Iron Deficit Calculator (Ganzoni Formula)
Estimate total iron deficit from body weight, hemoglobin gap, and storage-iron needs.
Enter your body weight in kilograms.
Enter the current hemoglobin value. Normal 12-17
Enter the desired target hemoglobin value. Normal 12-17
Iron stores
A default value of 500 mg is provided. You can edit this value if a different assumption is appropriate for your calculation.
Default: 500 mg.
Estimated iron deficit
0 mg
Ganzoni formula
Iron deficit = weight (kg) × [target Hb − current Hb] × 2.4 + iron stores (mg)
This calculator provides an estimate for informational purposes and is not a substitute for clinical assessment or individualized medical advice.
How to Get an Accurate Iron Deficit Result
A few quick checks to make sure your iron deficit estimate is accurate and easy to understand.
Enter Your Current Body Weight
The formula uses your weight to estimate your blood volume. Use your actual current weight, not an old or estimated one — even a few kilos off will change the result.
Use the Target Hb Your Doctor Gave You
Target hemoglobin isn't the same for everyone. It depends on things like sex and pregnancy status, and it's usually somewhere between 12 and 15 g/dL. If your doctor gave you a specific number, use that instead of guessing.
Leave Iron Stores at 500 mg Unless Told Otherwise
This is extra iron the calculator adds to refill your body's iron reserves, not just fix your blood count. 500 mg is the standard amount for adults. Only change it if your doctor has told you to use a different number.
Double-Check Your Units
You can enter weight in kg or lb, and hemoglobin in g/dL or g/L. Picking the wrong unit is the easiest way to get a result that's way off — check both before you calculate.
Know What the Number Means
The result is how much iron (in milligrams) your body needs to fully recover — not a diagnosis. It's meant to be used after a blood test has already shown you're iron deficient, and it helps decide the size of an iron supplement course or IV iron dose.
Use It to Talk to Your Doctor, Not Instead Of One
This tool gives you a solid estimate, but only a doctor can turn that into a safe treatment plan — especially before starting IV iron.
What Is Total Body Iron Deficit?
In clinical hematology, total body iron deficit is the quantifiable amount of elemental iron, expressed in milligrams, that a patient needs to receive in order to fully correct anemia and rebuild the body's iron reserves. This number is the foundation of any repletion plan, whether the clinician is prescribing oral ferrous sulfate over several months or scheduling one or two infusions of an intravenous (IV) iron formulation such as ferric carboxymaltose or iron sucrose.
The deficit calculation matters most in parenteral (IV) iron planning, because IV products bypass the gut's natural absorption limits and deliver a fixed, calculated dose directly into circulation. An inaccurate estimate cannot be self-corrected by the body the way oral dosing can, which is why clinicians rely on a validated formula rather than a flat, one-size-fits-all dose.
"Iron Deficiency" vs. "Iron Deficit"
These two terms are frequently used interchangeably in casual conversation, but they describe fundamentally different things, and confusing them is a common source of dosing errors.
Iron deficiency is a physiological state — a description of a patient's iron status based on laboratory markers such as low serum ferritin or low transferrin saturation. It answers the question: does this patient have insufficient iron stores? It is a yes/no (or graded: mild/moderate/severe) clinical classification. To calculate TSAT from your lab values, use our TSAT Calculator.
Iron deficit, on the other hand, is a calculated quantity — a specific number of milligrams derived mathematically from a patient's weight, current hemoglobin, target hemoglobin, and estimated storage iron. It answers a completely different question: how much iron, in milligrams, does this specific patient need to receive to fully correct the problem?
In short: iron deficiency identifies that a problem exists; iron deficit calculates how big the problem is in dosable terms. A calculator like this one is built specifically to answer the second question, using an already-established diagnosis of iron deficiency (or iron deficiency anemia) as its starting point.
