The fish mercury calculator above does one narrow thing and refuses to do anything wider. It multiplies a published mercury concentration by a number of servings and a serving weight, divides by a reference dose published for body weight, and shows you the arithmetic. It does not tell you how much fish to eat, it does not label any species safe or unsafe, and it does not replace the advice of a clinician or of your own national food safety agency.
Arb Digital builds free calculators that cite their sources and state their limits. On this topic that discipline matters more than usual, because the people who search for it are frequently pregnant, breastfeeding, or feeding a young child — exactly the groups for whom methylmercury exposure is most consequential, and exactly the people who deserve a source rather than an opinion.
What This Fish Mercury Calculator Does
You choose a species, which fills in a mercury concentration you can then edit. You set how many servings a week, the serving size, your body weight, and the reference dose. The tool returns four figures: total methylmercury intake for the week in micrograms, the reference dose for that body weight expressed over a week, the daily intake per kilogram of body weight, and which of the EPA and FDA screening categories the concentration you entered falls into.
The headline is a percentage — weekly intake divided by the weekly reference dose. That is a ratio between two numbers, both of which are visible on the page, and it is presented as exactly that. It is not a score, not a warning, and not a permission.
Every input is editable, including the concentration and the reference dose, because both differ by source, by country and by year. Nothing on this page is a figure we made up.
Where the Numbers Come From
The species concentrations pre-loaded in the dropdown are mean total mercury values in parts per million from the United States Food and Drug Administration's published dataset Mercury Levels in Commercial Fish and Shellfish (1990–2012). That table is the reference most English-language fish advice is built on. We name it rather than link it, and we would encourage you to open the FDA's own copy and check any value that matters to you, because the mean for a species is a summary of a wide distribution.
The reference dose default of 0.1 micrograms of mercury per kilogram of body weight per day is the US Environmental Protection Agency's reference dose for chronic oral exposure to methylmercury. The EPA-FDA fish advice technical information page states it in exactly those terms and sets out how it was derived and how the agencies used it to build their advice categories.
The serving sizes are the ones EPA and FDA use in their own advice, published on the EPA-FDA advice about eating fish and shellfish: four ounces for adults and for children aged 11 and over, three ounces at ages 8 to 10, two ounces at ages 4 to 7 and one ounce at ages 1 to 3. That page, last updated in November 2025, is addressed specifically to people who might become pregnant, are pregnant or are breastfeeding, and to parents and carers feeding children.
The Formula / How It's Calculated
Concentration in parts per million is the same number as micrograms per gram, which makes the arithmetic straightforward. Weekly intake (µg) = servings per week × serving weight in grams × concentration in ppm. The reference dose is expressed per day, so the weekly allowance is reference dose × body weight in kg × 7.
Work through the defaults. Canned light tuna at the FDA mean of 0.126 ppm, three 4-ounce servings a week, at a body weight of 70 kg. Each serving is 113.4 grams, so each contributes 113.4 × 0.126 = 14.29 µg. Three servings give 42.9 µg a week. The weekly reference dose at 0.1 µg/kg/day is 0.1 × 70 × 7 = 49 µg. The intake is therefore 87.5% of the reference dose, or 0.0875 µg per kg of body weight per day.
Change one input and see how sensitive it is. The same three servings of swordfish at 0.995 ppm come to 338.5 µg a week — roughly 691% of the same reference dose. Nothing about the person changed; only the species did. That is why species-level concentration, rather than the amount of fish, is what drives the published advice.
One simplification is worth stating. The FDA table reports total mercury, while the reference dose is for methylmercury. In most finfish the great majority of total mercury is methylmercury, so treating them as equal is the conservative convention the agencies themselves work with, but it is an approximation and not an identity.
What EPA and FDA Publish, and What This Page Does Not Add
The EPA-FDA technical information page describes how the two agencies sorted fish into categories by calculating the highest average mercury concentration a fish could have and still be eaten one, two or three times a week without exceeding the maximum acceptable intake for an average pregnant woman. It gives the resulting screening values: Best Choices at 0.15 µg/g or below, Good Choices in the range from 0.23 to 0.46 µg/g, and Choices to Avoid above 0.46 µg/g. The fourth grid item on this page reports which of those bands the concentration you entered falls into. That is a statement about what those two agencies published, in that jurisdiction, on that date.
It is not this page's recommendation, and we do not extend it. We do not tell you how many servings to eat. We do not tell you a fish is fine. We do not tell you a fish is dangerous. Those are decisions for you with your clinician, informed by the advice your own national food safety agency issues.
That distinction is not pedantry. Fish is a significant source of protein, iodine, selenium and long-chain omega-3 fatty acids, and the published advice from EPA and FDA is framed around eating fish, not avoiding it. A tool that produced a scary percentage and stopped there would be giving a partial picture, and a tool that produced a reassuring one would be giving a different partial picture. Neither is our call to make.
Advice Differs by Country and Changes Over Time
There is no single global standard here, and assuming there is one is a real source of error. The reference dose in the United States is set by EPA at 0.1 µg/kg body weight per day. Other bodies publish provisional tolerable weekly intakes on a different basis and at different values, and national food agencies issue their own species lists reflecting the fish their populations actually eat and the mercury levels measured in their own waters. A species that appears in one country's lowest category may not appear on another's list at all.
