Probiotics Dosage Calculator: Which Strain and Dose for Your Goal

Probiotics are live microorganisms, usually bacteria or yeast, that help when taken in the right amount. The amount is counted in CFU (Colony Forming Units), the number of live organisms in a dose.

Why this page will not give you a CFU number

Most dose charts online pair an age with a CFU count: 5 billion for a child, 10 for an adult, 30 after antibiotics. The World Gastroenterology Organisation’s 2023 guideline says plainly that no such general dose exists, because the dose that works depends on the strain and the product.
The clearest example is the IBS strain Bifidobacterium longum subsp. infantis 35624, which the same guideline cites as working at 100 million CFU a day. That is a hundredth of the 10 billion printed on many shelves. Another strain at 10 billion may do nothing at all.
So the useful question is which strain has been tested for your problem. The dose is then whatever the tested product delivered, which is what its label says.

What the guidelines say, goal by goal

• After antibiotics: the best case for probiotics. The WGO guideline for children (2016) strongly recommends Lactobacillus rhamnosus GG or Saccharomyces boulardii when the risk of diarrhea is high. The American Gastroenterological Association (AGA, 2020) suggests S. boulardii or certain multi-strain products to prevent C. difficile infection in adults and children on antibiotics.
• IBS: weak. The American College of Gastroenterology (2021) recommends against probiotics for overall IBS symptoms, and the AGA (2020) advises them only within clinical trials. If you try one anyway, give it four weeks.
• Colds and immunity: a 2022 Cochrane review found fewer respiratory infections with probiotics, but at low certainty.
• Travelers’ diarrhea: the International Society of Travel Medicine’s 2017 consensus found too little evidence to recommend them.
• Vaginal yeast infections: low-quality evidence that probiotics help alongside antifungal treatment. Little for prevention on their own.
• General gut health in a healthy person: there is no good evidence that you need one.

Two groups should not start probiotics on a calculator recommendation. Anyone genuinely immunocompromised, which includes chemotherapy, transplant medication and an indwelling central line, faces a real if uncommon risk of the organisms themselves causing a bloodstream infection. And infants, particularly premature ones, are a paediatrician decision. In 2023 the FDA warned after a premature baby died of an infection caused by the organism in a probiotic product, and in 2014 a contaminated probiotic powder killed another. Both cases come from the product being alive, which is the whole point of it.

Reading the label: look for the strain code (the letters and numbers after the species, such as GG or CNCM I-745) and a CFU count guaranteed at expiry, not at manufacture.