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Check vitamin B12 intake vs RDA — see mcg/day, % of RDA and food sources.
Check vitamin B12 intake vs RDA — see mcg/day, % of RDA and food sources.
Enter values above and click Calculate — results will appear here with the formula explained.
Vitamin B12 RDA is 2.4 mcg daily for adults 19+ (2.6 mcg pregnancy, 2.8 mcg lactation) — a tiny amount with outsized jobs in DNA synthesis, red-cell formation and myelin maintenance. This calculator checks estimated intake as percent of RDA; typical omnivore diets deliver 5–7 mcg, so adequacy is the default outside specific risk groups.
Animal foods are essentially the only reliable sources: clams (~84 mcg per 3 oz), liver, fish, meat, dairy and eggs. Plants contain effectively zero B12 — nori, mushrooms and fermented foods carry analogues that don't count. Vegans must use fortified foods (nutritional yeast, plant milks) or supplements; 'natural' plant diets without them reliably produce deficiency within 2–5 years as liver stores deplete.
Absorption is the bottleneck, not intake: stomach acid and intrinsic factor must cleave B12 from food protein, then ileal receptors absorb it — a chain broken by age over 50 (atrophic gastritis affects 10–30%), PPIs and metformin, bariatric surgery, celiac/Crohn's and pernicious anemia (autoimmune intrinsic-factor loss). Older adults and metformin users should test, not assume.
Deficiency unfolds slowly then irreversibly: fatigue and macrocytic anemia first, then paresthesia, gait disturbance, cognitive fog and, if prolonged, permanent subacute combined degeneration of the spinal cord. Folate can mask the anemia while neurology worsens — another reason to test B12, MMA and homocysteine rather than treating numbers alone.
Supplement strategy by cause: food-bound malabsorption responds to high oral doses (500–1,000 mcg, ~1% passive absorption bypasses intrinsic factor); pernicious anemia needs injections or high-dose sublingual; vegans do well on 25–100 mcg daily or 2,500 mcg weekly cyanocobalamin. No UL exists — excess excretes — but potassium monitoring matters when treating severe deficiency.
Pregnancy, infancy and vegan children are the vigilance zones: maternal deficiency transmits through breast milk with limited infant stores, risking developmental harm. Vegan pregnancies need verified B12 status plus reliable daily fortified intake — this is one nutrient where ideology must yield to biochemistry.
Check vitamin B12 intake vs RDA — see mcg/day, % of RDA and food sources. Formula: RDA: 2.4 mcg 19+ (2.6 pregnancy, 2.8 lactation). Example: With 5 mcg intake: 208% of the 2.4 mcg RDA — adequate with margin.
Vitamin B12 RDA is 2.4 mcg daily for adults 19+ (2.6 mcg pregnancy, 2.8 mcg lactation) — a tiny amount with outsized jobs in DNA synthesis, red-cell formation and myelin maintenance. This calculator checks estimated intake as percent of RDA; typical omnivore diets deliver 5–7 mcg, so adequacy is the default outside specific risk groups.
Animal foods are essentially the only reliable sources: clams (~84 mcg per 3 oz), liver, fish, meat, dairy and eggs. Plants contain effectively zero B12 — nori, mushrooms and fermented foods carry analogues that don't count. Vegans must use fortified foods (nutritional yeast, plant milks) or supplements; 'natural' plant diets without them reliably produce deficiency within 2–5 years as liver stores deplete.
Absorption is the bottleneck, not intake: stomach acid and intrinsic factor must cleave B12 from food protein, then ileal receptors absorb it — a chain broken by age over 50 (atrophic gastritis affects 10–30%), PPIs and metformin, bariatric surgery, celiac/Crohn's and pernicious anemia (autoimmune intrinsic-factor loss). Older adults and metformin users should test, not assume.
Deficiency unfolds slowly then irreversibly: fatigue and macrocytic anemia first, then paresthesia, gait disturbance, cognitive fog and, if prolonged, permanent subacute combined degeneration of the spinal cord. Folate can mask the anemia while neurology worsens — another reason to test B12, MMA and homocysteine rather than treating numbers alone.
Supplement strategy by cause: food-bound malabsorption responds to high oral doses (500–1,000 mcg, ~1% passive absorption bypasses intrinsic factor); pernicious anemia needs injections or high-dose sublingual; vegans do well on 25–100 mcg daily or 2,500 mcg weekly cyanocobalamin. No UL exists — excess excretes — but potassium monitoring matters when treating severe deficiency.
Pregnancy, infancy and vegan children are the vigilance zones: maternal deficiency transmits through breast milk with limited infant stores, risking developmental harm. Vegan pregnancies need verified B12 status plus reliable daily fortified intake — this is one nutrient where ideology must yield to biochemistry.
With 5 mcg intake: 208% of the 2.4 mcg RDA — adequate with margin. A vegan with zero fortified intake sits near 0% regardless of calories — the gap supplements must fill, since plants contribute effectively nothing.
Last reviewed: September 2026 · Report an error