These figures are also revised. Datasets are updated as new sampling is done, advice pages are re-issued, and categories move. The EPA-FDA advice page carries a last-updated date for exactly that reason. Anything you take from this page should be checked against the current publication from the agency with jurisdiction where you live, not against a calculator's cached copy.
If you are pregnant, breastfeeding, trying to conceive, or feeding a young child, the agency to follow is your own national food safety authority, and the person to ask is your own clinician. That is not a disclaimer bolted on at the end; it is the correct answer to the question this page is most often used to ask.
Why the Mean Is Only Part of the Story
Every pre-loaded value is a mean, and mercury concentration within a species varies enormously around it. Methylmercury bioaccumulates: it builds up in tissue over a fish's life and concentrates further up the food chain. So a large, old, predatory fish carries far more than a small, young one of the same species, and the published mean sits somewhere between them.
Practically, that means an individual fish can be several times the tabulated mean. It also means the same market name can cover different things — "tuna" spans canned light tuna near the bottom of the table and bigeye tuna several times higher, which is why the dropdown lists them separately rather than offering one figure for tuna.
Locally caught fish is a separate case again, and the national tables do not cover it. Freshwater and coastal catch from a specific water body is governed by local advisories issued by state, provincial or regional authorities that have sampled that water. The commercial-fish table has nothing to say about a lake near you, and this calculator has nothing to add to a local advisory. If you are estimating the size of a fish you caught rather than its chemistry, our fish weight calculator is the tool for that.
Working With the Units
Parts per million, micrograms per gram and milligrams per kilogram are all the same quantity written three ways, which is a common source of hundredfold errors when figures are copied between sources. A value of 0.126 ppm is 0.126 µg/g and 0.126 mg/kg. If your source publishes in a different unit, convert it before it goes in the box — our concentration converter handles that, and the weight converter covers ounces, grams and pounds if your serving size or body weight is in a different system.
The other unit trap is the reference dose itself. It is per kilogram of body weight per day. Comparing a weekly intake against a daily figure without multiplying by seven understates the allowance sevenfold, which is the single most common arithmetic error on this topic. The calculator does that conversion explicitly and shows both the weekly and the per-kilogram-per-day figures so the step is visible.
Arb Digital designs and builds free interactive calculators that cite their sources, state their limits, and stay on the right side of the line between information and advice. Browse what we have already published, or tell us what your audience keeps searching for.
Browse the Free Tools Hub Talk to Arb DigitalCommon Mistakes to Avoid
- Comparing a weekly intake to a daily reference dose. The reference dose is per day. Multiply by seven before comparing, or the allowance is understated sevenfold.
- Treating a species mean as the value of one fish. Mercury bioaccumulates with age and size, so an individual fish can be well above or below the published mean.
- Applying commercial-fish tables to locally caught fish. Freshwater and local coastal catch is covered by local advisories based on sampling that specific water, not by national commercial datasets.
- Assuming one country's figures apply everywhere. Reference doses, species lists and categories differ between authorities and are revised over time.
- Confusing "tuna" with one number. The market name spans species with very different concentrations, from canned light tuna to bigeye.
Related Free Tools From Arb Digital
Use the concentration converter for ppm, mg/kg and µg/g, the weight converter for servings and body weight in other units, the fish weight calculator for estimating a caught fish from its measurements, the micronutrient intake calculator for the nutrition side of a diet, the percentage calculator for working through the ratios by hand, and the food carbon footprint calculator for the environmental side of food choices. Everything else is on the free online tools hub.
Frequently Asked Questions
The US Environmental Protection Agency's reference dose for chronic oral exposure to methylmercury is 0.1 micrograms of mercury per kilogram of body weight per day, as stated in the EPA-FDA fish advice technical information. Other authorities publish different values on different bases, so use the figure that applies where you live.
They are mean total mercury values in parts per million from the US Food and Drug Administration dataset Mercury Levels in Commercial Fish and Shellfish, covering 1990 to 2012. Every value is editable, and any figure that matters should be checked against the FDA's own published table.
No. It reports the arithmetic of intake against a published reference dose and states which category EPA and FDA place a given concentration in. It sets no serving recommendation and issues no verdict on whether any fish is safe to eat.
The two agencies used screening values of 0.15 micrograms per gram or below for Best Choices, a range from 0.23 to 0.46 for Good Choices, and above 0.46 for Choices to Avoid. Those values apply to their advice in the United States and were derived from the reference dose and an assumed body weight and serving size.
No. Reference doses, tolerable intakes, species lists and categories differ between national authorities and are revised as new sampling and new assessments are published. Follow the current advice from the food safety agency with jurisdiction where you live.
Methylmercury bioaccumulates in tissue over a fish's life and concentrates up the food chain, so larger, older and more predatory individuals carry more. A published mean summarises a wide distribution rather than describing any particular fish.
No. National commercial-fish datasets do not describe a specific lake, river or stretch of coast. Locally caught fish is covered by advisories from the state, provincial or regional authority that has sampled that water.
This page is an arithmetic tool for general information only. It is not medical or dietary advice, it does not recommend how much fish anyone should eat, and it does not assess whether any fish is safe. If you are pregnant, breastfeeding, trying to conceive, or feeding a young child, follow the current advice of your national food safety agency and discuss your own situation with a qualified clinician.
