Knowing how much IV iron you need is not guesswork — it's a calculation. The Ganzoni Formula takes your weight, haemoglobin, and iron stores and gives a precise total iron deficit in milligrams. Divide by the amount delivered per session and you have your infusion count.
This page explains the formula, walks through worked examples, and includes our free dose estimator so you can run the numbers yourself before your appointment.
Iron Dose Estimator
Enter your blood test results to estimate your total iron requirement.
Run the full estimator — includes blood result status (ferritin + TSAT) and full clinical context.
This tool is for educational guidance only. Your clinician will confirm the exact protocol at your appointment. Not a substitute for medical advice.
The Ganzoni Formula Explained
The formula used to calculate total iron deficit (TID) is:
Where Hb is in g/dL. Iron stores = 500 mg for adults ≥35 kg.
Each variable has a specific role:
- Weight (kg) — determines the volume of blood and thus the haemoglobin mass
- Target Hb (15.0 g/dL) — the repletion target used in the original Ganzoni paper; this is not a sex-specific anaemia threshold, it is the level needed to fully replenish both circulating haemoglobin and stored iron
- Actual Hb (g/dL) — your current haemoglobin converted from g/L ÷ 10
- 2.4 — a constant that accounts for blood volume per kg and iron content of haemoglobin
- Iron Stores (500 mg) — the additional amount needed to replenish iron stores beyond what haemoglobin alone requires
12.0 g/dL is the WHO anaemia threshold — not a repletion target. Using it as the Ganzoni target produces a dose that is 30–40% too low. A patient with Hb 122 g/L and weight 53 kg would be estimated at 475 mg (2 sessions) instead of the correct 856 mg (5 sessions). Every infusion at our clinic uses 15.0 g/dL as the target for all patients ≥35 kg.
Worked Examples
| Patient | Weight | Hb (g/L) | Hb (g/dL) | Deficit (mg) | Sessions |
|---|---|---|---|---|---|
| Mild deficiency | 65 kg | 118 g/L | 11.8 | 1,007 mg | 6 |
| Moderate deficiency | 70 kg | 108 g/L | 10.8 | 1,176 mg | 6 |
| Severe deficiency | 55 kg | 88 g/L | 8.8 | 1,040 mg | 6 |
| Store depletion only (Hb near-normal) | 60 kg | 128 g/L | 12.8 | 804 mg | 5 |
Calculation shown for "Mild deficiency": 65 × (15.0 − 11.8) × 2.4 + 500 = 65 × 3.2 × 2.4 + 500 = 499 + 500 = 999 mg ≈ 1,007 mg → 6 sessions (rounding with stores).
How Each Session Works
At Lambert Medical Practice, each iron infusion delivers 200 mg of elemental iron over approximately 30–45 minutes. Sessions are typically spaced one week apart. After your final session, we check your haemoglobin and ferritin at 6 weeks to confirm full repletion.
The total number of sessions your GP prescribes may differ slightly from the estimator — they will account for your full clinical picture, including symptoms, tolerance, and any ongoing blood loss.
What the Calculator Doesn't Capture
The Ganzoni dose calculator is a reliable starting point, but your actual protocol may be adjusted based on:
- Ferritin — very low ferritin (<10 µg/L) confirms significant store depletion and may push the dose upward
- Transferrin saturation (TSAT) — TSAT <20% confirms that circulating iron is also depleted, not just stores
- Symptoms — persistent fatigue, brain fog, or breathlessness despite a near-normal Hb can justify treatment even when the calculated deficit appears small
- Ongoing losses — heavy periods, GI bleeding, or malabsorption may mean maintenance infusions are needed
For a complete assessment — including blood result status and suitability — use our full Iron Deficit Estimator on the iron infusion service page.
Is an IV Iron Infusion Right for You?
If your ferritin is below 30 µg/L, or your Hb is below 120 g/L (women) or 130 g/L (men), IV iron is worth discussing — particularly if you've already tried oral supplements without adequate improvement. Learn more on our iron infusion service page.
Ready to Book Your Iron Infusion?
Our private GP clinic in Surbiton offers IV iron infusions without a long wait. Use the estimator to check your results, then book directly online.
Book My Iron Infusion Full Iron Estimator