A significant-figures tool rounds a number to a chosen number of significant digits, following standard scientific rounding rules. The Toolenza calculator handles both standard decimal and scientific-notation outputs, and shows the trailing zeros that significance requires.

Why sig figs matter (it's about honesty)

Writing 12.30 instead of 12.3 is a claim: that the trailing zero is measured, not assumed. The first form says "I measured to ±0.005"; the second says "I measured to ±0.05". Mixing these up makes results look more precise than they are.

The convention is arithmetic doesn't add precision. A weight of 2.50 kg (3 sig figs) divided by a volume of 1.20 L (3 sig figs) gives 2.08 g/cm³, not 2.083333… — the trailing digits are spurious.

The rules in three lines

• Non-zero digits are always significant. 1234 has 4 sig figs.
• Zeros between non-zeros are significant. 1002 has 4.
• Leading zeros never count; trailing zeros count only if the number has a decimal point. 0.00120 has 3; 1200 is ambiguous (write as 1.20 × 10³ for 3 sig figs, or 1200. with the trailing decimal point for 4).

Where it shows up

• Lab reports — every measured quantity must match instrument precision.
• Engineering specs — tolerances are sig-fig statements in disguise.
• Scientific papers — referees flag over-precision ("reported to 7 sig figs from an instrument accurate to 3").
• Statistics — reporting a mean of 4.32178 from n = 12 is dishonest; 4.3 ± 0.4 is honest.

Pitfalls

Don't round intermediate results. Carry full precision through calculations; only round the final answer. Rounding partway corrupts the math ("rounding error compounds") and makes the result less accurate than the inputs deserve.