Pick a vitamin or mineral and who it is for. You get the daily amount the National Academies set (the RDA, or an AI where there is not enough data for one) and the upper limit — and, if you type what your label says, where that amount sits between the two.
Vitamin B12 · female, 19–30 years
2.4 mcg a day RDA · upper limit: none set
| Age | Male | Female | Upper limit |
|---|
* an Adequate Intake (AI). ND: not determinable for this age — food only.
This is not medical advice and it does not diagnose anything — whether you are low in a nutrient is a blood test, not a table. If you take medication (especially blood thinners, antidepressants or blood-pressure medication), are pregnant, or have been prescribed a supplement, follow your doctor’s or pharmacist’s amount.
How much of a vitamin or mineral you need a day is set by the National Academies’ Food and Nutrition Board: the RDA (or an AI where the evidence is thinner), and the upper limit (UL), the most a day that is unlikely to cause harm. For adults 19–30, vitamin B12’s RDA is 2.4 mcg with no upper limit; zinc’s is 11 mg for men and 8 mg for women, with a UL of 40 mg; iron’s is 8 mg or 18 mg, with a UL of 45 mg.
How to use it
Pick the vitamin or mineral, the age group, and — from age 9 — sex, and whether you are pregnant or breastfeeding. You get the RDA or AI and the upper limit for exactly that row of the tables, the tables’ own footnotes about that nutrient, and the nutrient at every age. Type the amount on your supplement label, and the page places it: under the RDA, between the RDA and the upper limit, at the limit, or over it. It never suggests an amount.
What the numbers mean
- RDA. “An RDA is the average daily dietary intake level sufficient to meet the nutrient requirements of nearly all (97–98 percent) healthy individuals in a group.” It is for food and supplements together.
- AI (marked *). Used where there is not enough evidence to set an RDA — for babies under 7 months, for every nutrient but iron and zinc from 7 to 12 months, and for vitamin K, biotin, pantothenic acid, choline, chromium, manganese and potassium at every age.
- UL. “The highest level of daily nutrient intake that is likely to pose no risk of adverse health effects to almost all individuals in the general population.” It is a ceiling, not a target, and “unless otherwise specified, the UL represents total intake from food, water, and supplements.”
- No UL. Vitamin K, thiamin, riboflavin, vitamin B12, pantothenic acid, biotin, chromium and potassium have none — for lack of data, not because more is better: “In the absence of a UL, extra caution may be warranted in consuming levels above recommended intakes.”
- Limits that count only supplements. Magnesium’s UL (350 mg for adults) is for magnesium from supplements and medicines only — which is why it can be lower than the RDA. The ULs for vitamin E, niacin and folate count only the synthetic forms in supplements and fortified foods; vitamin A’s counts only preformed vitamin A, not beta-carotene.
- Where official bodies disagree. The US UL for vitamin B6 is 100 mg; in 2023 the European Food Safety Authority set 12 mg for adults. The page shows both, named.
Every vitamin and mineral, adults 19–30
| Nutrient | Men | Women | Upper limit |
|---|---|---|---|
| Vitamin A | 900 mcg RAE | 700 mcg RAE | 3,000 mcg RAE |
| Vitamin C | 90 mg | 75 mg | 2,000 mg |
| Vitamin D | 15 mcg | 15 mcg | 100 mcg |
| Vitamin E | 15 mg | 15 mg | 1,000 mg |
| Vitamin K | 120* mcg | 90* mcg | none set |
| Thiamin (vitamin B1) | 1.2 mg | 1.1 mg | none set |
| Riboflavin (vitamin B2) | 1.3 mg | 1.1 mg | none set |
| Niacin (vitamin B3) | 16 mg NE | 14 mg NE | 35 mg NE |
| Pantothenic acid (vitamin B5) | 5* mg | 5* mg | none set |
| Vitamin B6 | 1.3 mg | 1.3 mg | 100 mg |
| Biotin (vitamin B7) | 30* mcg | 30* mcg | none set |
| Folate (vitamin B9) | 400 mcg DFE | 400 mcg DFE | 1,000 mcg of folic acid |
| Vitamin B12 | 2.4 mcg | 2.4 mcg | none set |
| Choline | 550* mg | 425* mg | 3,500 mg |
| Calcium | 1,000 mg | 1,000 mg | 2,500 mg |
| Chromium | 35* mcg | 25* mcg | none set |
| Copper | 900 mcg | 900 mcg | 10,000 mcg |
| Iodine | 150 mcg | 150 mcg | 1,100 mcg |
| Iron | 8 mg | 18 mg | 45 mg |
| Magnesium | 400 mg | 310 mg | 350 mg |
| Manganese | 2.3* mg | 1.8* mg | 11 mg |
| Potassium | 3,400* mg | 2,600* mg | none set |
| Selenium | 55 mcg | 55 mcg | 400 mcg |
| Zinc | 11 mg | 8 mg | 40 mg |
For other ages, pregnancy and breastfeeding, pick them in the tool above: some figures change at 31, 51 and 71 (magnesium, B6, calcium, iron, vitamin D), and pregnancy raises iron to 27 mg and folate to 600 mcg DFE.
