Significant Figures Counter
Count significant figures, see exactly which digits count, and round to N sig figs. Everything runs in your browser — no uploads, no tracking.
1 · Count significant figures
2 · Round to N significant figures
3 · The rules
- Non-zero digits are always significant. 1002 → 4 sig figs
- Zeros between significant digits are significant (captive zeros). 1002 → 4 sig figs
- Leading zeros are never significant — they only place the decimal point. 0.00450 → 3 sig figs
- Trailing zeros after a decimal point are significant. 2.5000 → 5 sig figs
- Trailing zeros with no decimal point are ambiguous (counted as not significant here). 300 → 1 sig fig; write 300. → 3 sig figs, or use scientific notation
- In scientific notation every mantissa digit counts. 1.230×10⁵ → 4 sig figs
4 · Classic examples
| Number | Sig figs | Why |
|---|---|---|
| 0.00450 | 3 | leading zeros don't count; trailing 0 after the point does |
| 1002 | 4 | captive zeros are significant |
| 100. | 3 | explicit decimal point makes trailing zeros significant |
| 1.230×10⁵ | 4 | every mantissa digit counts |
| 0.050 | 2 | 5 and trailing 0 |
| 300 | 1 | ambiguous trailing zeros |
| 0.004567 → 3 sf | = 0.00457 (round half up) | |
| 999.9 → 3 sf | = 1000 | |
| 12345 → 2 sf | = 12000 | |
Frequently asked questions
How many significant figures are in 0.00450?
3. The leading zeros (0.00) only set the decimal place and are never significant. The digits 4, 5, and the trailing 0 after the decimal point are all significant.
Why does 100 have 1 significant figure while 100. has 3?
Trailing zeros in a plain integer like 100 are ambiguous — they may just be placeholders — so only the 1 counts (1 significant figure). Writing 100. with an explicit decimal point declares the zeros measured, giving 3 significant figures.
How many significant figures are in 1.230×10⁵?
4. In scientific notation every digit written in the mantissa counts, including the trailing zero: 1, 2, 3, and 0 are all significant.
How do I round a number to N significant figures?
Count N digits starting from the first non-zero digit, look at the next digit, and round half up (5 rounds up). Example: 0.004567 rounded to 3 significant figures is 0.00457, and 999.9 rounded to 3 significant figures is 1000.
Are zeros between non-zero digits significant?
Yes. Captive zeros are always significant: 1002 has 4 significant figures.
Free tool by NRTechStudio