Magnification calculator
Enter any two of magnification, image size and actual size, and this tool calculates the third, handling the unit conversions for you.
Magnification = image size ÷ actual size. Enter any two values (with units) and the tool finds the third, the calculation SPM asks for in microscopy and drawing questions.
How to use it
- Decide which two of the three values you already know: image size, actual size, or magnification.
- Enter those two values in their boxes, choosing millimetres or micrometres for each size as needed.
- Leave the third box empty; press "Compute" and the tool fills it in.
- Check the units shown next to the result, since image and actual size can be entered in different units before the tool converts them.
- Re-enter fresh values for each new question rather than reusing the previous result, since mixing values from two different diagrams gives a meaningless answer.
- Round your final answer to a sensible number of significant figures once you have it, matching the precision the question's own measurements use.
How to read the result
Magnification is image size divided by actual size, so the tool always converts both sizes to the same unit first before dividing or multiplying, which is why you can mix millimetres and micrometres between the two size boxes without doing the conversion yourself.
If you are given magnification and one size, watch which way the calculation goes: actual size equals image size divided by magnification, while image size equals actual size multiplied by magnification. Confusing these two directions is the most common way marks are lost in this type of question, so check that your answer for actual size comes out smaller than the image size, not larger.
Because the formula only involves one division or multiplication, small rounding differences between your own working and the tool's answer are usually a sign that a unit was left in millimetres instead of being converted to micrometres, rather than a sign of a wrong method.
What it does not do
The calculator only performs the magnification formula on the numbers you give it; it cannot read a diagram's scale bar for you, and it does not check whether the units in the original question match the units you have entered. It also does not know which specific structure a question is describing, so it cannot tell you whether an answer of, say, 300 micrometres is a realistic size for that structure, that judgement still needs your own biology knowledge.
Frequently asked questions
Which unit should I use, millimetres or micrometres?
How do I know which value is missing in a question?
Why is my calculated actual size larger than the image size?
What if the question gives magnification as a ratio like 400:1 instead of a plain number?
Can this tool handle drawings that are not to a stated magnification?
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