IGCSE Physics / Practical and maths skills
Significant figures and rounding revision notes
Edexcel International GCSE Physics (4PH1), chapter 29: Practical and maths skills, specification point M2A, M1A. Free lesson.
Watch the video lesson · IGCSE Physics past papers
Significant figures show how precisely a measurement is known. Use them to report answers at a sensible precision, and round only at the end of a calculation.
How to count significant figures
Count from the first digit that is not zero. The digits after that carry meaning, including some zeros. This helps you show the precision of the number without counting zeros that only set the place value. 2.46 has 3 significant figures because every digit counts. 0.052 has 2. The zeros before the 5 only hold the place. 4.07 has 3. A zero between other digits counts. 1.60 has 3. A zero at the end after the decimal point counts because it shows the measurement's precision.
How to round a calculated answer
Give a calculated answer to the same number of significant figures as the data, usually 2 or 3. If the question tells you exactly how many significant figures to use, follow that instruction. Keep the calculator value before rounding so you do not lose accuracy.
Why you should not round in the middle
Rounding an intermediate value can change the final answer. For example, 7 ÷ 3 is a recurring decimal. Replacing it with 2.3 before calculating the force gives a different result, so keep every figure or use the calculator memory until the end. Using 1200 × 2.3 gives 2760 N. Using 1200 × 7/3 gives 2800 N. Rounding the acceleration to 2.3 too early changes the force by 40 N.
How to estimate without a calculator
An estimate is a rough answer. Round each number to one significant figure first, then calculate. This makes the arithmetic easier while keeping the result close enough for an estimate. For 1900, use about 2000. For 98, use about 100. Then multiply the rounded values to estimate the energy.
Check your rounding
When rounding, first find the digit in the place you want to keep. Look at the next digit: if it is 5 or more, round up. Zeros may then be needed to hold the place value. To 2 significant figures, 0.00487 becomes 0.0049 because the 7 rounds the 8 up. To 3 significant figures, 52 649 becomes 52 600. The zeros hold the place.