Revision Notes

2.4 Size of Specimens

Calculating Magnification & Specimen Size: Basics

Calculating magnification and specimen size using millimetres as units:

  • Magnification is calculated using the following equation:
Magnification = Drawing size ÷ Actual size
  • A better way to remember the equation is using an equation triangle:


Magnification Equation, IGCSE & GCSE Biology revision notesMagnification equation


  • Rearranging the equation to find things other than the magnification becomes easy when you remember the triangle – whatever you are trying to find, place your finger over it and whatever is left is what you do, so:
    • Magnification = image size / actual size
    • Actual size = image size / magnification
    • Image size = magnification x actual size
  • Remember magnification does not have any units and is just written as ‘x 10’ or ‘x 5000’
  • Let’s look at an example:

An image of an animal cell is 30 mm in size and it has been magnified by a factor of x 3000. What is the actual size of the cell?

To find the actual size of the cell:

Worked Example Using Magnification Equation, IGCSE & GCSE Biology revision notesWorked example using the magnification equation

Exam Tip

This skill most frequently comes up in paper 5 and 6 (although it also comes up in the multiple choice and occasionally the theory paper) and you will definitely have to calculate either magnification, drawing size or actual size in a least one paper.

To ensure you do not lose marks:

  1. Always look at the units that have been given in the question – if you are asked to measure something, most often you will be expected to measure it in millimetres NOT in centimetres – double check the question to see!
  2. Learn the equation triangle for magnification and write it on the page straight away
  3. Don’t forget that magnification has NO UNITS – students often lose a mark because they put one in
Extended Only

Calculating Magnification & Specimen Size

Using millimetres and micrometres as units:

  • The table below shows how millimetres are related to two other measures of length

Size of Specimens table, IGCSE & GCSE Biology revision notes

  • What this basically means is that 1mm = 1000µm and 1cm = 10,000µm
  • This usually comes up in questions where you have two different units and you need to ensure that you convert them both into the same unit before proceeding with the calculation
  • For example:

Example extended magnification question, IGCSE & GCSE Biology revision notesExample extended magnification question

  • Remember 1mm = 1000µm
  • 2000 / 1000 = 2 so the actual thickness of the leaf is 2mm and the drawing thickness is 50mm
  • Magnification = image size / actual size = 50 / 2 = 25
  • So the magnification is x 25 (NO UNITS)

Exam Tip

If you are given a question with two different units in it, make sure you convert them to the same unit before doing your calculation.

If you don’t, there is a good chance that your answer will be the same as one of the incorrect options in a multiple choice question so you may think you got it right when, in fact, you haven’t!

The following diagram may help with unit conversion between mm and µm:

Converting Units, IGCSE & GCSE Biology revision notesConverting units

Author: Jenna

Jenna studied at Cardiff University before training to become a science teacher at the University of Bath specialising in Biology (although she loves teaching all three sciences at GCSE level!). Teaching is her passion, and with 10 years experience teaching across a wide range of specifications – from GCSE and A Level Biology in the UK to IGCSE and IB Biology internationally – she knows what is required to pass those Biology exams.

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