The formula
Magnification = Image size ÷ Actual size (M = I ÷ A)
Rearranged: Image size = Magnification × Actual size (I = M × A)
Rearranged: Actual size = Image size ÷ Magnification (A = I ÷ M)
Use the triangle method: cover the variable you want to find, and the remaining two tell you the calculation.
Unit conversions
1 mm = 1000 μm. Both values must be in the same unit before you calculate. Common approach: convert everything to μm first, then divide.
Example: Image = 48 mm, Actual = 120 μm. Convert image to μm: 48 × 1000 = 48 000 μm. Magnification = 48 000 ÷ 120 = ×400.
Worked examples
Example 1: A cell is 50 μm long. A drawing of the cell is 100 mm long. What is the magnification?
Image = 100 mm = 100 000 μm. Magnification = 100 000 ÷ 50 = ×2000.
Example 2: A photograph has magnification ×500. The image of a cell is 25 mm long. What is the actual size?
Actual = 25 mm ÷ 500 = 0.05 mm = 50 μm.