Probiotic CFU vs mg: How to Read a Probiotic Label
TPBy the TrustedPurchaseReview Editorial Team
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The short answer
CFU counts live cells. Milligrams (mg) count weight, and that weight includes dead cells too. US rules only require the weight, so many gut supplements print mg and skip the CFU count. When you compare probiotics, look for three things: a CFU count, a count guaranteed until the expiration date, and a full strain name. If one of them is missing, you cannot tell how much live probiotic you are paying for.
Probiotic labels are hard to read for a simple reason: the number buyers care about is optional. We read the NIH Office of Dietary Supplements fact sheet on probiotics, the FDA guidance on how these products may be labeled, and the labels of three gut supplements we reviewed. This guide explains what each number means and how to check a label in under a minute. It is general information, not medical advice.
What CFU Means on a Probiotic Label
CFU stands for colony forming units. It estimates how many live bacteria or yeast cells are in one serving. Labels often write it in scientific notation: 1 x 109 is 1 billion CFU, and 1 x 1010 is 10 billion CFU.
The count matters because probiotics only help while they are alive. A capsule full of dead cells still has weight, but it is not doing the job a probiotic is sold for.
Why Probiotic Labels Show mg Instead of CFU
Under current US rules, the Supplement Facts panel has to show the total weight of the microorganisms, in mg. Listing CFU is voluntary. In 2018 the FDA published a draft guidance saying it does not intend to object if companies add the CFU count next to the weight, as long as certain conditions are met. That guidance is still a draft.
The result: some brands print both numbers, some print only mg, and many hide everything inside a “proprietary blend” with a single total weight.
Can You Convert mg to CFU?
No. The NIH fact sheet points out that the weight includes live and dead cells, so it says nothing about how many live cells you get. There is no formula that turns 36 mg into a CFU count. Two capsules with the same mg can hold very different numbers of live bacteria.
So when a label shows only mg, treat the probiotic part as unknown, not as small or large.
“At Time of Manufacture” vs “Through Expiration”
Probiotics can die while the bottle sits in a warehouse, a truck or your kitchen. A count measured when the capsules were made can be higher than what is left months later. NIH advises looking for products that state the CFU count at the end of the shelf life, not at manufacture.
| Label wording | What it tells you |
|---|---|
| “10 billion CFU through expiration” | The brand claims at least that many live cells until the date on the bottle. The strongest wording. |
| “10 billion CFU at time of manufacture” | The count when it was made. It may be lower when you take it. |
| “500 mg probiotic blend” | Weight only. The number of live cells is unknown. |
How Many CFU Do You Need?
There is no single number. According to NIH, many probiotic supplements contain 1 to 10 billion CFU per dose, and some contain 50 billion or more. In research, the dose changes with the strain and the problem being studied.
NIH also says higher CFU counts are not necessarily more effective. A product with 50 billion CFU of unnamed strains is not automatically better than one with 5 billion CFU of a well studied strain. The better question is: does the label name a strain, and is its dose close to the dose that strain was tested at?
Why the Strain Name Matters
A full probiotic name has three parts: genus, species and a strain code. For example, Lacticaseibacillus rhamnosus GG. The code at the end is what lets you look up the research, because effects are often specific to one strain. NIH gives the example of skin studies where some species helped and others did not.
When a label says only “L. reuteri” or “Lactobacillus blend”, you cannot match it to a study. When it says “Bifidobacterium lactis BL-04″, you can.
A 5 Point Probiotic Label Checklist
| Check | Good sign | Red flag |
|---|---|---|
| 1. CFU count | A number in billions per serving | Only mg, or no number at all |
| 2. When it was counted | “Through expiration” or “at end of shelf life” | “At time of manufacture”, or not stated |
| 3. Strain names | Genus, species and strain code | Species only, or just “probiotic blend” |
| 4. Dose per strain | Each strain listed with its own amount | One total for a proprietary blend |
| 5. Storage | Clear storage instructions | None, for a product that needs them |
Three Gut Supplement Labels We Checked
Here is how the checklist works on real products. All three are sold online as gut supplements, and we went through their published labels in our reviews.
| Product | Probiotic on the label | CFU stated? | Strain codes? |
|---|---|---|---|
| GutSave | Two blends of 12.5 mg each | 1.25 billion for the first blend, at manufacture. None for the second. | Only one (B. lactis BL-04). L. reuteri has no code. |
| SodaTide | 36 mg blend: B. infantis, C. butyricum, A. muciniphila | No | No |
| JavaTide | Same 36 mg blend as SodaTide | No | No |
None of the three passes all five checks. GutSave comes closest, because it gives a count for half of its probiotic and names one strain. SodaTide and JavaTide list modern species, but with only a weight, there is no way to know how many live cells each capsule holds. The full details are in each review.
Probiotic Side Effects and Who Should Ask a Doctor
NIH says probiotics are unlikely to harm healthy people. The usual side effects are mild gut symptoms, like gas, that go away on their own. Serious infections linked to probiotics have been reported, mostly in people who were very ill or had a weakened immune system. The FDA has also warned about probiotics in premature babies.
Talk to a doctor first if you have a serious illness or a weakened immune system, or if the product is for an infant. New or lasting gut symptoms deserve a checkup, not just a supplement.
FAQ
What does CFU mean on a probiotic label?
Is a probiotic with more CFU better?
Can I convert mg to CFU?
What does “CFU at time of manufacture” mean?
How many CFU should a probiotic have?
Sources
- NIH Office of Dietary Supplements: Probiotics, fact sheet for health professionals, updated March 25, 2025.
- NIH Office of Dietary Supplements: Probiotics, fact sheet for consumers.
- US Food and Drug Administration: Draft guidance on quantitative labeling of dietary supplements containing live microbials, September 2018.
- TrustedPurchaseReview: GutSave review, SodaTide review and JavaTide review, labels checked October 2026.
About the editorial team
Written by the TrustedPurchaseReview editorial team. We read official labels and sales pages and check them against published research and government sources. We do not buy or test the products ourselves. We earn affiliate commissions on some links in our reviews, and ratings are not for sale.
Questions or corrections? Contact us.
This guide is informational and not medical advice. It has no affiliate links. Some of the reviews it links to contain affiliate links.