Vitamin D Dose Calculator
Estimate the daily vitamin D3 dose needed to raise your blood 25(OH)D level from where it is now to your target, using the ~100 IU per ng/mL rule of thumb.
How it works
Daily dose ≈ (target − current) × 100 IU · (≈100 IU/day lifts serum 25(OH)D by ~1 ng/mL over months)
Blood 25(OH)D responds to supplementation at roughly 1 ng/mL for every 100 IU/day of vitamin D3, reaching a new steady state after about two to three months. This rule of thumb estimates the top-up dose to move from your current level toward a target, usually 30–50 ng/mL. Response varies with body weight, baseline level and absorption, and the general tolerable upper limit for adults is 4,000 IU/day, so high estimates should be checked with a clinician and a follow-up blood test.
Worked example
Going from 20 ng/mL to 40 ng/mL is a 20 ng/mL gain. At about 100 IU per ng/mL that is 20 × 100 = 2,000 IU/day for two to three months — comfortably under the 4,000 IU upper limit — after which a smaller maintenance dose holds the level.
Frequently asked questions
How long until my level rises?
Blood 25(OH)D climbs gradually and reaches a new steady state after roughly 8–12 weeks of consistent daily dosing, which is why retesting sooner can be misleading.
What is a good target level?
Most labs consider 30–50 ng/mL (75–125 nmol/L) sufficient. Higher is not automatically better, and levels well above this range offer no proven extra benefit.
Should I take D2 or D3?
D3 (cholecalciferol) raises and sustains blood levels more effectively than D2 for most people, and is the form these estimates assume.
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Educational estimate, not medical advice. Talk to a doctor, pharmacist or registered dietitian before starting or changing any supplement, especially if you are pregnant, nursing, on medication or managing a health condition.