Iron Intake Calculator

Background.

Ask how much iron an adult needs each day and the official answer runs from 8 mg to 27 mg, depending on who is asking. A man of any adult age needs 8 mg. A woman between 19 and 50 needs 18 mg, more than twice as much, almost entirely to replace the iron lost in menstrual blood. Pregnancy raises the figure to 27 mg, and breastfeeding brings it down to 9 mg. These are the Recommended Dietary Allowances set by the US Institute of Medicine in 2001, and they are the values the NIH Office of Dietary Supplements still publishes.

Diet adds a second layer. Food iron comes in two forms. Heme iron, found in meat, poultry and seafood, is absorbed readily. Non-heme iron, from plants, eggs, dairy, fortified foods and supplements, is absorbed far less reliably, and how much gets through depends on what else is in the meal. The RDAs assume a mixed diet. For people who eat no meat, poultry or fish, the Institute of Medicine set the requirement 1.8 times higher, so a vegetarian woman of 28 is advised to take in 32.4 mg a day rather than 18.

You enter age, sex, pregnancy status and whether you eat a vegetarian or vegan diet, and you get both figures: the RDA from the table, and an absorption-adjusted target that applies the 1.8 factor when you answer yes. Ages 19 to 100 are covered; children and teenagers have their own allowances.

These are intake targets for healthy people planning what to eat. They cannot tell you whether you are short of iron now; only a blood test, usually hemoglobin and ferritin, can do that. And because the body has no way to get rid of surplus iron, supplements beyond an ordinary multivitamin are worth discussing with a doctor before you start them.

What is iron intake calculator?

Most of the iron in the body sits in hemoglobin, the oxygen-carrying protein in red blood cells, with smaller amounts in myoglobin in muscle and in storage as ferritin, mainly in the liver. The Recommended Dietary Allowance is the daily intake that meets the needs of nearly all healthy people in a group, set to cover about 97.5% of them. It sits above the Estimated Average Requirement, the intake that meets the needs of half. For men the average requirement is 6 mg and the RDA 8 mg. For women aged 19 to 50 the average is 8.1 mg but the RDA is 18 mg, because menstrual losses vary so widely that covering the women who lose the most takes a much higher figure. Values are milligrams of elemental iron per day from food and supplements combined. The absorption-adjusted target is the same allowance scaled for a diet without meat, poultry or fish, and it equals the RDA when you answer no to the vegetarian question.

How to use this calculator.

1. 01Enter your age in whole years, from 19 to 100. For women the table changes at 51; for men it does not change with age at all.
2. 02Choose your biological sex. The 18 mg allowance for women assumes regular periods, so read the result with your own situation in mind if menopause came well before 51 or has not arrived after it.
3. 03Set Pregnancy status. Pregnant or Lactating replaces the age-and-sex value entirely, giving 27 mg or 9 mg. These two settings are for women only: with Male selected the calculator asks you to choose Not pregnant.
4. 04Answer yes to the vegetarian or vegan question only if you eat little or no meat, poultry or fish. Eating fish or meat on most days puts you on the mixed-diet figure.
5. 05Compare the figure that applies to you with a typical day's food. US Nutrition Facts labels show iron as a percentage of an 18 mg Daily Value, so a serving marked 50% supplies 9 mg.

The formula.

There is no equation in the usual sense: the allowance is read from the Institute of Medicine table in a fixed order. Pregnancy gives 27 mg and breastfeeding 9 mg. Otherwise a woman aged 19 to 50 gets 18 mg, and everyone else, meaning men of any adult age and women 51 and over, gets 8 mg. The absorption-adjusted target is that value multiplied by 1.8 when you answer yes to the vegetarian question, and the same value when you answer no.

Each number in the table is built from losses and absorption. Adults lose roughly 1 mg of iron a day in shed gut and skin cells, sweat and urine. The body cannot excrete extra iron, so the diet only has to replace those losses, but only part of dietary iron is absorbed: about 18% from a typical mixed Western diet, in the IOM's working assumption. A 1 mg loss at 18% absorption needs about 1 ÷ 0.18 ≈ 5.6 mg of food iron; the average requirement for men is 6 mg, and a margin for people who lose more or absorb less brings the allowance to 8 mg. Menstruation adds a loss that varies widely from woman to woman, which is why the allowance for women of reproductive age jumps to 18 mg. Pregnancy adds the iron needed to expand the mother's red-cell mass and to build the fetus and placenta, raising it to 27 mg. While breastfeeding, periods are usually suspended and milk carries little iron, so the allowance falls to 9 mg.

The 1.8 factor comes from the same absorption reasoning. The IOM estimated absorption from a vegetarian diet at about 10%, against 18% from a mixed diet, and 18 ÷ 10 = 1.8, so multiplying intake by 1.8 is meant to deliver the same absorbed iron.

Two simplifications are built into the table. Age 51 stands in for menopause, so the drop from 18 mg to 8 mg happens on a birthday rather than when periods stop. And the vegetarian factor is a population average: someone who eats vitamin C-rich foods at most meals and drinks tea between rather than with them may absorb more than 10%, while someone relying on unfortified whole grains and legumes may absorb less.

A worked example.

Frequently asked questions.

References& sources.

1. [1]Institute of Medicine (2001). Dietary Reference Intakes for Vitamin A, Vitamin K, Arsenic, Boron, Chromium, Copper, Iodine, Iron, Manganese, Molybdenum, Nickel, Silicon, Vanadium, and Zinc. National Academies Press.
2. [2]National Institutes of Health, Office of Dietary Supplements (2023). Iron: Fact Sheet for Health Professionals.
3. [3]Bothwell, T.H., Charlton, R.W., Cook, J.D., & Finch, C.A. (1979). Iron Metabolism in Man. Blackwell Scientific Publications. ISBN 978-0632004687
4. [4]Hurrell, R. & Egli, I. (2010). Iron bioavailability and dietary reference values. American Journal of Clinical Nutrition 91(5):1461S-1467S. doi:10.3945/ajcn.2010.28674F
5. [5]World Health Organization (2020). Guideline on Iron Supplementation in Postpartum Women.