Pick the mineral and the form on your label, and type the milligrams. You see how much of it is the mineral itself — worked out from the compound’s formula and checked against the NIH where it prints a figure — the same amount in another form, and where it sits against the RDA and the upper limit.
The mineral itself
325 mg of ferrous sulfate is 65 mg of iron 20% iron by weight (NIH ODS: 20%) · the same 65 mg of iron is 199 mg of ferrous fumarate
- RDA
- 363% of the 18 mg RDA for a woman, 19–50
- Upper limit
- 145% of the 45 mg upper limit
| Form | Iron by weight | mg of the form |
|---|
This is not medical advice and it does not diagnose anything. It does the arithmetic on a label and places the result against official limits; whether you need a supplement at all, and how much, is a conversation with your doctor or pharmacist — especially with iron.
Only part of a mineral supplement’s compound is the mineral. NIH ODS: “ferrous fumarate is 33% elemental iron by weight, whereas ferrous sulfate is 20% and ferrous gluconate is 12%”, and “calcium carbonate is 40% calcium by weight, whereas calcium citrate is 21% calcium”. So a 325 mg ferrous sulfate tablet holds 65 mg of iron, and 1,250 mg of calcium carbonate about 500 mg of calcium.
How to use it
Pick iron, zinc or calcium, then the form named on your label, and type the milligrams. Say which number it is: most Supplement Facts panels already print the mineral itself — “Iron 65 mg (as ferrous sulfate)” — and that number must not be converted again; the compound’s weight (325 mg of ferrous sulfate) is what pharmacy boxes and some labels print. You get the mineral in it, the same amount of mineral in a second form, every form side by side, and where the amount sits against the RDA and the upper limit for the person it is for. It does not tell you what to take: that is the conversation with your doctor or pharmacist.
Where the percentages come from
Each percentage is the mineral’s share of the compound’s formula weight, from the IUPAC atomic weights — the same arithmetic as the magnesium page. Where NIH ODS prints a figure, the formula agrees with it to within one point: ferrous sulfate (as the heptahydrate, the US 325 mg tablet) 20.1% against ODS’s 20, fumarate 32.9 against 33, gluconate 11.6 against 12, calcium carbonate 40.0 against 40, calcium citrate 21.1 against 21. ODS prints no percentage for the chelates or for any zinc form — its zinc sheet says the label “declares the amount of elemental zinc in the product, not the weight of the entire zinc-containing compound” — so those numbers are the formula’s alone, and the table says so.
| Form | Iron by weight | mg of the form |
|---|---|---|
| Ferrous sulfate | 20% | 324 mg |
| Ferrous fumarate | 33% | 198 mg |
| Ferrous gluconate | 12% | 561 mg |
| Ferrous bisglycinate (chelate) | 27% | 237 mg |
| Form | Zinc by weight | mg of the form |
|---|---|---|
| Zinc gluconate | 14% | 348 mg |
| Zinc picolinate | 21% | 237 mg |
| Zinc citrate | 32% | 156 mg |
| Zinc sulfate | 23% | 220 mg |
| Zinc acetate | 30% | 168 mg |
| Zinc bisglycinate | 31% | 163 mg |
| Zinc oxide | 80% | 62 mg |
| Form | Calcium by weight | mg of the form |
|---|---|---|
| Calcium carbonate | 40% | 1,249 mg |
| Calcium citrate | 21% | 2,372 mg |
| Calcium lactate | 13% | 3,846 mg |
| Calcium gluconate | 8.9% | 5,594 mg |
What the sources say about the forms
Iron. NIH ODS: “Because of its higher solubility, ferrous iron in dietary supplements is more bioavailable than ferric iron.” Chelates and other forms “might have fewer gastrointestinal side effects than ferrous or ferric salts”, and “High doses of supplemental iron (45 mg/day or more) may cause gastrointestinal side effects, such as nausea and constipation.”
