Elemental Iron in Oral Iron Salts Converter
Elemental Iron in Oral Iron Salts Converter
Convert the tablet strength printed on an oral iron pack into the elemental iron it actually delivers.
Elemental Iron in Oral Iron Salts Converter
Salt → elemental ironFerrous sulfate 325 mg tablet
Formula
- ferrous sulfate
- 20% elemental iron as the heptahydrate, 30% when dried
- ferrous fumarate
- 33% elemental iron
- ferrous gluconate
- 12% elemental iron
Worked example
Ferrous sulfate 325 mg tablet
325 × 20 ÷ 100 = 65 mg elemental iron
A 200 mg ferrous sulfate tablet delivers 65 mg by the dried-salt calculation used in some markets
Elemental iron content of common salts
| Salt | Iron content | Common tablet | Elemental iron |
|---|---|---|---|
| Ferrous sulfate (heptahydrate) | 20% | 325 mg | 65 mg |
| Ferrous sulfate (dried) | 30% | 200 mg | 60 mg |
| Ferrous fumarate | 33% | 210 mg | 69 mg |
| Ferrous gluconate | 12% | 300 mg | 36 mg |
| Carbonyl iron | 100% | 45 mg | 45 mg |
Why the distinction matters
Oral iron packs quote the mass of the salt, while guidelines, prescriptions and dose recommendations are written in terms of elemental iron. A 325 mg ferrous sulfate tablet and a 300 mg ferrous gluconate tablet look comparable on the box but deliver 65 mg and 36 mg of iron respectively — nearly a two-fold difference. Substituting one for the other tablet-for-tablet under-doses the patient badly.
The percentages come from the stoichiometry of each salt. Ferrous fumarate is a small molecule, so a third of its mass is iron; ferrous gluconate is a much larger one, so only about an eighth is. Carbonyl iron and polysaccharide-iron complex are elemental preparations where the two numbers coincide.
How the dose is divided has turned out to matter as much as its size. A dose of oral iron raises hepcidin for roughly 24 hours, and that rise blocks absorption of anything taken in the interim. Studies measuring fractional absorption directly have shown that a single daily dose, or a dose on alternate days, delivers more absorbed iron than the same total split three times daily — while causing considerably less gastrointestinal upset.
Frequently asked questions
How much elemental iron is in a 325 mg ferrous sulfate tablet?
About 65 mg. Ferrous sulfate heptahydrate is 20% iron by mass.
Which iron salt has the most elemental iron?
Among the conventional salts, ferrous fumarate at 33%. Carbonyl iron and polysaccharide-iron complex are elemental preparations, so their stated mass is the iron content.
Should oral iron be taken once daily or three times a day?
Once daily, or on alternate days. A dose raises hepcidin for about 24 hours and blocks further absorption, so divided doses within a day absorb less total iron and cause more side effects.
Does taking iron with vitamin C help?
The effect is smaller than long assumed. Trials comparing iron with and without ascorbic acid have found little difference in haemoglobin response. Avoiding tea, coffee, calcium and antacids near the dose matters more.
Related calculators
References
- Stoffel NU et al. Iron absorption from oral iron supplements given on consecutive versus alternate days. Lancet Haematol. 2017;4(11):e524–33.
- British National Formulary, iron salts — elemental iron content.
Medical Disclaimer: The tools and content provided here are for educational and reference purposes only. They are not intended to substitute for professional medical advice, diagnosis, or treatment. Clinical decisions should always be based on the comprehensive assessment of a qualified healthcare professional.