Microscope Scale Calculator
Calculate the actual size of a specimen from the image size and total microscope magnification. Select eyepiece and objective lens presets or enter custom values. Results shown in millimetres, micrometres, and nanometres. No signup, runs entirely in your browser.
Eyepiece Magnification
Objective Magnification
Common Magnification Combinations
| Eyepiece | Objective | Total | Typical use |
|---|---|---|---|
| ×10 | ×4 | ×40 | Low power — whole organism overview |
| ×10 | ×10 | ×100 | Low-medium — tissues, large cells |
| ×10 | ×40 | ×400 | Medium — individual cells |
| ×10 | ×100 | ×1000 | High (oil immersion) — bacteria |
Microscopy Tips
Measuring from a diagram
Use a ruler to measure the length of the structure shown in your textbook diagram. The magnification is usually printed below the image (e.g. "× 400"). Divide the measured length in mm by 400 to get actual size in mm.
Scale bar method
If given a scale bar (e.g. 10 µm), measure the scale bar length in mm on the image. Magnification = scale bar mm / scale bar real size mm. Then use actual size = image size / magnification.
Common cell sizes
Human red blood cell: ~8 µm diameter. E. coli bacterium: ~2 µm long. Chloroplast: ~4–8 µm. Mitochondrion: ~1–3 µm. Plant cell: ~40–80 µm. These help you check whether your calculated size is realistic.
Resolution vs magnification
Magnification increases the apparent size of a specimen; resolution is the ability to distinguish two nearby points as separate. A light microscope resolves ~200 nm; an electron microscope resolves ~0.2 nm. Higher magnification is only useful up to the resolution limit.
Frequently Asked Questions
What is the formula for actual size?
Actual size = Image size / Magnification. The image size is measured on the micrograph (in mm), and divided by the total magnification to get the real size. To convert: 1 mm = 1,000 µm = 1,000,000 nm.
How do I calculate total magnification?
Total magnification = eyepiece lens power × objective lens power. A 10× eyepiece and 40× objective gives 400× total magnification. This is the value to enter in the magnification field.
What units should I measure the image in?
Measure the image or diagram in millimetres using a ruler. If the image is printed at a specific scale, measure the feature you want to calculate and enter that measurement in mm.
Why is µm used for cells?
A micrometre (µm, 1 × 10⁻⁶ m) is the most practical unit for cells. A typical human cell is 10–20 µm. Bacteria are 1–10 µm. Viruses and organelles are measured in nanometres (nm, 1 × 10⁻⁹ m).
What is a scale bar?
A scale bar on a micrograph shows a known physical length (e.g. 10 µm) as a line drawn to scale. You can use it to calculate magnification: measure the scale bar length in mm on the image, then divide by the stated real length.
Is my data stored?
No. All calculations are done locally in your browser with no data sent to any server.