Iron Deficit (Ganzoni Formula) Calculator
Iron Deficit (Ganzoni Formula) Calculator
Calculate the total iron deficit in milligrams from body weight and haemoglobin, the standard basis for intravenous iron dosing.
Iron Deficit (Ganzoni Formula)
Weight + Hb → mg ironWeight 70 kg, Hb 9.5 g/dL, target 15 g/dL
Formula
- 2.4
- combines blood volume per kg (70 mL) with the iron content of haemoglobin (3.4 mg per g)
- iron stores
- 500 mg for body weight ≥ 35 kg; 15 mg/kg below 35 kg
- target Hb
- conventionally 15 g/dL in adults
Worked example
Weight 70 kg, Hb 9.5 g/dL, target 15 g/dL
70 × (15 − 9.5) × 2.4 = 70 × 5.5 × 2.4 = 924 mg
924 + 500 mg stores = 1424 mg
Where the 2.4 comes from
| Component | Value | Note |
|---|---|---|
| Blood volume | 70 mL/kg | Adult estimate |
| Iron in haemoglobin | 3.4 mg per g Hb | — |
| Combined | 0.070 × 3.4 × 10 = 2.4 | The factor of 10 converts g/dL to g/L |
| Iron stores | 500 mg | 15 mg/kg if under 35 kg |
Using the Ganzoni deficit
The Ganzoni formula converts a haemoglobin shortfall into the amount of elemental iron needed to correct it, plus an allowance to refill storage. It has been the basis of intravenous iron dosing for decades and remains the reference calculation in most product labels and guidelines.
Its two components are worth separating. The first term is the iron locked up in the haemoglobin the patient has not made: body weight gives blood volume, the haemoglobin gap gives the deficit per unit volume, and 3.4 mg of iron per gram of haemoglobin converts it to milligrams. The second term, conventionally 500 mg in adults, replaces depleted stores so the patient does not become deficient again as soon as the marrow draws on them.
In practice most units have moved to simplified weight-and-haemoglobin dosing tables published with each preparation, because they are faster and avoid transcription errors. The Ganzoni result remains useful as a sanity check on those tables and in situations they do not cover — very low or very high body weight, or a target haemoglobin other than 15 g/dL. Note that using actual body weight in obesity overestimates the deficit, since fat mass carries proportionally less blood volume.
Frequently asked questions
What is the Ganzoni formula?
Iron deficit in mg = body weight in kg × (target haemoglobin − actual haemoglobin in g/dL) × 2.4, plus 500 mg for iron stores in adults over 35 kg.
Why 2.4?
It combines an adult blood volume of 70 mL/kg with the 3.4 mg of iron contained in each gram of haemoglobin, and a factor of 10 to convert g/dL into g/L.
What target haemoglobin should I use?
15 g/dL is conventional for adults regardless of sex, because the formula is designed to restore normal haemoglobin plus stores rather than to reach a sex-specific threshold.
Should I use actual or ideal body weight?
The formula is written for actual body weight, but it overestimates the deficit in obesity because adipose tissue carries less blood volume per kilogram. Many centres cap the calculated dose in this situation.
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References
- Ganzoni AM. Intravenous iron-dextran: therapeutic and experimental possibilities. Schweiz Med Wochenschr. 1970;100(7):301–3.
- Ferinject (ferric carboxymaltose) Summary of Product Characteristics.