| Parameter | Iron Deficiency (Physiological State) | Iron Deficit (Calculated Value) |
| Definition | A clinical state of depleted iron stores, identified by lab markers | A specific milligram quantity of elemental iron needed for full repletion |
| Clinical Measurement | Serum ferritin, TSAT, serum iron, hemoglobin (qualitative/threshold-based) | Body weight × Hb gap × constant + storage iron (formula-based, quantitative) |
| Primary Diagnostic Goal | Confirm whether a patient's iron status is abnormally low | Determine the exact dose required to normalize hemoglobin and stores |
| Role in Treatment Planning | Triggers the decision to treat and guides the choice of oral vs. IV route | Directly determines the number of infusions, vials, or the oral course duration |
Iron Deficiency at a Glance
Before getting into the formula itself, it's worth grounding the topic in scale — iron deficiency is not a niche concern. The World Health Organization's most recent global figures give a simple sense of how widespread it is:
| Population | Share Affected by Anaemia | Source |
| Children aged 6–59 months | ~40% | WHO, Anaemia Fact Sheet, 2025 |
| Pregnant women | ~37% | WHO, Anaemia Fact Sheet, 2025 |
| Women aged 15–49 (all) | ~30% | WHO, Anaemia Fact Sheet, 2025 |
| Leading cause of anaemia worldwide | Dietary iron deficiency | WHO, Anaemia Fact Sheet, 2025 |
| Global disability burden from anaemia (2019) | ~50 million years of healthy life lost | WHO, Anaemia Fact Sheet, 2025 |
| U.S. women aged 20–59, anaemia prevalence (2021–2023) | ~14.0% | CDC, NCHS Data Brief No. 519, 2024 |
These figures describe anaemia broadly (iron deficiency is its single largest cause worldwide), which is why an accessible, accurate calculator for translating a diagnosis into a specific treatment dose has genuine everyday value rather than being a purely academic exercise.
If you're evaluating your own iron panel, start with the TSAT Calculator and UIBC Calculator before calculating replacement dose.
The Ganzoni Equation: Mathematical Breakdown
The Formula
The most widely used method for calculating total iron deficit is the Ganzoni equation, first described by Ganzoni in 1970 and still cited today across intravenous iron product labeling, hematology references, and calculator tools such as MDCalc:
Total Iron Deficit (mg) = Body Weight (kg) × (Target Hb − Actual Hb) (g/dL) × 2.4 + Depot Iron (mg)
Why the Constant Is 2.4
The 2.4 multiplier is not an arbitrary rounding — it is derived from three physiological constants multiplied together:
- 0.07 — average blood volume as a fraction of body weight (approximately 7% of body weight is blood, expressed in liters per kilogram).
- 0.0034 — the iron content of hemoglobin, since iron makes up approximately 0.34% of the hemoglobin molecule's mass.
- 10,000 — a unit-conversion factor that turns a hemoglobin difference expressed in g/dL, combined with a blood volume in liters, into a final result in milligrams rather than grams.
Multiplying these together: 0.07 × 0.0034 × 10,000 = 2.38, which rounds to the commonly published constant of 2.4. This derivation is confirmed in Canada's health technology assessment review of ferric derisomaltose (Monoferric) dosing, which lays out the same three-factor breakdown. Because this constant is fixed by human physiology rather than by convention, it should not be altered regardless of which patient population the calculator is being used for — what changes from patient to patient is the target hemoglobin and the storage-iron assumption, not this multiplier.
Target Hemoglobin Standards
The "Target Hb" term represents the hemoglobin level the clinician wants the patient to reach after full repletion — it is not necessarily the top of the normal range, but rather a clinically reasonable endpoint. The classic Ganzoni convention, still used in many IV iron dosing protocols and clinical trial documents, sets the target hemoglobin at 15.0 g/dL for adult patients weighing 35 kg or more. However, target hemoglobin is ultimately a clinical judgment, not a fixed constant, and many modern calculators and prescribers use a somewhat lower, more individualized target — often in the 12–13 g/dL range for non-pregnant adults, or around 11 g/dL during pregnancy — depending on the patient's baseline severity, age, and clinical context. This is precisely why a well-designed calculator leaves target hemoglobin as an adjustable, clinician-entered field rather than hard-coding a single number.
Depot (Storage) Iron Allocation
The final additive term in the equation, "Depot Iron," accounts for the iron needed not just to correct circulating hemoglobin but to rebuild the body's iron reserves so the correction is durable rather than transient. Clinical convention allocates this as:
- 500 mg (fixed) for adult patients or any patient weighing 35 kg or more — this reflects a physiologically reasonable lower-bound estimate of storage iron in a small adult.
- 15 mg per kilogram of body weight for patients under 35 kg, which scales the storage allocation down appropriately for smaller adults, adolescents, and pediatric patients rather than assigning them the same flat 500 mg used for a full-sized adult.
Worked Clinical Calculation Examples
Case A — Standard Adult Female (65 kg): A 65 kg woman presents with an actual hemoglobin of 9.0 g/dL. Her clinician selects a target hemoglobin of 15.0 g/dL (the classic adult Ganzoni target) and, because her weight exceeds 35 kg, a fixed 500 mg storage allocation.
