count too, and get the correctly rounded result. It's built for chemistry and physics students who need to report measurements and calculated results with the correct precision. Everything runs locally in your browser. Scroll down to enter your number.
Features
- Counts significant figures following standard rules
- Rounds a number to a target number of significant figures
- Handles decimals and leading/trailing zeros correctly
- Instant recalculation as you type
Why use this significant figures calculator?
- Avoids manual sig-fig counting mistakes
- Useful for lab reports and homework
- Fully private — nothing uploaded
- Free with unlimited use
Frequently asked questions
What are significant figures?
Significant figures are the digits in a number that carry meaningful precision — all non-zero digits are significant, zeros between non-zero digits are significant, and trailing zeros after a decimal point are significant, but leading zeros are not.
Why isn't 100 always 3 significant figures?
Trailing zeros in a whole number without a decimal point are ambiguous — they might just be placeholders — so this calculator doesn't count them as significant unless a decimal point is present (e.g. "100." or "1.00 × 10²").
How does rounding to sig figs differ from rounding to decimal places?
Rounding to decimal places counts digits after the decimal point regardless of magnitude; rounding to significant figures counts meaningful digits from the first non-zero digit, which matters more for very small or very large numbers.
Why do trailing zeros after a decimal point count?
A value like 2.50 explicitly indicates precision to the hundredths place — the trailing zero communicates that the measurement wasn't just "2.5" but specifically "2.50".
Does this work with scientific notation input?
Yes, you can enter numbers in scientific notation (e.g. 6.022e23); use the site's Scientific Notation Converter alongside this tool if you need to convert formats first.