Case 1
- Image height
- 20
- Object height
- 5
4.00
Open with these valuesMagnification Calculator
4.00
Result: 4.00Magnification is the image height divided by the object height. Above 1 the image is enlarged, below 1 it is reduced, and exactly 1 means it is the same size. Both heights must use the same unit; the units cancel and the answer is a plain number.
Worked examples
Case 2
- Image height
- 5
- Object height
- 20
0.25
Open with these valuesCase 3
- Image height
- -15
- Object height
- 5
-3.00
Open with these valuesHow it's calculated
m = image height ÷ object height
- StepMeasure the object and the image in the same unit — millimetres, centimetres or inches all work.
- StepEnter the image height first, the object height second.
- ResultRead the ratio; multiply it by 100 if you want a percentage.
Reference table
| Image height, object height | Reading | Magnification |
|---|---|---|
| 20, 5 | Enlarged four times, 400 % | 4 |
| 150, 25 | Enlarged six times, 600 % | 6 |
| 10, 10 | Same size, 100 % | 1 |
| 3, 4 | Reduced to three quarters, 75 % | 0.75 |
| 5, 20 | Reduced to a quarter, 25 % | 0.25 |
| -15, 5 | Inverted and three times larger | -3 |
Questions
How do I calculate magnification?
Divide the image height by the object height, both measured in the same unit. An image 20 mm tall of an object 5 mm tall gives 20 ÷ 5 = 4, so the image is four times larger.
What does a magnification above or below 1 mean?
Above 1 the image is enlarged, as in a microscope or a magnifying glass. Below 1 it is reduced, as when a camera lens shrinks a distant scene onto a sensor. Exactly 1 means image and object are the same size.
How do I turn magnification into a percentage?
Multiply by 100. A magnification of 4 is 400 %, and one of 0.25 is 25 %. The ratio and the percentage describe the same thing.
Does it matter which unit I use?
No, as long as both heights use the same one. Magnification is a ratio, so the units cancel: 20 mm ÷ 5 mm gives the same 4 as 2 cm ÷ 0.5 cm. Mixing units is the only way to get a wrong answer here.
Why can the result be negative?
A negative image height marks an inverted image, and the ratio carries that sign through. Enter −15 for an image that is upside down against an object of 5 and you get −3: three times larger and inverted.
Why must the object height not be zero?
The formula divides by the object height, and division by zero is undefined. An object of zero height has no image to compare against, so the calculator shows no result.
Sources and last check
Information, not professional advice.