Iron Toxicity Dose Calculator
Iron Toxicity Dose Calculator
Convert a number of iron tablets into ingested elemental iron per kilogram against the 20 and 60 mg/kg thresholds, and avoid the salt-versus-elemental error that understates every dose.
Ingested elemental iron
Tablets → mg/kg elemental20 tablets of ferrous sulfate 325 mg taken by a 70 kg adult
Formula
Result = elemental iron (mg) ÷ body weight (kg)
- elemental fraction
- ferrous sulfate 20%, ferrous gluconate 12%, ferrous fumarate 33% — the proportion of the salt that is actually iron
- tablet strength
- the number printed on the bottle, which is almost always the mass of the salt and not of elemental iron
- 20 mg/kg
- up to this figure toxicity is unlikely; 20–60 mg/kg is mildly to moderately toxic
- 60 mg/kg
- above this, severe symptoms and significant morbidity can follow
Worked example
20 tablets of ferrous sulfate 325 mg taken by a 70 kg adult
Salt ingested = 20 × 325 = 6500 mg of ferrous sulfate
Elemental iron = 6500 × 20% = 1300 mg
1300 ÷ 70 = 18.6 mg/kg elemental iron → toxicity unlikely
Using the salt figure instead would give 6500 ÷ 70 = 92.9 mg/kg and put the same ingestion in the severe band — the error runs in the other direction for a real ingestion reported as elemental
Elemental iron in common oral preparations
| Salt | Elemental content | Common tablet | Elemental iron per tablet |
|---|---|---|---|
| Ferrous sulfate | 20% | 325 mg | 65 mg |
| Ferrous sulfate | 20% | 200 mg | 40 mg |
| Ferrous gluconate | 12% | 300 mg | 36 mg |
| Ferrous fumarate | 33% | 200 mg | 66 mg |
| Ferrous fumarate | 33% | 325 mg | 107 mg |
The five stages of iron poisoning
| Stage | Timing | Features |
|---|---|---|
| 1 — Gastrointestinal | 0 – 6 hours | Vomiting, diarrhoea, abdominal pain, haematemesis, melaena |
| 2 — Quiescent | 6 – 24 hours | Apparent improvement while occult hypoperfusion and acidosis progress |
| 3 — Shock and acidosis | 6 – 72 hours | Metabolic acidosis, shock, coagulopathy, multi-organ failure |
| 4 — Hepatotoxicity | 12 – 96 hours | Transaminitis and hepatic failure |
| 5 — Strictures | 2 – 8 weeks | Gastric outlet obstruction and intestinal strictures from healing corrosive injury |
Salt, elemental iron and the deceptive second stage
Iron toxicity is judged on the elemental iron ingested per kilogram of body weight, and the single commonest error is to use the number printed on the bottle. That figure is almost always the mass of the salt: ferrous sulfate is only 20% elemental iron, ferrous gluconate 12% and ferrous fumarate 33%. Taking the salt weight as elemental therefore overstates a dose by three to eight times, and taking an elemental figure as salt understates it by the same factor. Twenty ferrous sulfate 325 mg tablets are 6500 mg of salt but only 1300 mg of elemental iron.
Up to 20 mg/kg of elemental iron toxicity is unlikely. Between 20 and 60 mg/kg the ingestion is mildly to moderately toxic, and above 60 mg/kg severe symptoms and significant morbidity can follow. The classic presentations are a small child who has found a bottle of adult iron tablets, and a child who has eaten prenatal vitamins, which are often brightly coloured, sweet-coated and stored where a toddler can reach them. A modest number of tablets is a large mg/kg dose in a 12 kg child.
Iron poisoning runs through five stages, and the second is the one that catches people out. After the initial vomiting, diarrhoea, abdominal pain and haematemesis of the first 6 hours, there is a quiescent period in which the child appears to improve while hypoperfusion and acidosis progress unseen. Shock, metabolic acidosis and coagulopathy follow, then hepatotoxicity, and weeks later gastric outlet obstruction or intestinal strictures from healing corrosive injury. Apparent recovery at 8 hours is not recovery.
Investigations support the assessment rather than settling it. Iron tablets are radio-opaque and may be visible on an abdominal radiograph, but a normal film does not exclude ingestion, particularly for liquid, chewable or prenatal preparations. Serum iron peaks 4 to 6 hours after ingestion, so an early concentration can be falsely reassuring, and the decision to give deferoxamine is driven by the clinical picture — persistent vomiting, acidosis, shock, lethargy — rather than by a single number. Poisoning management is time-critical and directed by a poisons centre or clinical toxicology service, which this calculator supports rather than replaces.
Frequently asked questions
How much iron is toxic?
Up to 20 mg/kg of elemental iron is not expected to be toxic, 20 to 60 mg/kg is mildly to moderately toxic, and above 60 mg/kg severe symptoms and significant morbidity can follow. The figure must be elemental iron, not the weight of the salt.
Why does the iron salt matter?
Because only part of the salt is iron: ferrous sulfate is 20% elemental, ferrous gluconate 12% and ferrous fumarate 33%. The strength on the bottle is the salt, so treating it as elemental iron misstates the dose by three to eight times.
The child seems better a few hours after taking the tablets — is that reassuring?
No. The second stage of iron poisoning is a quiescent period between roughly 6 and 24 hours in which the child appears to improve while hypoperfusion and acidosis progress. Deterioration into shock and metabolic acidosis follows.
Does a normal abdominal radiograph exclude an iron ingestion?
No. Tablets are often radio-opaque and a positive film is useful, but liquid, chewable and prenatal preparations frequently are not visible, so a normal film excludes nothing.
When is serum iron measured, and who decides about deferoxamine?
Serum iron peaks 4 to 6 hours after ingestion, so an earlier sample can be falsely low. Deferoxamine is directed by the clinical picture rather than one concentration, and poisoning management is time-critical and directed by a poisons centre or clinical toxicology service that this calculator supports rather than replaces.
Related calculators
References
- Yuen HW, Gossman W. Iron toxicity. In: StatPearls. Treasure Island (FL): StatPearls Publishing.
- Manoguerra AS, Erdman AR, Booze LL, et al. Iron ingestion: an evidence-based consensus guideline for out-of-hospital management. Clin Toxicol. 2005;43(6):553–570.
- Iron poisoning. In: Merck Manual Professional Edition. Merck & Co.