Iron Deficit Calculator (Ganzoni Formula) — Total Iron Dose & IV Iron Products

How to Use This Calculator

1. Enter weight, current hemoglobin, and target hemoglobin (default 15 g/dL).
2. Total iron deficit in mg calculates via Ganzoni formula.
3. Use the IV Iron Tables tab to calculate number of doses by product.
4. The Professional tier provides indication-specific IV iron guidance.

Formula

Example

Frequently Asked Questions

• What is the iron deficit and how is it calculated? ▼
  The iron deficit is the total amount of elemental iron (in milligrams) that needs to be replaced to correct iron deficiency anaemia and replenish iron stores. The Ganzoni formula calculates this as: Iron deficit (mg) = Body weight (kg) × (Target Hgb − Current Hgb g/dL) × 2.4 + Iron stores (mg). The constant 2.4 is derived from the fact that each gram of hemoglobin requires approximately 3.4 mg of iron, and blood volume is approximately 70 mL/kg with a hemoglobin content of approximately 0.34 g/g — working through the algebra gives the 2.4 factor. Iron stores are estimated at 500 mg for adults weighing 35 kg or above, and 15 mg/kg for patients under 35 kg (paediatric correction). The target hemoglobin is typically set at 15 g/dL for adults. A simplified version used in some trials and product labelling assumes a fixed target Hgb of 15 g/dL; this is the version embedded in the approved prescribing information for several IV iron products. The Ganzoni formula was developed by Ganzoni in 1970 and remains the standard method for calculating IV iron requirements.
• When is intravenous iron preferred over oral iron? ▼
  Intravenous (IV) iron is preferred over oral supplementation in several clinical situations where oral iron is inadequate, not tolerated, or contraindicated. In inflammatory bowel disease (Crohn's, ulcerative colitis), intestinal inflammation impairs oral iron absorption and exacerbates GI symptoms; ACG guidelines recommend IV iron as first-line in active IBD with Hgb below 10 g/dL. In chronic kidney disease (CKD), particularly patients on erythropoiesis-stimulating agents (ESAs), IV iron is required to provide sufficient substrate for ESA-driven erythropoiesis; KDIGO guidelines recommend IV iron targeting ferritin 200–500 ng/mL and transferrin saturation 20–50%. In perioperative anaemia, IV iron given 2–6 weeks preoperatively significantly reduces transfusion requirements in major surgeries (PREVENTT trial). Post-bariatric surgery patients have impaired absorption in the proximal duodenum where iron is absorbed. Patients with celiac disease, atrophic gastritis, or H. pylori infection may have impaired absorption. IV iron is also appropriate when the Ganzoni deficit exceeds what can be reliably delivered orally (oral iron replaces approximately 20–30 mg/day at best) and when rapid correction is required.
• What is the Ganzoni formula and why is it used? ▼
  The Ganzoni formula is the gold-standard equation for calculating the total IV iron dose required to correct iron deficiency anaemia and replenish tissue iron stores. Developed by AM Ganzoni and published in Schweizerische Medizinische Wochenschrift in 1970, the formula is: Iron deficit (mg) = Body weight (kg) × (Target Hgb − Current Hgb) × 2.4 + 500 mg (stores). It is used because underdosing IV iron — a common clinical error — results in incomplete correction and early recurrence. Multiple studies, including the FIND-CKD and FERINJECT-HF trials, demonstrated that Ganzoni-calculated doses produce superior outcomes compared to empirical fixed doses. The formula is incorporated into the approved prescribing information for ferric carboxymaltose (Ferinject), iron sucrose (Venofer), and ferric derisomaltose (MonoFer). One practical limitation of the formula is that it requires two inputs that may be estimated — target hemoglobin and iron stores — but the standard assumptions (15 g/dL target, 500 mg stores) are validated in published literature and appropriate for most adults. For patients in whom ferritin is severely depressed (below 5 ng/mL) or who have significant ongoing blood loss, the formula may underestimate total requirements.
• How should IV iron doses be spaced? ▼
  The dosing interval for IV iron depends on the product used and the total calculated deficit. Ferric carboxymaltose (Ferinject, Injectafer) allows doses up to 1,000 mg per infusion given over 15 minutes. If the Ganzoni deficit exceeds 1,000 mg, repeat dosing is needed at a minimum interval of 7 days, as iron redistribution and erythropoietic utilisation require time. Iron sucrose (Venofer) is given as 200 mg doses two to three times per week; each infusion takes 15–60 minutes. The slower dosing schedule makes iron sucrose practical for dialysis patients who receive infusions at each session. Ferric derisomaltose (MonoFer, Monoferric) allows single high doses up to 20 mg/kg or 3,000 mg in a single infusion over 20–60 minutes, making it attractive for patients who cannot return for multiple visits. Low molecular weight iron dextran (CosmoFer) also permits total-dose infusion. For all products, a 30-minute observation period after the first dose is recommended to monitor for hypersensitivity reactions, though anaphylaxis rates with modern formulations are very low (estimated 1:200,000 for ferric carboxymaltose). Baseline vital signs and resuscitation equipment should be available.
• Why should patients receiving ferric carboxymaltose be monitored for hypophosphatemia? ▼
  Hypophosphatemia (low serum phosphate) is a recognised complication of ferric carboxymaltose (FCM) and ferric derisomaltose, occurring in up to 75% of patients receiving FCM in some prospective studies. The mechanism involves FCM stimulating fibroblast growth factor 23 (FGF-23), a phosphaturic hormone that increases renal phosphate excretion. In most patients, the hypophosphatemia is asymptomatic and self-limiting, resolving within 8–12 weeks. However, in patients receiving repeated high doses — such as those with chronic IBD, CKD, or recurrent post-surgical iron deficiency — cumulative hypophosphatemia can be severe and prolonged, causing bone pain, fatigue, muscle weakness, and in extreme cases, hypophosphataemic osteomalacia. Risk factors include baseline lower phosphate, high FCM doses, and multiple repeat doses within months. Guidelines recommend monitoring serum phosphate before and 4 weeks after FCM administration in high-risk patients. Iron sucrose and iron dextran have a much lower risk of FGF-23-mediated hypophosphatemia, making them preferable alternatives in patients at risk. Oral phosphate supplementation is used for symptomatic cases.

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