- Hb difference: 15.0 − 9.0 = 6.0 g/dL
- Base deficit: 65 kg × 6.0 g/dL × 2.4 = 936 mg
- Plus storage iron: 936 + 500 = 1,436 mg total iron deficit
Case B — Underweight/Adolescent Patient (30 kg): A 30 kg adolescent patient has an actual hemoglobin of 8.0 g/dL. Given the patient's smaller frame, the clinician selects a more conservative target hemoglobin of 12.0 g/dL, and because body weight is under 35 kg, storage iron is calculated at 15 mg/kg rather than the flat adult allocation.
- Hb difference: 12.0 − 8.0 = 4.0 g/dL
- Base deficit: 30 kg × 4.0 g/dL × 2.4 = 288 mg
- Storage iron: 30 kg × 15 mg/kg = 450 mg
- Total: 288 + 450 = 738 mg total iron deficit
Iron Deficit Calculator Logic
The following flow describes, step by step, how the four calculator inputs move through the Ganzoni engine to a final clinical decision:
Input Stage → Patient Weight (kg or lb) · Current Hb (g/dL, g/L, or mmol/L) · Target Hb (same unit options)
↓
Calculation Engine (Ganzoni Formula Factor Processing)
- Normalize weight to kilograms (convert from pounds if needed).
- Normalize both hemoglobin values to g/dL (convert from g/L or mmol/L if needed).
- Subtract Actual Hb from Target Hb to get the hemoglobin gap.
- Multiply: Weight (kg) × Hb gap (g/dL) × 2.4.
- Determine storage-iron allocation: flat 500 mg if weight ≥ 35 kg, or 15 mg/kg if weight < 35 kg.
- Add the storage-iron allocation to the base result.
↓
Output Stage → Total Iron Deficit, in milligrams (a single validated number)
Related Iron Panel Calculators:
Evaluating complete iron status? Use our specialized tools to convert lab values:
- Transferrin Saturation (TSAT) Calculator: — Check transferrin saturation to confirm iron deficiency before calculating replacement dose.
- Unsaturated Iron-Binding Capacity (UIBC) Calculator: — Calculate unsaturated iron-binding capacity and derive TIBC from your iron panel.
Frequently Asked Questions
How do I calculate iron deficit manually without the calculator?
Use the Ganzoni equation directly: subtract actual hemoglobin from target hemoglobin (both in g/dL), multiply by body weight in kilograms, multiply that result by 2.4, then add 500 mg (or 15 mg per kg for patients under 35 kg) for storage iron.
What is the Ganzoni equation formula, exactly?
Total Iron Deficit (mg) = Body Weight (kg) × (Target Hb − Actual Hb, in g/dL) × 2.4 + Depot Iron (mg).
What's the difference between iron deficit and iron deficiency?
Iron deficiency is the diagnosis — a physiological state of depleted iron stores identified through lab testing. Iron deficit is the calculated number of milligrams of iron needed to correct that state. Iron deficiency is confirmed with lab markers like ferritin and transferrin saturation.
Should I use actual body weight or ideal body weight in the formula?
For most adult patients, actual body weight is used. However, several clinical reviews recommend substituting an estimated ideal body weight for patients with significant obesity, since the classic formula's use of actual weight can overstate the true blood-volume-driven iron requirement in this population. This adjustment is a matter of clinical judgment rather than something a general calculator applies automatically.
References
- Ganzoni AM. Intravenous iron-dextran: therapeutic and experimental possibilities. Schweiz Med Wochenschr. 1970;100(7):301-303.
- MDCalc. Ganzoni Equation for Iron Deficiency Anemia Calculator. https://www.mdcalc.com/calc/10139/ganzoni-equation-iron-deficiency-anemia
- Canadian Agency for Drugs and Technologies in Health (CADTH). Pharmacoeconomic Review Report: Iron Isomaltoside 1000 (Monoferric), Table 12 — Ganzoni Formula for Iron Isomaltoside 1000 Dosing. https://www.ncbi.nlm.nih.gov/books/NBK564172/table/pe.app5.tab4/
- World Health Organization. Anaemia (fact sheet). 2025. https://www.who.int/news-room/fact-sheets/detail/anaemia
- Centers for Disease Control and Prevention, National Center for Health Statistics. NCHS Data Brief No. 519, December 2024. https://www.cdc.gov/nchs/products/databriefs/db519.htm
Disclaimer: We are developers and health information researchers, not medical professionals. Our calculators are educational tools built from published formulas, not a substitute for clinical assessment or confirmation of a medication dose. Always consult a qualified healthcare provider for medical advice, diagnosis, or treatment.