Significant figures are the digits that carry real precision. The rules: non-zero digits always count; zeros between them count; leading zeros never count; trailing zeros after a decimal point count. To round to N sig figs, keep the first N meaningful digits from the first non-zero one and round. So 3.14159 → 3 sig figs is 3.14.
Reviewed: June 20, 2026 · Author: Naveen P N, Founder — AI Calculator · Verified against: sig-fig rules and toPrecision rounding, recomputed in code.
The rules
Counting starts at the first non-zero digit. 0.00789 has three sig figs (7, 8, 9) — the leading zeros just place the decimal. 100.0 has four, because the trailing zeros come after a decimal point. A bare 1200 is ambiguous; scientific notation (1.2 × 10³ vs 1.200 × 10³) makes the intended precision explicit.
Worked examples
Rounding π to 3 and 4 sig figs:
A small number, 0.00789 to 2 sig figs:
A large number, 12345 to 3 sig figs:
So π keeps three meaningful digits as 3.14, 0.00789 becomes 0.0079, and 12345 rounds to 12300 — the first three significant digits with trailing place-holders.
Frequently Asked Questions
The meaningful precision-carrying digits. 3.14159 has 6; 0.00789 has 3.
Keep the first N meaningful digits, round the rest. 3.14159 → 3 sf = 3.14; 12345 → 3 sf = 12300.
No. 0.0456 has 3 sig figs (4,5,6). Leading zeros only set the decimal place.
Ambiguous. 1200 could be 2–4 sig figs. Scientific notation removes the doubt.
Decimal places count after the point; sig figs count meaningful digits from the first non-zero. 0.0456 = 4 dp but 3 sf.