Quick answer
CFU stands for colony-forming unit, the standard count of live microbes in a probiotic. A lab counts it by growing a sample on agar and counting the colonies that form. Many probiotic supplements list 1 to 10 billion CFU per dose, and a higher count does not by itself mean a product works better.1,2
Key facts
- FDA describes a CFU as a measure of viable microbial cells that can multiply on agar plates and form colonies, which are then counted.1
- Many probiotic supplements contain 1 to 10 billion CFU per dose, and some contain 50 billion CFU or more.2
- The weight of a probiotic ingredient counts live and dead cells alike, so it does not show how many are alive.1,2
- NIH advises choosing products that state CFU at the end of shelf life, not at the time of manufacture.2
- A product's effects, if any, depend on both its specific strains and its number of CFU.3
How CFU are counted
A sample is spread on agar, a nutrient gel. Each living cell, or small group of cells, that can multiply grows into a visible colony, and the colonies are counted. That is why the unit is a colony-forming unit rather than a cell count: it is the smallest number of cells that can produce one detectable colony.1,4
Dead cells do not form colonies, so a CFU count reflects only live, culturable microbes. FDA has noted that researchers are working on other methods that may count viable cells more accurately.1
Reading CFU on a label
Amounts are often written in scientific notation: 1 x 10^9 CFU is 1 billion, and 1 x 10^10 CFU is 10 billion.2
The Supplement Facts panel must show the weight of each probiotic ingredient. FDA has said it intends to allow a CFU figure as well, provided the weight comes first, the CFU figure is clearly separate, and it counts only live microbes.1
Probiotics die gradually during storage, so the time point matters. A count measured at manufacture can be well above what is left by the expiry date, which is why the scientific group ISAPP advises listing CFU through the end of shelf life, ideally for each strain.1,2
A higher count is not automatically better
NIH says products with higher CFU counts are not necessarily more effective than those with lower counts. The sensible amount depends on the strain and the product, and the World Gastroenterology Organization advises using the strains, doses and durations that have been studied in people. Storage matters too: some products need refrigeration.2
Who should ask a doctor first
NIH notes that probiotics have been linked to rare bloodstream infections, mostly in people who were severely ill or immunocompromised, and FDA has warned about giving probiotics to preterm infants. Ask a doctor before using probiotics in these situations.2,5
Questions people ask
How many CFU should a probiotic have?
There is no single right number. Many supplements contain 1 to 10 billion CFU per dose, but the useful amount depends on the strain and on what it was studied for. Look for the strain and amount used in studies, and a count stated to the expiry date.2
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Sources
- U.S. Food and Drug Administration. Policy Regarding Quantitative Labeling of Dietary Supplements Containing Live Microbials: Draft Guidance for Industry. September 2018. Opened 2026-10-07.
- Office of Dietary Supplements, National Institutes of Health. Probiotics: Fact Sheet for Health Professionals. Updated March 25, 2025. Opened 2026-10-07.
- Office of Dietary Supplements, National Institutes of Health. Probiotics: Fact Sheet for Consumers. Updated November 17, 2025. Opened 2026-10-07.
- NCI Thesaurus, NCI Enterprise Vocabulary Services, National Institutes of Health. Colony Forming Unit, code C68742. Opened 2026-10-07.
- National Center for Complementary and Integrative Health. Probiotics: What You Need To Know. Opened 2026-10-07.
This page is information, not medical advice. Talk to your doctor or pharmacist about your own situation.