Iron Intake Calculator
Determine your recommended daily iron intake based on age, gender, pregnancy status, and dietary pattern. Includes vegetarian multiplier and upper limits.
Results
Visualization
How It Works
Iron is the central atom of hemoglobin, myoglobin, and many cytochrome and electron-transport enzymes. NAM RDA: 8 mg/day for men and post-menopausal women, 18 mg/day for menstruating women, 27 mg/day in pregnancy, 9-10 mg during lactation. UL is 45 mg/day from all sources. Dietary iron exists in two forms with markedly different absorption: heme iron from meat/poultry/fish (15-35% bioavailable) and non-heme iron from plants and fortified foods (2-20% bioavailable, modulated heavily by enhancers and inhibitors). Vegetarians and vegans need an estimated 1.8x the standard RDA because their iron is exclusively non-heme.
The Formula
Variables
- Base RDA — NAM Recommended Dietary Allowance for age, sex, and pregnancy status
- Vegetarian Multiplier — 1.8x adjustment per IOM 2001 to account for lower non-heme iron bioavailability
- UL — Tolerable Upper Intake Level — 45 mg/day adults; gastrointestinal distress is the primary endpoint
Worked Example
Profile: 35-year-old vegan woman with menstrual losses, marathon training. Base RDA = 18 mg/day; vegetarian multiplier = 1.8; Adjusted RDA = 32.4 mg/day. Athletes lose an extra 1-2 mg/day through hemolysis, GI bleeding, and sweat (Peeling et al., Eur J Appl Physiol 2008), so functional needs are closer to 33-34 mg/day. UL is 45 mg, so dietary intake stays in range. If serum ferritin is below 30 ng/mL on this regimen, a 12-week course of 65 mg elemental iron (325 mg ferrous sulfate) every other day raises ferritin more efficiently and with less GI upset than daily dosing — Stoffel et al., Lancet Haematol 2017 showed alternate-day regimens double fractional absorption by lowering hepcidin.
Practical Tips
- Pair non-heme iron with 50-100 mg vitamin C in the same meal — ascorbate reduces ferric to ferrous iron and forms a soluble chelate, raising absorption 2-4x (Cook & Monsen, AJCN 1977).
- Tannins in tea and coffee inhibit non-heme iron absorption by 60-90%; phytates in whole grains and legumes inhibit by 50-65%. Separate these from iron-rich meals by 2 hours.
- Calcium 300 mg or more competes with iron at the DMT1 transporter — do not co-administer iron supplements with dairy, calcium pills, or calcium-fortified foods.
- Take ferrous sulfate, gluconate, or fumarate on an empty stomach for maximum absorption; switch to with-food dosing if nausea persists. Ferrous bisglycinate has lower GI side-effect rates at equivalent absorption.
- Alternate-day dosing (every 48 hours) outperforms daily dosing for hemoglobin recovery — daily iron transiently spikes hepcidin and blocks the next dose's absorption (Moretti et al., Blood 2015).
- Hemochromatosis (HFE C282Y homozygosity) affects roughly 1 in 200 Northern Europeans. Never start iron supplementation without a serum ferritin and transferrin saturation if there is family history of liver disease.
Frequently Asked Questions
Should I take iron with vitamin C?
Yes — for non-heme sources. Cook & Monsen (AJCN 1977) showed 75 mg ascorbate raises non-heme iron absorption 3-4x. Concrete recommendation: take ferrous sulfate or gluconate with 4 oz orange juice or a 100 mg vitamin C tablet on an empty stomach. Heme iron from meat absorbs efficiently regardless. The effect of vitamin C diminishes once stores are replete because hepcidin downregulates absorption.
Why is alternate-day dosing better than daily?
Stoffel et al. (Lancet Haematol 2017) randomized iron-deficient women to 60 mg elemental iron daily versus alternate-day. Alternate-day dosing produced 25-49% higher cumulative absorption and equivalent ferritin recovery in fewer total doses with less GI distress. Mechanism: each oral dose elevates serum hepcidin for 24 hours, suppressing the next day's enterocyte uptake. Skip a day, hepcidin falls, absorption recovers.
Will cooking in cast iron meaningfully raise my iron intake?
Modestly. Acidic foods like tomato sauce simmered in cast iron pick up 2-5 mg iron per serving (Brittin & Nossaman, J Am Diet Assoc 1986). Non-acidic foods gain less than 0.5 mg per serving. Useful as a passive supplement for menstruating women but not sufficient to correct deficiency.
What ferritin level indicates I need to supplement?
Adult serum ferritin under 30 ng/mL signals depleted iron stores per WHO criteria; under 15 ng/mL is overt deficiency. The American Society of Hematology cutoff for premenopausal women is 50 ng/mL given high prevalence of iron-deficient erythropoiesis with normal hemoglobin. Ferritin is also an acute-phase reactant — interpret with CRP if there is concurrent infection or inflammation.
How long until iron supplementation reverses anemia?
Hemoglobin rises ~1 g/dL per 2-3 weeks of effective oral therapy; full restoration takes 8-12 weeks. Ferritin lags — count on 3-6 months at maintenance dosing to refill stores. Retest at week 4 to confirm hemoglobin response, then at week 12. If hemoglobin has not risen 1 g/dL by week 4, suspect ongoing blood loss, malabsorption (celiac, H. pylori), or non-adherence.
What does pregnancy do to iron requirements?
Plasma volume expands ~50% and the fetus plus placenta sequester ~300 mg iron, so RDA jumps from 18 mg to 27 mg/day. Most prenatal multivitamins provide 27-28 mg ferrous fumarate or sulfate. CDC recommends low-dose oral iron (30 mg/day) starting at the first prenatal visit, with screening for anemia at intake and 28 weeks. Untreated maternal iron-deficiency anemia raises risk of preterm birth and low birth weight (Stoltzfus, Food Nutr Bull 2003).
Are heme and non-heme iron equally counted by the RDA?
No. The 18 mg/day RDA for women assumes ~10% bioavailability based on a mixed Western diet (~10-15% heme, 85-90% non-heme). Vegetarians average ~5-12% absorption from a fully plant-based pattern, hence the 1.8x adjustment. Heavy tea drinkers, those with achlorhydria, or post-bariatric surgery patients absorb even less and may need clinical iron infusion despite adequate oral intake.
When is intravenous iron warranted?
Indications per ASH 2020: oral iron failure or intolerance, ongoing blood loss exceeding GI absorptive capacity, malabsorption (celiac, IBD, post-bariatric), CKD with EPO therapy, third-trimester anemia where rapid correction is needed, and chemotherapy-induced anemia. Modern preparations (ferric carboxymaltose, iron sucrose) deliver 500-1000 mg in 15-60 minutes with low anaphylaxis risk versus older iron dextran.
Sources
- NIH Office of Dietary Supplements — Iron Health Professional Fact Sheet
- NAM/IOM Dietary Reference Intakes for Iron (2001)
- Stoffel et al., Alternate-day vs Daily Iron (Lancet Haematol 2017)
- WHO — Iron Deficiency Anaemia Assessment, Prevention and Control
- Cook & Monsen, Vitamin C and Non-heme Iron (AJCN 1977)