The rules
- Every non-zero digit counts. In
47.3 all three are significant.
- Zeros between other significant digits count. In
4007 both zeros are significant.
- Zeros in front of the first non-zero digit never count. In
0.0058 only the 5 and 8 are significant.
- Trailing zeros count when a decimal point is written. In
2.500 and 250. every digit is significant.
- Trailing zeros with no decimal point are placeholders. In
4500 only the 4 and 5 are significant.
Rule 5 is a convention. Written as 4500 the zeros are ambiguous, so this app treats them as placeholders — the way most chemistry courses do. Write 4.50 × 10³ to remove the doubt.