Worked examples
- 1,000 mcg of B12, a man over 70. Far above the 2.4 mcg RDA, and there is no upper limit. The tables add a footnote for over-50s: “it is advisable for those older than 50 years to meet their RDA mainly by consuming foods fortified with B12 or a supplement containing B12.”
- 50 mg of B6, a woman of 31–50. Between the RDA (1.3 mg) and the US upper limit of 100 mg — but above the 12 mg a day the European Food Safety Authority set in 2023.
- A 65 mg iron tablet, a woman of 51–70. That is 325 mg of ferrous sulfate (20% iron, per NIH ODS), and 144% of the 45 mg upper limit. The limit “is not meant to apply to individuals who are treated with the nutrient under medical supervision” — a prescribed course is exactly that case, and a reason to ask before taking one unprescribed.
- 400 IU of natural vitamin E. 268 mg of alpha-tocopherol: above the 15 mg RDA and below the 1,000 mg upper limit, which is 1,500 IU of the natural form.
- 10,000 IU of vitamin A as retinol. 3,000 mcg RAE — exactly the adult upper limit. The same 10,000 IU as beta-carotene is not held against it.
- 400 mcg of folic acid, with food. 667 mcg DFE — above the 400 mcg DFE RDA — and 40% of the 1,000 mcg upper limit, which is in mcg of folic acid, not DFE.
Common mistakes
- Counting one bottle. A multivitamin, a B-complex and a fortified cereal can each carry the same nutrient. Add them up before comparing.
- Reading the upper limit as a target. It is the most a day that is unlikely to cause harm, not an amount to aim for.
- Mixing units. IU is not mcg: 1 mcg of vitamin D is 40 IU, 1 IU of retinol is 0.3 mcg RAE, and vitamin E’s IU depends on the form. The tool converts IU for A, D and E.
- Folic acid vs DFE. A label that says “680 mcg DFE (400 mcg folic acid)” is 680 against the RDA and 400 against the upper limit.
- “No upper limit” as permission. It means the data were not enough to set one.
Frequently asked questions
How much B12 should I take a day?
The RDA for adults is 2.4 mcg a day, 2.6 mcg in pregnancy and 2.8 mcg while breastfeeding, from food and supplements together. No upper limit has been set: the Food and Nutrition Board found its potential for toxicity low. Over 50, the tables advise getting it mainly from fortified foods or a supplement.
What is the B12 dosage for seniors?
The same 2.4 mcg RDA as younger adults. What changes is the source: because “10 to 30 percent of older people may malabsorb food-bound B12”, the National Academies advise those over 50 to meet it mainly from fortified foods or a supplement.
How much B6 is too much?
The US upper limit for adults is 100 mg a day, from food and supplements. In 2023 the European Food Safety Authority set 12 mg a day for all adults, based on reviews of B6 and peripheral neuropathy. Both are official; they disagree.
How much zinc is too much?
The upper limit for adults is 40 mg a day (34 mg at 14–18), counting food and supplements. NIH ODS warns that chronic, excessive use “can cause copper deficiency and neurologic diseases”.
How much iron should I take a day?
The RDA is 8 mg for men and for women over 50, 18 mg for women 19–50 and 27 mg in pregnancy. The upper limit is 45 mg for adults. Whether you need a supplement at all is a blood test and a conversation with your doctor.
What is the difference between the RDA and the upper limit?
The RDA is enough for nearly all healthy people; the upper limit is the most a day unlikely to cause harm. Anything between the two is inside the published range; above the upper limit is not.
Why is the magnesium upper limit lower than the RDA?
Because they count different things. The RDA is for magnesium from all sources; the 350 mg upper limit, in the National Academies’ words, represents “intake from a pharmacological agent only and do[es] not include intake from food and water”.
Read next
- Vitamin B12 as a nootropic
- Vitamin B6 as a nootropic
- Folic acid for women and men
- Multivitamins for adults: how to choose
- The best vitamins for women at 30+
- The Vitamin D Dose Calculator — IU, mcg and mg, and where a dose sits.
This is not medical advice, and nothing here diagnoses or treats anything. Talk to your doctor or pharmacist before starting a supplement, especially if you take blood thinners, antidepressants or blood-pressure medication, are pregnant, or have been prescribed a nutrient. Some links on this site are affiliate links: I buy what I review at full price, and a commission never changes a verdict.