Iron Deficiency Calculator.

When you are iron deficient, the shortfall has two parts: the iron missing from your red blood cells (which is what drops your hemoglobin and causes anemia) and the iron missing from your storage depot (mostly ferritin, which buffers future needs). The Ganzoni equation adds both into a single number — the total milligrams of iron you would need to fully correct the anemia and refill the tank.

The formula is: deficit (mg) = body weight (kg) × (target − current hemoglobin, g/dL) × 2.4 + depot iron. The 2.4 factor comes from the iron content of hemoglobin and your blood volume; the depot term (500 mg for a typical adult) is the store you want to rebuild. A 70 kg person at hemoglobin 10 aiming for 15 needs about 1,340 mg.

The number matters because the two repletion routes work at very different speeds. Intravenous iron delivers 500–1000 mg per infusion, so most deficits are closed in one or two visits — which is why IV iron is preferred for large deficits, poor oral tolerance, or ongoing blood loss. The exact IV dose follows the specific product's weight-and-hemoglobin chart, not the Ganzoni figure directly.

Oral iron is slower: even taken well (on an empty stomach, with vitamin C, ideally every other day to blunt the hepcidin blockade), the gut absorbs only about 10–20 mg of elemental iron per day. Closing a four-figure deficit by mouth therefore takes months of consistent dosing — and many people stop early because of the stomach side effects.

Before trusting any iron calculation, confirm the deficiency is real. Ferritin is the usual marker — under about 30 ng/mL means depleted stores — but ferritin is also an acute-phase reactant, so infection, obesity, liver disease, or any inflammation can falsely raise it and mask a true deficiency. That is why iron studies are best read alongside a marker of inflammation like hs-CRP.

This tool estimates a total to replace; it does not diagnose why you are deficient (blood loss, malabsorption, diet, or increased demand all matter) or prescribe a regimen. Iron overload is harmful, so repletion should be planned and monitored with a clinician.