Magnification & scale bar calculator

Magnification = image size ÷ actual size, with both lengths in the same unit. A red blood cell 7.5 µm wide drawn 7.5 mm wide is magnified 7,500 µm ÷ 7.5 µm = 1,000×. Magnification has no units, and the same formula rearranges to actual size = image size ÷ magnification or image size = actual size × magnification.

Multiply millimeters by 1,000, because 1 mm = 1,000 µm (and 1 µm = 1,000 nm). A 20 mm drawing of a cell at 400× is 20,000 µm on paper, so the real cell is 20,000 ÷ 400 = 50 µm across. Forgetting the conversion is the most common exam error and gives an answer 1,000 times too large or too small.

Multiply the eyepiece by the objective: a 10× eyepiece with a 40× objective gives 400×, and with a 100× oil-immersion objective gives 1,000×. Higher magnification shrinks the field of view, so at 400× a typical field is about 0.45 mm (450 µm) across and roughly nine 50 µm cells would fit in a row. Add a scale bar to a drawing by multiplying the real length by the magnification: 20 µm at 400× is an 8 mm bar.