What Is the Ganzoni Formula?
The Ganzoni Formula is a mathematical equation developed to calculate a patient's total body iron deficit in milligrams. It was originally described by Alberto Ganzoni in 1970 and has since become the standard basis for dosing IV iron replacement therapy in clinical practice. It is referenced in the dosing guidance for IV iron preparations and used in NHS and private clinical settings worldwide.
The formula solves a practical clinical problem: iron replacement needs to correct not just anaemia (low haemoglobin), but also replenish the body's iron stores (ferritin). Without the formula, clinicians are either guessing the dose or applying a blanket amount that may significantly over- or under-treat individual patients.
The Formula
Total Iron Deficit (mg) =
Weight (kg) × (Target Hb − Actual Hb) g/dL × 2.4 + Iron Stores
For patients weighing less than 35 kg, iron stores = 15 mg/kg body weight rather than the fixed 500 mg.
Breaking Down Each Term
1. Weight (kg)
The formula scales with body weight. A heavier patient has more blood volume and therefore more haemoglobin to replenish — and more tissue to supply with iron. Weight is the most direct multiplier in the calculation.
2. (Target Hb − Actual Hb) × 2.4
This term quantifies the haemoglobin deficit — how much haemoglobin needs to be raised, and therefore how much iron is needed to produce those additional red blood cells. The constant 2.4 comes from the relationship between haemoglobin concentration and blood volume in a standard adult (approximately 3.4 mg iron per gram of Hb × 0.7 L blood/kg body weight ≈ 2.4).
⚠ Common mistake: using the wrong target Hb
The target Hb in the Ganzoni Formula is 15.0 g/dL — a repletion target, not the anaemia threshold. Using 12.0 g/dL (the lower limit of normal for women) as the target incorrectly produces a much lower (or even negative) Hb deficit term, and significantly underestimates the iron requirement. A 53 kg woman with Hb 122 g/L gets 856 mg with the correct target — and only ~475 mg if the wrong target is used. That's the difference between 5 infusions and 2.
3. Iron Stores (500 mg)
This is the amount of iron needed to replenish the body's iron depot — the reserves stored as ferritin and haemosiderin. Even after haemoglobin is fully corrected, the body's stores need to be topped up. If this term is omitted, patients who have their Hb corrected may feel better initially but relapse quickly as depleted stores are rapidly consumed. For adults weighing ≥35 kg, this is a fixed 500 mg.
Worked Examples
| Patient | Weight | Hb (g/L) | Hb (g/dL) | Calculation | Total | Infusions |
|---|---|---|---|---|---|---|
| Woman A | 65 kg | 105 g/L | 10.5 | 65 × (15.0−10.5) × 2.4 + 500 = 702+500 | 1,202 mg | 6 |
| Woman B | 53 kg | 122 g/L | 12.2 | 53 × (15.0−12.2) × 2.4 + 500 = 356+500 | 856 mg | 5 |
| Man A | 85 kg | 110 g/L | 11.0 | 85 × (15.0−11.0) × 2.4 + 500 = 816+500 | 1,316 mg | 7 |
| Woman C | 48 kg | 135 g/L | 13.5 | 48 × (15.0−13.5) × 2.4 + 500 = 173+500 | 673 mg | 4 |
Notice that Woman B — with a near-normal haemoglobin but confirmed iron deficiency via ferritin and TSAT — still has a calculated deficit of 856 mg (5 infusions). This illustrates why the Ganzoni Formula is important for patients with iron deficiency without anaemia, not just those with low Hb.
How Many Infusions Does the Formula Suggest?
Once the total iron deficit is calculated, divide by the iron delivered per infusion and round up:
Try the Calculator — Using Your Own Blood Results
We've built a free Iron Deficit Estimator that runs the Ganzoni Formula using your blood test results. Enter your weight, haemoglobin, ferritin, and TSAT to get your estimated iron requirement and infusion count:
Results are estimates. All treatment plans are confirmed by a GMC Registered GP after reviewing your full blood results.