O-Level Physics Significant Figures: Record Readings and Round Results

Identify the kind of number

A Secondary 4 student asked how to choose between decimal places and significant figures after losing marks in a Physics prelim. Begin by identifying what you are writing: a raw reading, an intermediate calculator result or a final answer. The justification differs for each. Check any instruction on the question or paper before applying a general habit.

Decimal places count the digits after the decimal point. Significant figures count from the first non-zero digit. For example, 0.0240 has four decimal places and three significant figures. The final zero matters because it reports precision. By contrast, the leading zeros indicate the position of the first significant digit.

Use the instrument scale for a raw reading

In its 2026 O-Level Physics 6091 practical notes, SEAB gives a ruler example in which a length coinciding with the 12 cm mark is recorded as 12.0 cm when the ruler has millimetre divisions. The decimal place shows that the measurement was recorded to a millimetre in centimetre units.

Before making a table, identify what the instrument can resolve and how you will express it. Keep entries for the same measured quantity consistent when they come from the same scale. Label the heading with the quantity and unit, so that a column of numbers remains interpretable even if it is read on its own.

Keep working digits and round at the end

Suppose an invented practical calculation uses a potential difference of 2.46 V and a current of 0.38 A. Dividing gives about 6.47368 Ω. To two significant figures this is 6.5 Ω. If that resistance is needed in another step, retain the unrounded calculator value during the calculation so that rounding does not build up across steps.

The 6091 practical notes discuss using the least significant figure count in the raw data for calculated quantities and give specific guidance for some outputs, including ratios and gradients. Check the relevant instruction. Do not turn a worked example into a rule that every theory answer or practical entry must have the same number of figures.

Review errors separately

Take one completed question and annotate three lines: raw reading, unrounded working and final answer. Check whether the unit follows from the equation and whether a conversion was needed. Then ask whether the final number follows the requested precision.

If a school answer key differs, bring the marked example to your teacher and ask which instruction controlled the answer. Recording a measurement with a missing zero and using the wrong unit are different mistakes, so correct them separately.

Check the role of a trailing zero

Imagine that one table gives a reading of 3.00 s and another gives 3.0 s. Both represent the same numerical value, but they report different precision. Rewriting both as 3 s discards information. Likewise, 0.0700 A has three significant figures, while 0.07 A has one. When you convert from amperes to milliamperes, preserve that distinction in your notation and check the unit before entering a value into a formula.

Choose a suitable practice question from our Secondary Pure Physics papers.

Sources and further reading

• physics significant figures (8 September 2026).
• Official syllabus reference. Checked on 5 November 2026.