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Significant figures, standard form and units

Edexcel IAL Biology · WBI13 Practical Skills in Biology I

Processing marks are often lost on rounding, missing units or a wrong unit conversion. These rules are quick to learn and appear in every paper.

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Significant figures and decimal places

Count significant figures from the first non-zero digit: 0.0450 has 3 s.f.; 1200 is ambiguous (write 1.2 x 10³ for 2 s.f.).
A calculated answer should usually have the same number of significant figures as the least precise data used, unless the question says otherwise.
In a table, record every reading in a column to the same number of decimal places, matching the resolution of the instrument (a balance to 0.01 g: 2.50 g, not 2.5 g).
Round only at the end; do not round 12.5 to 13.0 (the June 2024 report).

Standard form

Very large or small numbers: a x 10ⁿ with 1 ≤ a < 10.

3 400 000 bacteria cm⁻³ = 3.4 x 10⁶ cm⁻³
0.000 025 m = 2.5 x 10⁻⁵ m

If a question asks for standard form, an answer of 100 000 instead of 1 x 10⁵ loses the mark.

Units and conversions

Length: 1 mm = 1000 µm; 1 µm = 1000 nm.
Volume: 1 dm³ = 1000 cm³; 1 cm³ = 1000 mm³; 1 cm³ = 1 ml.
Time: 1 min = 60 s.
Rates use 'per': mm³ min⁻¹, cm³ s⁻¹. Write units as 'cm³ s⁻¹' or 'cm³ per s'.

Units go in column headings and on axis labels, separated by a solidus: 'Time / s'. Never put units in each cell of a table.

Convert	Method	Result
0.45 mm to µm	x 1000	450 µm
2500 mm³ to cm³	÷ 1000	2.5 cm³
0.38 dm³ min⁻¹ to cm³ s⁻¹	x 1000 ÷ 60	6.3 cm³ s⁻¹

Worked example

A cell is 0.032 mm long. Convert this to micrometres and write the answer in standard form in metres.

0.032 mm x 1000 = 32 µm
32 µm = 32 x 10⁻⁶ m = 3.2 x 10⁻⁵ m

Answer: 32 µm; 3.2 x 10⁻⁵ m

Key idea: Count s.f. from the first non-zero digit; match the least precise data; same d.p. down a column; round only at the end. Standard form a x 10ⁿ. 1 mm = 1000 µm, 1 cm³ = 1000 mm³, 1 dm³ = 1000 cm³.

Check yourself

How many significant figures are there in 0.0450?
Show answer
3. Start at 4: 4, 5 and the final 0 are significant.
Convert 0.085 mm to micrometres.
Show answer
85 µm. 0.085 x 1000 = 85 µm.
Convert 3750 mm³ to cm³.
Show answer
3.75 cm³. 3750 ÷ 1000 = 3.75 cm³.
Convert a ventilation rate of 0.42 dm³ min⁻¹ to cm³ s⁻¹. Give your answer to 2 significant figures.
Show answer
7.0 cm³ s⁻¹. 0.42 x 1000 = 420 cm³ min⁻¹; ÷ 60 = 7.0 cm³ s⁻¹.
A culture contains 4 600 000 cells per cm³. Written in standard form this is a x 10⁶. What is a?
Show answer
4.6. 4 600 000 = 4.6 x 10⁶.
Which set of masses is recorded correctly from a balance that reads to 0.01 g?
- 2.5, 2.47, 2.6
- 2.50, 2.47, 2.60
- 2.500, 2.47, 2.6
- 3, 2, 3
Show answer
2.50, 2.47, 2.60. Same number of decimal places, matching the balance resolution.
Where should the units go in a results table?
- After every value in each cell
- In a footnote only
- Units are not needed in tables
- In the column heading, for example 'Time / s'
Show answer
In the column heading, for example 'Time / s'. The January 2025 report noted candidates who put units in every cell.

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Last updated 8 November 2026. Spotted a mistake? Tell us.