Count the Significant Figures

Type a measured value and see which of its digits actually carry information. Green digits are significant; grey ones are only there to hold the place. Each digit below gets a card stating the reason. Try the tricky cases — trailing zeros are where everyone gets caught.

Type any number, with or without a decimal point Each digit gets a card with its reason Scientific form like 4.2e3 is allowed

Try these

Why this count

The rules

1–9Every non-zero digit is significant.
0 betweenZeros between non-zero digits are significant.
0 leadingZeros before the first non-zero digit are not — they only fix the decimal place.
0 trailingTrailing zeros count only if there is a decimal point in the number.

The trailing-zero trap

Plain 100 is ambiguous — it could be 1, 2 or 3 significant figures, and the rule counts it as 1. Writing 100. with a decimal point makes all three significant. Scientific notation removes the doubt entirely: 1.00×10² is clearly 3 figures. This is why scientists prefer scientific notation for measured values.