How the drawings stay honest
Every length in a drawing is the length in the key. Micrographs come with a millimetre ruler drawn under them, and the line X–Y and the scale bar start and end on whole millimetres. On screen the picture shrinks or grows, but so does its ruler. Printed at 100% the ruler is life size, so a real ruler reads the same. Graticule lines meet the stage micrometer exactly where the key says, and the cells at high power fill the field edge to edge.
Example: a plant cell with a scale bar
In set BAR7GC (GCSE), the scale bar runs from 79 mm to 104 mm on the ruler, so it is 25 mm long, and it is labelled 20 µm. Magnification = 25,000 µm ÷ 20 µm = ×1,250. X–Y across the cell runs from 50 mm to 91 mm, 41 mm, so the cell is 41,000 ÷ 1,250 = 32.8 µm long, or 3.28 × 10⁻² mm.
What each course includes
GCSE: light and electron micrographs of cells, bacteria and mitochondria, sizes in µm and mm, standard form, and graticule calibration with 10 µm stage divisions (AQA lists the graticule at A-level, so leave it off if your GCSE course skips it). A-level and IB add virus particles in nm, the 0.1 mm stage micrometer, a ×100 objective, switching objectives and the magnification of a drawing; IB sets here also ask how long to draw a scale bar. US high school: total magnification, field of view and cell size, and the eyepiece graticule (in US labs, the ocular micrometer or reticle), in mm and µm, without standard form.
Questions
How do you calculate magnification?
Magnification = image size ÷ actual size, with both lengths in the same unit. A cell 62.5 µm long drawn 50 mm long: 50 mm = 50,000 µm, and 50,000 ÷ 62.5 = ×800. Rearranged, actual size = image size ÷ magnification.
How do you use a scale bar?
Measure the bar in mm and change it to µm, then divide by the length written on it: a 20 mm bar labelled 25 µm gives 20,000 ÷ 25 = ×800. For a specimen, measure it too: it is (specimen ÷ bar) times the bar’s label.
How do you calibrate an eyepiece graticule?
Line up the zeros of the graticule and the stage micrometer, then find where two lines meet again. If 40 eyepiece units match 16 divisions of 10 µm, one eyepiece unit is 160 ÷ 40 = 4 µm. Calibrate again for every objective lens.
How do you estimate cell size from the field of view?
Measure the low-power field with a clear ruler, scale it to high power (field × low magnification ÷ high magnification), then divide by the number of cells across it. A 4.5 mm field at 40× is 450 µm at 400×; 6 cells across makes each about 75 µm.
How do you convert between mm, µm and nm?
1 mm = 1,000 µm and 1 µm = 1,000 nm. To go to a smaller unit multiply by 1,000; to go to a bigger unit divide by 1,000. Convert before you divide, or the magnification comes out 1,000 times too small.
How do you write a size in standard form?
Write one non-zero digit before the decimal point and a power of ten: 0.0625 mm is 6.25 × 10⁻² mm, and 25,000 is 2.5 × 10⁴. Type it here as 6.25 × 10^-2, 6.25x10-2 or 6.25e-2.
Sources, checked 25 October 2026: AQA GCSE Biology 8461 specification, version 1.0 (21 April 2016), 4.1.1.5 and required practical 1; AQA AS and A-level Biology 7401/7402 specification, version 1.5 (26 November 2021), 3.2.1.3; Eduqas, Improving microscopy skills (July 2023); WJEC, Calibration of the light microscope (June 2015); LibreTexts, Practice for Microscopy Calculations (Julie Robinson), licensed CC BY 4.0; OpenStax Biology 2e (Clark, Douglas and Choi, 2018), 4.2 Prokaryotic Cells (cell sizes), licensed CC BY 4.0. All drawings here are made by this page’s own code; no figures or questions are copied.