---
title: "Magnification and Scale Calculations"
description: "Use magnification as image size divided by actual size, and convert between millimetres, micrometres and nanometres without losing a factor of a thousand. Includes reading a scale bar and choosing a l"
canonical: https://lightmysky.com/learn/science/magnification-and-scale-calculations-mt_0Iowx29ddQ
source: https://lightmysky.com/learn/science/magnification-and-scale-calculations-mt_0Iowx29ddQ.md
retrieved: 2026-09-12
---

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# Magnification and Scale Calculations

Use magnification as image size divided by actual size, and convert between millimetres, micrometres and nanometres without losing a factor of a thousand. Includes reading a scale bar and choosing a light or electron microscope by the detail needed.

Subject: Science · Area: Organisms & Life Processes · Ages 14 to 15
Page: https://lightmysky.com/learn/science/magnification-and-scale-calculations-mt_0Iowx29ddQ

## Ready when they can

- Rearranges the magnification relation to find any one of the three quantities.
- Converts a length between millimetres, micrometres and nanometres and keeps the units on the answer.
- Uses a scale bar on a micrograph to state the real width of a cell.

## Lesson: How big is that cell really

A flea photo can show it as big as your hand, but the flea has not grown. Magnification tells you how many times bigger the image is than the real thing. You find it with one rule: magnification equals image size divided by actual size. If you know any two of the three, the rule gives you the third.

**Example.** A photo shows a cell 30 mm across, and the real cell is 0.1 mm across. Divide 30 by 0.1 and you get 300, so the image is 300 times life size. The rule also runs backwards: actual size equals image size divided by magnification.

Tiny lengths need tiny units. One millimetre holds 1000 micrometres, and one micrometre holds 1000 nanometres. A mitochondrion image 32 mm across at 8000 times gives 0.004 mm, which is 4 micrometres. A scale bar shows real length: four spans of a 5 micrometre bar mean a 20 micrometre cell. Use a light microscope for living cells, which it keeps alive, and an electron microscope for finer detail when the sample need not stay alive. Always write the unit on your answer.

**Tip.** Read the scale bar before you trust your eyes. A long bar on a highly magnified photo can still mean a tiny cell. Pick a light microscope for living cells, and an electron microscope when the sample need not stay alive.

**Recap.** Divide image by real to get magnification, convert in steps of 1000, and let the scale bar tell you the truth.

## Practice

14 questions on this page, each with its working shown.

## Needs first

- [Using a Microscope](https://lightmysky.com/learn/science/using-a-microscope-mt_lMLeNLDRO8)
- [Prokaryotic and Eukaryotic Cells](https://lightmysky.com/learn/science/prokaryotic-and-eukaryotic-cells-mt_vlL4GinuH6)

## Opens up

- [Fluorescent Tagging and Reading Cell Images](https://lightmysky.com/learn/science/fluorescent-tagging-and-reading-cell-images-mt_J0toch-ZZ3)