Rounding & Significant Figures Calculator
Round a number to a chosen number of decimal places or significant figures.
How rounding and significant figures work
Rounding to decimal places fixes how many digits appear after the decimal point. Rounding to significant figures instead fixes how many meaningful digits are kept overall, regardless of where the decimal point falls.
Enter a value plus the number of decimal places and significant figures you want. The calculator shows both roundings side by side so you can compare them.
Significant figures matter most in scientific and engineering contexts, where they communicate measurement precision: 1230 (3 sig figs) implies less precision than 1230.0 (5 sig figs), even though both display similarly.
The formula
Decimals: round(value × 10^d) ÷ 10^d. Sig figs: scale by magnitude, round, rescale.
Decimal rounding multiplies by 10 to the power of the decimal count, rounds to the nearest whole number, then divides back. Significant-figure rounding first finds the value's order of magnitude, scales so the desired digit count lands on a whole number, rounds, then rescales.
Worked example
1234.567, rounded to 2 decimals and to 3 significant figures
To 2 decimal places: 1234.57. To 3 significant figures: 1230 (the first three meaningful digits, 1-2-3, with the rest rounded away).
Frequently asked questions
What's the difference between decimal places and significant figures?
Decimal places count digits after the decimal point only. Significant figures count all meaningful digits in the number, so a large number like 1234.567 can be rounded to very few significant figures (1230) while still having zero decimal places.
Does this use standard "round half up" rounding?
Yes, with a floating-point-safe correction so common edge cases like 1.005 round to 1.01 as expected, rather than being caught by binary floating-point representation error.
Why would I need significant figures instead of decimal places?
Significant figures are the standard for reporting measurement precision in science and engineering, where the number of meaningful digits — not the position of the decimal point — is what communicates how precise a measurement really is.
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