Probiotic labels usually state a count in millions or billions of CFU. That figure tells you something useful, but only once you know what it measures and which point in the product's life it describes. This guide covers both, then shows how to put two labels side by side.
What does CFU mean?
CFU stands for colony forming units. It is an estimate of how many live microbes in a serving are able to multiply and form a visible colony when grown in a laboratory. Dead cells do not form colonies, so the count is designed to reflect live cultures rather than every cell in the product.
A widely used definition describes probiotics as live microorganisms that, when taken in adequate amounts, can benefit the person taking them. Because being alive is part of that definition, the live count is the figure that gets the most attention on a label.
How a CFU count is measured
The traditional method is a plate count. A sample is diluted, spread on a growth medium and incubated, then the colonies that appear are counted and scaled back up to the size of a serving. Each colony may have grown from one live cell or a small cluster of cells, which is why the result is an estimate.
A newer method, flow cytometry, counts live cells directly and can pick up some cells that do not readily form colonies. The two methods can give different results for the same product, so small differences between labels are less meaningful than they look.
CFU at manufacture vs to best-before
Live cultures decline gradually over time, and faster when the product meets heat, moisture or air. A count measured on the day of manufacture can therefore be higher than the number left by the best-before date. Labels may state either figure, and the wording next to the number tells you which.
Look for phrases such as "at end of shelf life" or "until best-before" beside the count. Manufacturers usually add extra cultures at the start, known as overage, so that the stated count is still met at the end. A label that says "at time of manufacture", or gives no indication at all, tells you less about what you will be taking. Our label reading guide shows where to find this wording.
Why a higher CFU is not automatically better
It is tempting to read CFU as a strength rating, but a bigger number does not make one product better than another. Research on probiotics is strain-specific, so the amount used in studies of one strain says nothing about a different strain. A clearly named strain at a modest count can be a more sensible choice than an unnamed blend at a very large one.
Format plays a part too, because capsules and powders can usually carry higher counts than gummies or chewables, which have less space and rely on heat-tolerant spore-formers. Our probiotic strain guide and probiotic types page explain how strains and formats fit together.
How to compare CFU counts fairly
Comparing CFU counts is only fair when you compare like with like. Work through these checks before you decide that one figure is higher than another.
| Check | What to look for |
|---|---|
| Per unit or per serving | Some labels give the count per capsule while the daily serving is more than one, so work out the daily figure |
| When the count applies | A count stated to the best-before date is more useful than one at manufacture |
| Per strain or in total | A single total can hide how much of each strain is present |
| Strain names | Compare products on named strains first, then on the size of the number |
| Storage | A product that should be chilled but has been kept warm may no longer meet its stated count |
If two products still look hard to compare, you are welcome to ask us a question and we will explain what each label is telling you. For more on how storage affects the count, read our guide to probiotic storage and shelf life.