Count significant figures
Plain decimal notation or scientific notation (1.50e3, 1.50 x 10^3) both work.
Enter a valid number, in plain decimal or scientific notation.
Round a calculation to the correct significant figures
Type the numbers exactly as measured, including trailing zeros such as 18.0, so the tool can read their precision.
Enter two valid numbers.
Division by zero is not defined.
Counting significant figures the way GCSE and A-level chemistry expects
Every non-zero digit is always significant. The part that catches most students out is the zeros: a zero sitting between two non-zero digits is significant, a zero at the start of a number is never significant, and a trailing zero only counts once a decimal point confirms it was actually measured rather than just marking place value. This is the exact rule tested in GCSE combined science and A-level chemistry practicals, where a titration reading of 24.50 cm3 needs to keep all five digits, not be rounded down to 24.5.
| Rule | Example | Significant figures |
|---|---|---|
| Non-zero digits | 4.56 | 3 |
| Zero between digits | 1002 | 4 |
| Leading zeros | 0.0045 | 2 |
| Trailing zeros with a decimal point | 0.00450 | 3 |
| Trailing zeros with no decimal point | 1200 | 2 (ambiguous) |
Where this tool falls short
This calculator applies the standard rounding conventions taught at school: decimal places for addition and subtraction, significant figures for multiplication and division. It does not carry out full measurement uncertainty propagation and cannot tell an exact counted number (such as 12 eggs) from a measured one typed the same way, so treat the result as a starting point for lab write-ups rather than a formal error analysis.
Applying the rule to a calculation
For addition and subtraction, the answer can only be as precise as the least precise decimal place among the inputs, for example 12.11 g plus 18.0 g gives 30.1 g, not 30.11 g. For multiplication and division, the answer keeps only as many significant figures as the input with the fewest, so 4.5 cm times 2.3 cm gives 10 cm2, not 10.35 cm2. Pair this with the Scientific Calculator for the raw arithmetic, or check exact reference values with the Scientific Constants Reference.