The decision tree
The calculator runs three branches in order: severity flags first, then the dose range from your indication, then the form and schedule from your tolerance.
1. Severity flags (gate first):
- Ferritin ≥300 ng/mL → stop. Iron overload (hereditary hemochromatosis) or chronic inflammation masking true iron status.
- Pregnancy + Hb <11 g/dL → escalate to obstetric provider. Pregnancy IDA needs split dosing through delivery + 3 months postpartum and prenatal-vitamin co-management — out of scope for a self-serve calculator.
- Men or post-menopausal women + ferritin <30 ng/mL + repletion indication → GI workup required first. New iron-deficiency in these cohorts is a red flag for occult GI bleeding (colorectal cancer is the most-feared cause).
- Ferritin <15 ng/mL → severe-deficiency advisory (non-blocking). Oral iron is appropriate, but if no reticulocyte response by day 7 or no Hb rise by 4 weeks, IV iron should be discussed.
2. Daily elemental iron range (by indication):
- Repletion (active IDA): 100–200 mg/day
- Non-anemic deficiency / maintenance: 60–100 mg/day
- Pregnancy prophylaxis: 30–60 mg/day throughout pregnancy + 3 months postpartum
- Athlete with low ferritin: 100–200 mg/day post-workout
The 100–200 mg range comes from the systematic review across 41 RCTs that established the dose-response curve for iron supplementation effect on ferritin and hemoglobin (PMID 22932280). Pregnancy ranges come from the prospective trials showing the 30 mg/day floor is enough to lower the incidence of low birth weight without GI side effects exceeding placebo (PMID 14522736).
3. Form (sulfate vs bisglycinate vs sucrosomial):
Ferrous sulfate is first-line for tolerant users — cheapest, most studied, 10–15% bioavailable. Ferrous bisglycinate kicks in for sensitive users, pregnancy, athletes, and elderly / post-menopausal: 20–30% bioavailable, fewer GI side effects, at 2–3× the cost. Sucrosomial iron is reserved for refractory cases (celiac flares, IBD, post-bariatric malabsorption) where standard oral iron fails.
4. Schedule (daily vs alternate-day vs 3×/week):
Tolerant users in active repletion get daily dosing per the 2026 RCT (PMID 41354563): 60 mg elemental daily for 14 days produced greater hematological gains than 120 mg every-other-day in iron-deficient women, despite the higher fractional absorption per dose claimed for the alternate-day regimen. The mean-corpuscular-volume bump (1.25 vs 0.50 fL, p = 0.043) and reticulocyte rise (0.32 vs 0.27 %, p = 0.055) both favour daily.
Maintenance and GI-sensitive cohorts get alternate-day. The hepcidin elevation softens between doses, GI side effects drop, and the absorption hit is small once you're past active repletion.
What the calculator does NOT cover
- IV iron formulations (FCM, iron sucrose, ferric derisomaltose). IV is the right answer for HF + iron deficiency, CKD anemia non-response, IBD flare, and oral-iron failure. The decision is hematology / cardiology / nephrology territory and outside what a self-serve calculator should attempt.
- Pediatric dosing (1–6 mg/kg/day, weight-scaled, liquid-preferred).
- Hereditary hemochromatosis (HFE C282Y) where the answer is therapeutic phlebotomy, not supplementation.
Where the math stops being safe to extrapolate
The non-anemic iron-deficiency cohort (ferritin <30, Hb normal, with fatigue) is the most contested indication: oral iron reduces self-reported fatigue (SMD −0.38, 95% CI −0.52 to −0.23) but does NOT improve objective measures of physical capacity in a meta-analysis of 18 RCTs (PMID 29626044). The calculator surfaces this distinction implicitly by recommending a maintenance dose for non-anemic ID, not a full repletion dose — the goal is symptom relief and ferritin reserve restoration, not hematological correction that's already normal.