Biology · Grades 7–11
Cells & microscopy · Formula reference
Formula reference
Rule and assumptions
Actual size from an image
Magnification is image size divided by actual size. Convert millimetres to micrometres consistently.
actual size (µm) = image size (mm) × 1000 / magnification
Surface-area to volume ratio
For a cube, surface area is 6a² and volume is a³. Their ratio decreases as side length increases.
SA/V = 6s²/s³ = 6/s for a cube
Percentage mass change
Use initial mass in the denominator. A negative percentage represents a decrease from the starting mass.
(final − initial)/initial × 100
Further reading and review references
These are reference links, not copied exercises, institutional endorsement or proof of independent review.
Learning preview · independent educational and translation review required. Suggested grades are not official curriculum alignment.