Rounding significant figures

The method for rounding to a given number of significant figures is almost identical to the method used for rounding to a given number of decimal places.

Look at the number after the one you're interested in to see whether it is greater or less than 5. If it's less than 5, round down. If it's 5 or more, round up.

Question

Q1. Round 0.0724591 to 3 significant figures (s.f.)

Q2. Round 0.2300105 to 4 significant figures (s.f.)

Q3. Round 648,942 to 2 significant figures (s.f.)

A1.

Diagram of the number 0.0724591

To round to three significant figures, look at the fourth significant figure. It's a 5, so round up.

Therefore, 0.0724591 = 0.0725 ( {3} s.f.)

A2.

Diagram of the number 0.2300105

To round to four significant figures, look at the fifth significant figure. It's a 1, so round down.

Therefore, 0.2300105 = 0.2300 ( 4 s.f.)

A3.

Diagram of the number 648,942 showing the 1st, 2nd and 3rd significant figures

To round to two significant figures, look at the third significant figure. It’s an 8, so round up.

Therefore, 648,942 = 650,000 ( 2 s.f.)

curriculum-key-fact
Even though 0.2300 is the same as 0.23, include the zeros to show that you have rounded to four significant figures.