Significant figures calculator
Count the significant figures in a number, and round it to the number you need.
Scientific notation works too, like 1.20e3.
The rules in short
Every nonzero digit counts. Zeros between nonzero digits count. Leading zeros never count, since they only place the decimal point. Trailing zeros count only when the number has a decimal point.
So 0.004050 has four significant figures, 100. has three, and 100 on its own has one. Check your instructor’s convention for whole numbers that end in zeros.