The Firmicutes:Bacteroidetes ratio compares the measured amounts of two broad bacterial groups. It is usually calculated by dividing Firmicutes by Bacteroidetes. The direction matters: a Bacteroidetes:Firmicutes ratio is the reverse calculation and cannot be interpreted using the same scale.
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Why the same ratio can hide different results
A ratio of 2 could come from 20 divided by 10 or from 200 divided by 100. Those examples have the same proportion but very different totals. On a stool report, read both component values and their units before focusing on the ratio. A very small denominator can also make a ratio unstable.
What the GI-MAP number means
The manufacturer reports this comparison alongside the two phylum measurements. It is not a count of species, a comprehensive diversity index or a direct measurement of microbial function. Firmicutes and Bacteroidetes each contain many organisms with different properties. [1]
Can the ratio diagnose obesity or dysbiosis?
No. Reviews of the research describe conflicting findings and differences in sampling, laboratory methods and study populations. There is no single ratio that can reliably diagnose obesity or define an ideal gut microbiome for everyone. [2]
A high ratio does not establish that Firmicutes caused weight gain, and a low ratio does not prove protection from metabolic disease. It also cannot identify the cause of bloating, diarrhea or constipation.
What to do instead of chasing a number
Use clinically meaningful goals: understanding symptoms, assessing nutrition and following up on established health risks. A diet that changes this ratio has not necessarily improved health. Comparing ratios across different laboratories adds further uncertainty because the underlying measurements may differ.
Common questions
Is a ratio of 1 always best?
No. An apparently balanced arithmetic ratio is not a universal medical target.
Is this a microbiome diversity score?
No. Diversity concerns the variety and distribution of organisms. A comparison of two broad groups does not measure that comprehensively.
Sources and review
Sources checked October 7, 2026. Manufacturer material describes the test; clinical guidance and research support the interpretation limits.
- Diagnostic Solutions Laboratory: GI-MAP interpretive guide (manufacturer specifications)
- The Firmicutes/Bacteroidetes ratio: a review of the evidence, 2020
Medical review of this article is pending. For a correction or source question, contact GI MAP Test.