Significant figures calculator
Round to significant figures, and find out how many the number you typed actually has — including when nobody can tell. Nothing you type here leaves your browser — it is worked out on your own device, so there is nothing to send and nothing to log.
Figures are not places
Two decimal places means “two digits after the point”. Two significant figures means “two digits of actual information”, counted from the first non-zero digit wherever it happens to be. So 0.00123 to two figures is 0.0012, while 123456 to two figures is 120000. The leading zeros in the first are placeholders telling you where the point is; they measure nothing.
1200 might be two figures, or three, or four
Write 1200 and there is no way to tell whether those zeros were measured or are just holding the place. A crowd of 1200 counted one by one has four significant figures. A crowd estimated at “about 1200” has two. The digits are identical.
Every other counter picks one reading silently. This one says so, and gives you the two ways of settling it:
| Scientific notation | 1.2 × 103 is two figures. 1.200 × 103 is four. This is the unambiguous way and the one to use in writing. |
|---|---|
| A trailing point | 1200. means all four
digits are significant. Older, terser, easy to mistake for a typo
— but this page reads it. |
Zeros after the point are never ambiguous.
0.001200 is four figures, full stop: nobody writes those
last two zeros unless they mean them.
The rules, in full
- Every non-zero digit counts.
- Zeros between non-zero digits count. 1002 is four figures.
- Leading zeros never count. 0.0025 is two figures, not four.
- Trailing zeros count if there is a decimal point. 2.50 is three figures; it says the hundredths were measured and found to be nothing.
- Trailing zeros before a decimal point are the ambiguous case above.
Carrying figures through a calculation
Multiplying and dividing: the answer gets the same number of significant figures as the least precise input. Adding and subtracting: the answer gets the same number of decimal places as the least precise input — a different rule, and the one people mix up. And round once, at the end, never at each step.
What it will not do
- It will not decide how many figures your measurement deserves. That comes from the instrument, not from the number.
- It will not write the answer in scientific notation for you. It says when you need it and why.
- It will not round to decimal places. That is the rounding calculator, which also explains what halfway does.