Significant Figures Calculator
Count the significant figures in a number, round to a given number of them, or run an operation and get the answer to the correct precision — with the rule that decided it shown alongside.
Counted digits: 450
- →Every non-zero digit is significant.
- →Leading zeros only place the decimal point, so they are not significant.
- →Trailing zeros after a decimal point record precision, so they are significant.
The rules, in short
Every non-zero digit counts.
123 has three.
Zeros between digits count.
1002 has four.
Leading zeros never count.
0.00450 has three — they only place the point.
Trailing zeros after a decimal point count.
2.50 has three; the zero is there to record precision.
Trailing zeros in a bare whole number are ambiguous.
100 is read as one by convention. Use 1.00 × 10² to mean three.
Which rule applies to which operation
× and ÷ — significant figures
The answer keeps as many significant figures as the input that has the fewest.
2.5 × 3.42 = 8.55 → 8.6
+ and − — decimal places
The answer keeps as many decimal places as the input that has the fewest.
2.5 + 3.42 = 5.92 → 5.9
Using one rule where the other belongs is the most common mistake in the topic, which is why the calculator above always names the rule it used.
Common questions
- How many significant figures does 0.00450 have?
- Three. The leading zeros only place the decimal point, so they do not count. The trailing zero is after the decimal point, so it does.
- Does 100 have one significant figure or three?
- Written plainly it is genuinely ambiguous, and no rule settles it. The usual convention reads it as one. Write 1.00 × 10² for three, or 100. with a trailing point.
- Why does 100.0 − 99.9 give only one significant figure?
- Because subtraction keeps decimal places, not significant figures. Both inputs are known to one decimal place, so the answer is 0.1 — four-figure inputs, a one-figure result. This is called loss of significance.
- Do exact numbers have significant figures?
- No. Counted quantities and defined conversions — 12 in a dozen, 2.54 cm in an inch — are exact and never limit the precision of a result.