Units: milli, micro, nano and picoSpec 1.5
In short
Milli, micro, nano and pico are units each 1000 times smaller than the one before: 1 mm = 1000 µm, 1 µm = 1000 nm and 1 nm = 1000 pm. To convert to a smaller unit multiply by 1000, and to a larger unit divide by 1000. Standard form uses negative powers of ten for small numbers.
Cells are far too small to measure in metres, so we use smaller units. Each unit below is 1000 times smaller than the one before it.
| Prefix | Symbol | Power of ten | Unit of length |
|---|---|---|---|
| milli | m | 10⁻³ | millimetre (mm) |
| micro | µ | 10⁻⁶ | micrometre (µm) |
| nano | n | 10⁻⁹ | nanometre (nm) |
| pico | p | 10⁻¹² | picometre (pm) |
To go to a smaller unit, multiply by 1000: mm to µm, µm to nm, nm to pm. To go to a larger unit, divide by 1000. So 1 mm = 1000 µm, 1 µm = 1000 nm and 1 nm = 1000 pm.
Higher tier (what this means)
Higher tier: only on the Higher tier papers. Foundation students can skip it. What the labels meanStandard form
Higher tier (what this means)
Higher tier: only on the Higher tier papers. Foundation students can skip it. What the labels meanStandard form writes a number as a number between 1 and 10 multiplied by a power of ten. For example, 0.000 045 m is 4.5 × 10⁻⁵ m, and 3 200 000 is 3.2 × 10⁶. A negative power means a small number. You must be able to do calculations with numbers written in standard form.
Higher tier (what this means)
Higher tier: only on the Higher tier papers. Foundation students can skip it. What the labels meanCalculating with standard form
A cell is 2.5 × 10⁻⁵ m long. It is viewed at a magnification of ×400. Calculate the length of the image in mm.
- image size = real size × magnification
- 2.5 × 10⁻⁵ × 400 = 1.0 × 10⁻² m
- 1.0 × 10⁻² m = 0.01 m
- 0.01 m × 1000 = 10 mm
Answer: 10 mm
Convert to one unit before you calculate. Mixing mm and µm in one calculation is the most common way to lose marks.
Written and checked against the Edexcel GCSE Biology (1BI0) specification · Updated October 2026