Iron Absorption Estimator
The iron on your plate is not the iron in your blood. Estimate what you actually absorb from a meal, and what would change it.
Only from meat, fish, and poultry. Roughly 40% of the iron in meat is heme; see the source table below.
Plants, eggs, dairy, fortified foods, supplements, plus the other ~60% of meat iron.
Absorption is regulated by hepcidin: low iron stores downregulate hepcidin and can more than double fractional absorption. Use a recent ferritin result if you have one.
Orange ~70mg, kiwi ~65mg, half a red bell pepper ~95mg, 500mg tablet = 500.
The "meat factor" boosts non-heme absorption. A palm-sized portion is ~100g.
Glass of milk ~300mg, yogurt cup ~250mg, calcium supplement 500-600mg. Inhibition matters above ~150mg.
Iron content of common sources
Use this to fill in the heme and non-heme fields. For meat, count ~40% of total iron as heme and the rest as non-heme.
| Food | Serving | Total iron | Heme portion |
|---|---|---|---|
| Beef, cooked | 100 g | ~2.6 mg | ~1.0 mg |
| Chicken breast | 100 g | ~0.9 mg | ~0.3 mg |
| Salmon | 100 g | ~0.6 mg | ~0.2 mg |
| Lentils, cooked | 1 cup | ~6.6 mg | 0 |
| Spinach, cooked | 1 cup | ~6.4 mg | 0 |
| Firm tofu | 1/2 cup | ~3.4 mg | 0 |
| Fortified breakfast cereal | 1 serving | 4-18 mg | 0 |
| Egg | 1 large | ~0.9 mg | 0 |
| Dark chocolate (70%+) | 28 g | ~3.4 mg | 0 |
How this works
Dietary iron comes in two forms with completely different absorption pathways. Heme iron (from hemoglobin and myoglobin in meat, fish, and poultry) is taken up as an intact porphyrin complex and absorbed at roughly 15-35% regardless of what else is on the plate; only co-ingested calcium meaningfully dents it. Non-heme iron (everything else) must be reduced to Fe2+ and carried across the enterocyte by DMT1, which makes its absorption low (typically 2-10% from mixed meals) and highly sensitive to the meal matrix.
This calculator follows the multiplicative-factor approach established by Hallberg and Hulthén's absorption algorithm and refined by later whole-diet regression models. It starts from a basal non-heme absorption set by iron status (6% replete, 10% borderline, 16% deficient; low ferritin suppresses hepcidin, which frees ferroportin to export more iron into circulation) and multiplies it by factor terms:
Enhancers. Ascorbic acid reduces Fe3+ to soluble Fe2+ and chelates it; the effect is dose-dependent and saturating, modeled here as up to ~3x at 200mg or more, and stronger (up to ~3.6x) in high-phytate meals where there is more inhibition to reverse. Muscle tissue contributes a "meat factor" worth up to ~2x at large portions, modeled as saturating around 100-200g.
Inhibitors. Phytates in whole grains, legumes, nuts, and bran are the strongest common inhibitor; even modest amounts cut non-heme absorption to 50%, and bran-heavy meals to 20-30% of baseline. Calcium inhibits both forms via a dose-dependent effect that begins around 150mg and plateaus near 50-60% inhibition of non-heme (about half that for heme) at 300-600mg. Tea polyphenols cut non-heme absorption by roughly 60% per cup (coffee about 40%), consistent with classic radioisotope meal studies.
Absorbed iron equals heme mg times its fraction plus non-heme mg times its adjusted fraction. Fractions are capped (non-heme at 15/25/40% by status, heme at 40%) because absorption saturates, and a damping term lowers the non-heme fraction for loads above ~15 mg, matching isotope data showing fractional absorption of large supplement doses falling roughly with the cube root of dose (about 12% at 40 mg but 6.5% at 160 mg in iron-depleted women). The output is shown as a range, not a point estimate: single-meal isotope studies show several-fold between-person variability even at matched ferritin, so treat the midpoint as a central tendency. The comparison target is absorbed iron needed to replace daily losses: about 1.0 mg for men and postmenopausal women, 1.5-2 mg for menstruating women, more with heavy periods.
References
Hallberg L, Hulthén L. Prediction of dietary iron absorption: an algorithm for calculating absorption and bioavailability of dietary iron. Am J Clin Nutr. 2000;71(5):1147-1160.
Armah SM, et al. A complete diet-based algorithm for predicting nonheme iron absorption in adults. J Nutr. 2013;143(7):1136-1140.
Stoffel NU, et al. Oral iron supplementation in iron-deficient women: How much and how often? Mol Aspects Med. 2020;75:100865.
Piskin E, et al. Iron Absorption: Factors, Limitations, and Improvement Methods. ACS Omega. 2022;7(24):20441-20456.