Solve every magnification question in biology: the total magnification of a microscope, the actual size of a cell from a drawing or micrograph, the image size, or the field of view when you switch objectives. Units convert between mm, µm and nm automatically.
Magnification formulas
Worked example
A drawing of a cheek cell is 40 mm wide and the microscope magnification is ×400. Actual size = 40 mm / 400 = 0.1 mm = 100 µm. If a micrograph shows a 25 mm mitochondrion at ×25,000, its real length is 25 / 25,000 = 0.001 mm = 1 µm.
Typical sizes and the magnification you need
| Specimen | Typical size | Seen with |
|---|---|---|
| Plant cell | 10–100 µm | Light microscope ×100–×400 |
| Animal cell | 10–30 µm | Light microscope ×400 |
| Red blood cell | about 7–8 µm | Light microscope ×400–×1000 |
| Bacterium (E. coli) | about 2 µm | ×1000 oil immersion |
| Mitochondrion | 0.5–1 µm | Electron microscope |
| Virus | 20–300 nm | Electron microscope |
Frequently asked questions
How do I calculate actual size from magnification?
Divide the image size by the magnification: actual size = image size / magnification. Convert the result to µm by multiplying millimetres by 1,000.
How do I calculate total magnification?
Multiply the eyepiece magnification by the objective magnification. A ×10 eyepiece with a ×40 objective gives ×400.
What does I = A × M mean?
Image size equals actual size times magnification. Cover the value you want in the I-AM triangle to see whether to multiply or divide.
Why does the field of view get smaller at high power?
Field of view is inversely proportional to magnification. Going from ×40 to ×400 makes the field of view 10 times smaller.
How many micrometres are in a millimetre?
There are 1,000 µm in 1 mm, and 1,000 nm in 1 µm.
Related tools
Free educational tool by HawkInc. Results are calculated in your browser and rounded for display; check critical work by hand or with a second method.