Iron Deficiency Calculator
This calculator uses the Ganzoni equation to estimate the total body iron deficit in milligrams. Enter the patient weight, current hemoglobin, and target hemoglobin to find the cumulative elemental iron dose needed to restore hemoglobin and replenish iron stores. Results apply to adults and children; the iron-stores rule switches automatically for patients under 35 kg. Switch between metric and imperial weight and between g/dL, g/L, and mmol/L for hemoglobin.
Formula
Worked example
A 70 kg woman with an actual Hb of 8.0 g/dL and a target of 12.0 g/dL: Hb difference = 12.0 - 8.0 = 4.0 g/dL. Hb correction = 70 x 4.0 x 2.4 = 672 mg. Iron stores (standard adult) = 500 mg. Total deficit = 672 + 500 = 1172 mg of elemental iron.
What is the Ganzoni equation?
The Ganzoni equation is the standard clinical formula for estimating the total body iron deficit in patients with iron-deficiency anemia. It was published by J. Ganzoni in 1970 and remains the reference method for calculating parenteral (intravenous) iron doses. The formula multiplies body weight by the hemoglobin gap and a factor of 2.4, then adds an iron-stores component to account for replenishing the body depot. The factor 2.4 is derived from the product of blood volume as a percentage of body weight (approximately 7%), the iron content of hemoglobin by mass (approximately 0.34%), and a milligrams-to-grams conversion, then scaled for the g/dL unit. The result is the cumulative elemental iron in milligrams needed to raise hemoglobin to the target and refill iron stores - not a single-dose instruction, because individual infusions are limited by product specifications and patient tolerance.
Iron stores: the 500 mg adult rule and the pediatric rule
Iron stores represent the depot iron held in the liver, spleen, and bone marrow that is mobilized when dietary intake falls short. In adult patients weighing 35 kg or more, the Ganzoni equation adds a fixed 500 mg to cover depot replenishment. For patients below 35 kg (typically children or very small adults), the standard practice is to calculate stores as 15 mg per kg of body weight, so a 20 kg child would add 300 mg. Some clinicians omit the stores component when treating acute surgical blood loss or when the stores are known to already be adequate from a recent ferritin measurement; this calculator provides a "no stores" option for those scenarios. For patients who do not fit the standard rules, a custom value can be entered directly.
Choosing a target hemoglobin and interpreting the result
The target hemoglobin should reflect the clinical goal for the specific patient population. For most adults, 12-13 g/dL for women and 13-15 g/dL for men are commonly used starting points. Patients with chronic kidney disease (CKD) are usually targeted to 10-12 g/dL to avoid cardiovascular risk from excessive erythropoiesis-stimulating agent use. Pregnant patients have trimester-specific lower limits. Once the total deficit is calculated, the clinician selects an IV iron product and divides the total dose accordingly: high-dose products such as ferric carboxymaltose (Ferinject) can deliver up to 1000 mg per infusion, while iron sucrose is typically given in 200 mg doses over multiple sessions. Oral iron is an alternative when anemia is mild, absorption is intact, and no urgency exists.
Hemoglobin unit conversion reference
Hemoglobin is reported in g/dL in the United States, Canada, and most of Europe. The United Kingdom and some European laboratories use g/L (multiply g/dL by 10). A small number of laboratories report in mmol/L; to convert, multiply g/dL by 0.6206 (or divide mmol/L by 0.6206 to go back). This calculator accepts all three units and converts internally to g/dL for the Ganzoni formula, so you can enter whatever your lab report shows without manual conversion.
Normal hemoglobin ranges by sex and age group
| Group | g/dL | g/L | mmol/L |
|---|---|---|---|
| Adult men | 13.5-17.5 | 135-175 | 8.37-10.87 |
| Adult women (non-pregnant) | 12.0-15.5 | 120-155 | 7.45-9.62 |
| Pregnant women (1st trimester) | 11.0+ | 110+ | 6.83+ |
| Pregnant women (2nd/3rd trimester) | 10.5+ | 105+ | 6.52+ |
| Children 6 months - 5 years | 11.0+ | 110+ | 6.83+ |
| Children 5-11 years | 11.5+ | 115+ | 7.14+ |
| Children 12-14 years | 12.0+ | 120+ | 7.45+ |
| CKD patients (common target) | 10.0-12.0 | 100-120 | 6.21-7.45 |
Reference values from WHO guidelines. Target hemoglobin for iron replacement is typically the lower end of the normal range for the patient population.
Frequently asked questions
What is the Ganzoni formula used for?
The Ganzoni formula calculates the total body iron deficit in milligrams for patients with iron-deficiency anemia. It is primarily used to determine the cumulative dose of intravenous (parenteral) iron needed to restore hemoglobin to a target level and replenish body iron stores. It is not designed to calculate oral iron doses.
Why does the formula use 2.4 as a factor?
The factor 2.4 is an empirical constant derived from physiological constants: blood volume is roughly 7% of body weight, hemoglobin contains roughly 0.34% iron by mass, and there are 1000 mg per gram. Multiplied together (0.07 x 0.0034 x 1000) and scaled for g/dL units (x10), the result rounds to approximately 2.4. When hemoglobin is expressed in g/L the coefficient becomes 0.24.
What iron stores value should I use?
The standard adult depot is 500 mg for patients weighing 35 kg or more. For pediatric patients or small adults below 35 kg, 15 mg/kg is the standard calculation. Use the "no stores" option if the patient has recently had stores confirmed as replete by ferritin measurement. Use "custom" to enter a protocol-specific value.
Can this calculator be used for children?
Yes, with caveats. The Ganzoni equation applies to children, and this calculator automatically switches to the pediatric iron-stores rule (15 mg/kg) when the entered weight converts to below 35 kg. However, pediatric iron dosing should always be supervised by a clinician, and some specialist guidelines prefer weight-based dosing tables over the Ganzoni equation for very young children.
What is the difference between elemental iron and iron salt dose?
The Ganzoni result is in milligrams of elemental iron, the actual amount of iron atoms delivered. IV iron products label their doses in elemental iron (e.g., 500 mg of ferric carboxymaltose means 500 mg elemental iron). Oral supplements, however, list the salt weight: a 325 mg ferrous sulfate tablet contains only about 65 mg of elemental iron (20%). Always check product labels to confirm the elemental iron content.
Is this calculator suitable for patients with chronic kidney disease?
The Ganzoni equation can be applied to CKD patients, but the target hemoglobin should be lower (typically 10-12 g/dL) than for the general population. Many nephrology guidelines now prefer the lower target to reduce cardiovascular events associated with high hemoglobin or excess erythropoiesis-stimulating agent doses. Enter the appropriate CKD target and the calculator will adjust the deficit accordingly.
Why does the result say 0 mg?
The calculator returns 0 mg when the target hemoglobin entered is equal to or lower than the actual hemoglobin, meaning no deficit exists based on the entered values. Check that actual and target hemoglobin are correct. If iron deficiency is suspected clinically (low ferritin, low transferrin saturation), discuss with a clinician - the patient may need stores repleted even without a hemoglobin gap.