How this calculator works
Methylmercury forms when bacteria act on inorganic mercury, and it moves up the food chain: small fish take it in, larger fish eat the small ones, and the concentration rises at each step. That is why long-lived predators such as shark, swordfish and tilefish carry the most and small fish such as sardines and shrimp carry the least. The US Food and Drug Administration has monitored mercury in commercial fish for decades; the values used here are the mean total mercury in milligrams per kilogram of wet weight from its 1990 to 2012 table. The health reference is the US Environmental Protection Agency reference dose for methylmercury of 0.1 micrograms per kilogram of body weight per day, which is a level considered unlikely to cause harm over a lifetime when eaten every day. Multiplying by seven gives a weekly amount, and dividing by the mercury in one serving gives the number of servings that fit. The table holds the serving size at 150 g so you can compare species.
Formula
mercury per serving (micrograms) = concentration (mg/kg) x serving (g), because 1 mg/kg in a gram is 1 microgram. weekly limit (micrograms) = 0.1 x body weight (kg) x 7. share of the limit = weekly mercury / limit. servings that fit = limit / mercury per serving.
Worked example
A 150 g serving of canned light tuna at 0.126 mg/kg contains 0.126 x 150 = 18.9 micrograms of mercury. For a 70 kg adult the weekly limit is 0.1 x 70 x 7 = 49 micrograms, so 49 / 18.9 = 2.6 servings a week fit. Two servings a week use 37.8 micrograms, or 77 percent of the limit.
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Frequently asked questions
Is this medical advice?
No. It is an arithmetic comparison with a public reference dose. Pregnant people, people who may become pregnant, nursing parents and young children are usually advised to avoid the highest-mercury fish altogether and to follow their health authority's specific guidance, which may be stricter than this calculation.
Is canned light tuna different from albacore?
Yes. Canned light tuna is mostly skipjack and yellowfin and averages about 0.126 mg/kg. Canned white tuna is albacore at about 0.35 mg/kg. The difference is why canned light is usually the recommended choice for frequent eaters.
Does cooking remove mercury?
No. Methylmercury is bound to the protein and is not destroyed by heat, so grilling, frying or canning does not reduce it. Trimming fat does not help much either, because the mercury is distributed through the muscle.
Are the benefits of fish still worth it?
For most people, yes. Fish provides omega-3 fatty acids and protein, and health agencies advise one to two servings a week of lower-mercury fish. The point of the calculator is to choose species and portions that keep mercury low while keeping the benefit.