Zinc. NIH ODS: “Absorption of zinc from supplements that contain zinc citrate or zinc gluconate is similar, at approximately 61% in young adults; the absorption from supplements that contain zinc oxide is 50%.” It does not compare zinc picolinate with the others, and we found no official source that does — so the page compares picolinate by its zinc content only.
Calcium. Both NIH ODS and MedlinePlus say carbonate is best taken with food and citrate can be taken without it — MedlinePlus: “It is taken up well by the body on a full or empty stomach.” Both say absorption is best at 500 mg or less at a time, and ODS adds that “Calcium carbonate appears to cause more of these side effects than calcium citrate, especially in older adults who have lower levels of stomach acid.”
Worked examples
- A 325 mg ferrous sulfate tablet (the page’s example): 65 mg of iron — the same iron as 198 mg of ferrous fumarate, and over the 45 mg adult upper limit, which is why iron at that dose is something a doctor decides.
- A label that says “Zinc 50 mg (as zinc picolinate)”: the 50 mg is already the zinc — switch the number to “of zinc” and you see it is 237 mg of the compound.
- 500 mg of calcium for a woman of 60: 1,249 mg of carbonate or 2,372 mg of citrate — 42% of her 1,200 mg RDA.
Common mistakes
- Converting the mineral twice. NIH ODS: “elemental iron is listed in the Supplement Facts panel, so consumers do not need to calculate the amount”. If the label says “Iron 65 mg”, that is the iron.
- Comparing tablets by their size. 500 mg of calcium citrate is not 500 mg of calcium; it is 105 mg.
- Taking calcium and iron together. NIH ODS: “experts recommend taking calcium and iron supplements at different times to avoid potential interference with the absorption of iron”.
- Forgetting that the upper limit counts food too. The National Academies’ UL is for total intake — food, water and supplements.
Frequently asked questions
What is the difference between calcium carbonate and calcium citrate?
Carbonate is 40% calcium by weight and is best taken with food; citrate is 21% calcium and can be taken with or without food (NIH ODS, MedlinePlus). You need about twice as much citrate for the same calcium: 1,249 mg of carbonate or 2,372 mg of citrate for 500 mg.
Is iron bisglycinate better than ferrous sulfate?
NIH ODS says iron amino-acid chelates “might have fewer gastrointestinal side effects than ferrous or ferric salts”. By formula the pure chelate is 27% iron against sulfate’s 20%, but chelate labels list the iron itself — compare that line, not the compound.
How much elemental iron is in ferrous sulfate?
NIH ODS puts ferrous sulfate at 20% iron by weight, so a 325 mg tablet holds 65 mg of iron. The heptahydrate’s formula gives 20.1%.
Is zinc picolinate better than zinc gluconate?
By weight, picolinate is about 21% zinc and gluconate about 14%, so less picolinate holds the same zinc. On absorption, NIH ODS compares only citrate, gluconate (about 61%) and oxide (50%); it says nothing about picolinate.
What is the best type of zinc supplement?
The official sources do not name one. NIH ODS reports similar absorption for citrate and gluconate and less for oxide, and notes that 25 mg of iron or more taken at the same time can reduce zinc absorption.
Is 65 mg of iron too much?
65 mg is above the 45 mg upper limit for adults. NIH ODS notes that “many” iron-only supplements provide 65 mg, and the National Academies say the upper limit is not meant for people treated with a nutrient under medical supervision — so a dose like that is your doctor’s call.
Read next
- How to get rid of anemia: the best 4 products
- Vitamin compatibility
- Multivitamins for adults: how to choose
- The best vitamins for women at 30+
- The Magnesium Form Finder, and the Vitamin & Mineral Dose Lookup for every age’s RDA and upper limit.
This is not medical advice, and nothing here diagnoses or treats anything. It does the arithmetic on a label; whether you need a supplement at all, and how much, is a conversation with your doctor or pharmacist. Some links on this site are affiliate links: I buy what I review at full price, and a commission never changes a verdict.