Iron Deficit Calculator (Ganzoni formula)

This calculator estimates the total amount of iron needed to correct anaemia and refill the body's iron stores, using the Ganzoni formula. That figure is the starting point for planning intravenous iron, and for judging how long oral iron might take.

How the Ganzoni formula works

iron deficit (mg) = weight (kg) × (target Hb − actual Hb) (g/dL) × 2.4 + iron stores (mg)

• The factor 2.4 converts a haemoglobin gap into milligrams of iron. It comes from blood volume (about 7% of body weight) and the iron content of haemoglobin (0.34%).
• Target Hb is 15 g/dL (150 g/L) for people weighing 35 kg or more, and 13 g/dL (130 g/L) below 35 kg. Some protocols use a lower target, for example in pregnancy, so the field is editable.
• Iron stores are 500 mg for 35 kg or more, and 15 mg/kg below 35 kg.
• If your haemoglobin is in g/L, the calculator converts it to g/dL first (divide by 10).

Which weight should be used?

Product information for IV iron advises using ideal body weight in overweight and obese patients and pre-pregnancy weight in pregnancy. Actual weight is used if the patient is underweight. Enter the weight that applies to your patient. Our ideal body weight calculator can help.

Using the result

• The calculator shows the exact figure and a value rounded to the nearest 100 mg, which is how many products' dosing tables work. Follow the rounding rule in the specific product's information.
• Every IV iron product has its own maximum single dose, maximum weekly dose and infusion time. A large deficit usually has to be given in more than one visit.
• The Ganzoni formula estimates the need. Haemoglobin and ferritin should be rechecked after treatment.

Frequently asked questions

Why does the formula add 500 mg?

That allowance is to replenish iron stores as well as correct the haemoglobin. Without it, haemoglobin might normalise but stores would stay empty and anaemia would return.

What target haemoglobin should I use?

The standard Ganzoni target is 15 g/dL for 35 kg and over, and 13 g/dL below 35 kg. Lower targets are sometimes used, for example in pregnancy. Follow your local protocol.

Can I use this for anaemia of chronic disease?

The formula is for confirmed iron deficiency. It doesn't apply to anaemia from other causes, and iron shouldn't be given without confirming deficiency.

Why does the calculator say Hb is at or above target?

If the current haemoglobin is already at the target, the formula gives no haemoglobin deficit. Check the numbers and units you entered.

Is the total given in one go?

Often not. Products limit each dose, so large deficits are split across visits according to the product information.

Medical disclaimer

Sources & references

• Ganzoni AM. “Intravenous iron-dextran: therapeutic and experimental possibilities.” Schweizerische Medizinische Wochenschrift, 1970;100(7):301–303.
• Medsafe (New Zealand). Venofer (iron sucrose) data sheet — dose calculation using the Ganzoni formula. medsafe.govt.nz
• NICE Clinical Knowledge Summaries. Anaemia — iron deficiency. cks.nice.org.uk