Why "Take Iron" Isn't a Specific Enough Answer
When a GP tells you that you're iron deficient, the natural next question is: how much do I need? Not in a vague "eat more red meat" sense — but in a precise, clinical sense. How many milligrams? Over how many infusions? And how does anyone actually calculate that?
The short answer is that iron replacement is not one-size-fits-all. A 45 kg woman with mild iron deficiency and a haemoglobin of 118 g/L needs significantly less iron than a 90 kg man with iron deficiency anaemia and a haemoglobin of 88 g/L. The calculation depends on your individual blood results.
The Markers That Tell Us How Iron Deficient You Are
When we assess iron status, we look at several markers together — not one in isolation:
| Marker | What It Measures | Normal Range |
|---|---|---|
| Ferritin (µg/L) | Iron stores in your body | 12–150 µg/L (adults) |
| Haemoglobin (g/L) | Oxygen-carrying capacity | 120–160 (women), 130–180 (men) |
| Transferrin Saturation (TSAT %) | How much iron is available in the blood | 20–50% |
| Serum Iron (µmol/L) | Iron circulating in plasma | 10–30 µmol/L |
Iron deficiency can exist even when haemoglobin is still normal — this is called iron deficiency without anaemia, and it still causes symptoms. Ferritin below 30 µg/L is considered absolute iron deficiency by most guidelines. Ferritin below 100 µg/L combined with TSAT below 20% indicates functional iron deficiency even if ferritin itself looks borderline.
The Ganzoni Formula: How Doctors Calculate Your Iron Requirement
For IV iron replacement, clinicians use the Ganzoni Formula to calculate total iron deficit in milligrams:
Total Iron Deficit (mg) =
Weight (kg) × (Target Hb − Actual Hb) g/dL × 2.4 + Iron Stores
Target Hb = 15.0 g/dL (standard Ganzoni convention for adults ≥35 kg) · Iron Stores = 500 mg (adults ≥35 kg) · Hb must be converted from g/L to g/dL by dividing by 10
The formula has two parts. The haemoglobin deficit term (weight × Hb gap × 2.4) estimates how much iron is needed to bring your haemoglobin up to target. The iron stores term (500 mg) adds what's needed to refill your body's reserves — because correcting Hb alone without restoring ferritin is an incomplete treatment.
A Worked Example
A 65 kg woman with Hb of 105 g/L (10.5 g/dL):
- Weight term: 65 × (15.0 − 10.5) × 2.4 = 65 × 4.5 × 2.4 = 702 mg
- Iron stores: + 500 mg
- Total iron deficit: 1,202 mg
- At 200 mg elemental iron per infusion: ≈ 6 infusions
The same woman with Hb of 118 g/L (11.8 g/dL) — mildly low but not anaemic:
- Weight term: 65 × (15.0 − 11.8) × 2.4 = 65 × 3.2 × 2.4 = 499 mg
- Iron stores: + 500 mg
- Total: 999 mg → 5 infusions
And a 53 kg woman with confirmed iron deficiency (ferritin 14, TSAT 12%) but near-normal Hb of 122 g/L:
- Weight term: 53 × (15.0 − 12.2) × 2.4 = 53 × 2.8 × 2.4 = 356 mg
- Iron stores: + 500 mg
- Total: 856 mg → 5 infusions
Notice that even patients with near-normal haemoglobin can have a significant calculated iron deficit — because the formula accounts for restoring depleted stores, not just correcting anaemia. This is particularly important for patients with confirmed iron deficiency via ferritin or TSAT who are still symptomatic despite an Hb that looks borderline normal.
Try the Iron Deficit Estimator
We've built a free Iron Deficit Estimator that runs the Ganzoni Formula using your own blood test results. Enter your weight, haemoglobin, ferritin and TSAT, and it returns:
- A GREEN / AMBER / GREY assessment of whether your results are consistent with iron deficiency
- Your estimated total iron deficit in mg
- The estimated number of infusions needed
Use the Iron Deficit Estimator →
Results are for guidance only. All treatment plans are confirmed by a GMC Registered GP after reviewing your full blood results.
What Happens If I Get Less Iron Than I Need?
Under-treating iron deficiency is one of the most common clinical mistakes. It leads to:
- Partial recovery of haemoglobin with ferritin remaining depleted
- Continued fatigue, brain fog, and exercise intolerance despite "finishing" treatment
- Rapid relapse of iron deficiency within months — particularly in patients with ongoing losses (heavy periods, gut bleeding)
This is why at Lambert Medical Practice, we use the Ganzoni Formula to guide dosing, and include a follow-up blood test at Week 6 to confirm that ferritin and haemoglobin have actually reached target — not just improved.
What Happens If I Get Too Much?
Iron overload — haemochromatosis or iatrogenic iron excess — is harmful to the liver, heart, and joints. This is why IV iron must always be prescribed by a doctor after reviewing blood results. At Lambert Medical Practice, we do not administer iron without a recent set of iron studies. The Ganzoni calculation ensures we're replacing what's actually missing, not a blanket high dose.
Ready to Find Out Your Iron Requirement?
If you have recent blood test results showing iron deficiency, use our free estimator to get a personalised Ganzoni calculation. Then book an appointment and our GP will review your results, confirm your treatment plan, and — if appropriate — administer your first infusion at the same visit.