Microscope Magnification Calculator
Work out the total magnification from your objective and eyepiece, the diameter of the field of view you are looking at, and the true size of a specimen from a drawing, a photograph or a scale bar. A fourth mode estimates cell size from the number of cells that fit across the field. The working is shown each time. Runs entirely in your browser. Free, no sign-up.
1 unless your microscope has an intermediate or zoom lens.
Printed on the eyepiece, often 18 to 26.5; 22 is typical for a 10× eyepiece.
Results
100×
Total magnification
2.2
Field of view diameter (mm)
2,200
Field of view diameter (µm)
3.801
Field area (mm²)
The working
total = objective × eyepiece × extra lens = 10 × 10 × 1 = 100×
field of view = field number ÷ (objective × extra lens) = 22 ÷ (10 × 1) = 2.2 mm
Field of view with each common objective
| Objective | Total magnification | Field of view (mm) | Field of view (µm) |
|---|---|---|---|
| 4× | 40× | 5.5 | 5,500 |
| 10× | 100× | 2.2 | 2,200 |
| 20× | 200× | 1.1 | 1,100 |
| 40× | 400× | 0.55 | 550 |
| 60× | 600× | 0.367 | 367 |
| 100× | 1,000× | 0.22 | 220 |
How to Use the Microscope Magnification Calculator
- 1. Pick the question. Total magnification and field of view; the actual size of something from its drawing or photo; the actual size from a scale bar; or the size of a cell from how many fit across the field.
- 2. Enter what you know. The objective is the number engraved on the lens barrel, the eyepiece is usually 10×, and the field number is printed on the eyepiece. Keep the extra lens factor at 1 unless your microscope has a zoom or intermediate lens.
- 3. Check the units. Image sizes can be in mm or cm when measured on a print, or in pixels for a scale bar on a screen; actual sizes are normally in micrometres.
- 4. Read the result and the working. The table shows the same eyepiece with each common objective, so you can see how fast the field shrinks as you zoom in.
Total Magnification and Field of View
Total magnification
objective × eyepiece (× any extra lens)
Field of view
FOV = field number ÷ (objective × extra lens)
Field area
area = π × (FOV ÷ 2)²
The field number is the diameter, in millimetres, of the circle you see at the eyepiece's intermediate image plane — Nikon's MicroscopyU defines it that way and gives the same formula. Divide it by the objective magnification (and by any extra lens in the path) to get the diameter of the circle you see on the specimen. The Microscope World guide lists typical field numbers: 26 mm for a 5× eyepiece, 22 mm for a 10×, and 7 mm for a 30×. The calculator opens with 22 mm and a 10× objective and eyepiece. Eyepieces vary, so read yours.
For those defaults the total magnification is 100× and the field of view is 22 ÷ 10 = 2.2 mm (2,200 µm), an area of 3.801 mm². With the same eyepiece, the 4× objective gives 40× and a 5.5 mm field, the 40× gives 400× and 0.55 mm, and the 100× gives 1000× and 0.22 mm. The field shrinks in proportion to the objective, which is why you should centre the specimen at low power before switching up.
Actual Size From an Image: The IAM Triangle
An image is the actual size multiplied by the magnification, I = A × M. Rearranged, the actual size is A = I ÷ M and the magnification is M = I ÷ A. Convert both sizes to the same unit before dividing. A cell drawn 36 mm long at 400× magnification has an actual length of 36 mm ÷ 400 = 0.09 mm, which is 90 µm (the calculator's default). Going the other way, a 90 µm cell drawn at 400× is 36 mm long.
Actual Size From a Scale Bar
A scale bar is better than a stated magnification, because it stays correct when a photograph is resized, cropped or printed. Measure the scale bar with a ruler (or in pixels on a screen), measure the feature the same way, and scale by the ratio:
Actual size
feature measured ÷ bar measured × length the bar stands for
If a bar labelled 100 µm measures 25 mm on your print and a cell measures 18 mm, the cell is 18 ÷ 25 × 100 = 72 µm long, and the image magnification is 25 mm ÷ 100 µm = 250×. In pixels there is no magnification to report, only a calibration of micrometres per pixel.
Estimating Cell Size From the Field
Count how many identical cells span the diameter of the field and divide the field diameter by that number. With a 2,200 µm field and 20 cells in a row, each cell is about 110 µm long. The method is only as accurate as the field number and your count, so use it for a first estimate and calibrate with a stage micrometer for measurements you will report.
Magnification is not resolution
Total magnification says how large the image looks, not how much detail it holds. Detail is limited by the objective's numerical aperture (NA) and the wavelength of light: the smallest resolvable separation is about 0.61 × wavelength ÷ NA. For green light of 550 nm and a 100× oil objective with NA 1.25, that is about 0.27 µm. Raising the magnification beyond what the optics can resolve (empty magnification) only enlarges the blur.
Frequently Asked Questions
- How do you calculate total magnification of a microscope?
- Multiply the magnification of the objective lens by the magnification of the eyepiece, and by any extra lens in the light path. A 40× objective with a 10× eyepiece gives 400×. The calculator does the multiplication and also gives the field of view.
- How do you calculate the field of view of a microscope?
- Divide the eyepiece's field number in millimetres by the objective magnification: FOV = field number ÷ objective. With a field number of 22 mm and a 40× objective, the field is 0.55 mm, or 550 µm across.
- How do you calculate the actual size of a specimen from a drawing?
- Divide the size of the drawing by the magnification: actual size = image size ÷ magnification. A drawing 36 mm long at 400× shows a specimen 0.09 mm, or 90 µm, long. Use the same length unit for both sizes before dividing.
- How do you find the magnification of an image with a scale bar?
- Measure the scale bar, then divide its measured length by the length it represents, in the same unit. A bar that measures 25 mm and is labelled 100 µm gives 25 mm ÷ 0.1 mm = 250×. The result is only valid for the size at which you measured the image.
- Why does the field of view get smaller at higher magnification?
- The eyepiece shows a fixed-diameter circle, the field number, but the objective enlarges the specimen more, so a smaller area of the slide fills it. The field of view falls in direct proportion to the objective magnification: a 40× objective shows a field one tenth the diameter of a 4× one.
- How accurate is cell size estimated from the field of view?
- Typically good to about 10–20%, limited by how well you know the field number and how many cells you can count across. For measurements that go in a report, calibrate with a stage micrometer or use a scale bar on a calibrated image.