Convert between millimetres and micrometres
Use one millimetre equals one thousand micrometres.
Section 2.2 · Size of specimens
Extended only (Supplement); Biology 0610, 2026–2028. Use the stated specimen features and evidence. Diagrams are schematic. Written practice is self-checked
Before you begin
- Recall the distinction between plant, animal and bacterial cells. Read the supplied evidence before drawing a conclusion.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which conversion preserves 0.08 mm?
What you will learn
- Convert in both directions.
- Check that numerical size changes appropriately when the unit changes.
The factor is one thousand
One millimetre (mm) equals 1000 micrometres (μm). Converting mm to μm multiplies the numerical value by 1000 because the smaller unit needs more units to describe the same length. Thus 0.08 mm = 80 μm. The specimen has not become larger.
Converting μm to mm divides by 1000: 250 μm = 0.25 mm. The symbol μ means micro; μm is a unit of length, not magnification. Do not use a factor of ten or confuse millimetres with centimetres.
Check direction and equivalence
A length of 2 mm is 2000 μm, whereas 2 μm is 0.002 mm. Writing the same number with a different unit changes the stated length. Each conversion should preserve the physical size and give a sensible numerical direction.
To compare two dimensions, convert them to one unit before adding, subtracting or forming a ratio. The Core lesson uses millimetres only; this conversion requirement is clearly labelled Extended (Supplement).
Pause and explain
Why does a length have a larger numerical value in μm than in mm?
Worked example
Convert 0.08 mm to μm and 250 μm to mm.
Show the worked solution
- 0.08 × 1000 = 80 μm.
- 250 ÷ 1000 = 0.25 mm.
Answer 80 μm; 0.25 mm.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Convert 0.035 mm to μm.
Give me a hint
Multiply by 1000.
Compare my reasoning
- 0.035 × 1000 = 35.
- Use μm.
35 μm.
Look for these in your work
- Correct factor.
- Correct unit.
Convert 640 μm to mm.
Give me a hint
Divide by 1000.
Compare my reasoning
- 640 ÷ 1000 = 0.64.
- Use mm.
0.64 mm.
Look for these in your work
- Correct decimal.
- Correct unit.
A cell is 0.12 mm long and another 90 μm long. Which is longer and by how much in μm?
Give me a hint
Put both in μm first.
Compare my reasoning
- 0.12 mm = 120 μm.
- 120 − 90 = 30 μm.
The first cell is longer by 30 μm.
Look for these in your work
- Match units.
- Correct comparison and difference.
Common mistakes
- Changing units does not change actual size.
- To μm multiply by 1000; to mm divide by 1000.
What can you explain now?
How many micrometres equal one millimetre?
Compare with the explanation
1000 μm.
The factor between these units is one thousand.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Explain both conversion directions and why the physical length stays the same.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.