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Vitamin D calculator, with or without a blood value

Pick how you start. The calculator works out the loading dose, the days it takes and the maintenance dose, from published formulas only.

A calculation aid for practitioners. It does not replace your own assessment and is not a prescription.

Formulas and sources checked on 15 October 2026.

From 15 ng/ml to 30 ng/mlGap 15 ng/ml = 37.4 nmol/l
Bands as the US Institute of Medicine (2011) describes vitamin D status: limits at 12 ng/ml · 20 ng/ml · 50 ng/ml.
Loading, total104,832 IU2,621 µg · 40 × 37.4 × 70 kg
Days 1 to 273,900 IU/day98 µg · at most 4,000 IU, the EFSA upper level
From day 281,500-2,000 IU/day38-50 µg · maintenance

The daily dose stays at or below 4,000 IU (100 µg), the EFSA upper level for adults. Higher loading doses are the practitioner's decision and need blood checks (BfR). Patients with obesity may need 2 to 3 times the maintenance dose; check the level (Endocrine Society).

With lab value · 70 kg · 15 ng/ml → 30 ng/ml3,900 IU × 27 days, then 1,500-2,000 IU

How it calculates

Formulas and sources

  1. Loading dose: 40 × (target − current level, in nmol/l) × body weight in kg. van Groningen et al. 2010 · Giustina et al. 2024
  2. Days: Loading dose ÷ 4,000 IU, rounded up. The daily dose is the total spread evenly over those days, rounded to 100 IU. EFSA 2023
  3. Maintenance: 1,500 to 2,000 IU a day for adults, the same for every body weight. Endocrine Society 2011
  4. Upper level: 100 µg (4,000 IU) a day for adults. Higher loading doses are the practitioner's decision and need blood checks. EFSA 2023 · BfR 2025
  5. No blood value: 16 ng/ml (40 nmol/l), the adult winter average from the Robert Koch Institute's DEGS1 study, converted to today's lab standard. RKI, DEGS1 · RKI, standardisation
  6. Status bands: Below 12, 12 to 20, 20 to 50 and above 50 ng/ml. IOM 2011
  7. Units: 1 ng/ml = 2.496 nmol/l. 1 µg = 40 IU.

Good to know

Questions about vitamin D dosing

Short answers to what people ask most.

Can you calculate vitamin D without a blood value?

Only as an estimate. The calculator then assumes 16 ng/ml (40 nmol/l), the average the Robert Koch Institute measured in adults in winter, converted to today's lab standard. Every number is then marked as an estimate. A lab value makes it exact.

How much vitamin D per day?

Once the target is reached, the Endocrine Society names 1,500 to 2,000 IU a day for adults. The estimate of the German Nutrition Society (DGE) is at least 800 IU (20 µg) a day when the skin makes none. EFSA sets the upper level at 4,000 IU (100 µg) a day.

How long does it take to fill up vitamin D?

With the default target of 30 ng/ml and at most 4,000 IU a day, 1 to 7 weeks for most adults, depending on body weight and the gap. The calculator shows the days for your values.

Why is the result lower than other vitamin D calculators?

Many calculators use a rule of 140 IU per kg body weight and ng/ml. We found no published study behind it. This calculator uses the formula van Groningen and colleagues tested on 208 adults in 2010, which comes to about 100 IU per kg and ng/ml. The 2024 consensus in Endocrine Reviews uses the same formula.

How do I convert IU to µg?

Divide by 40. 1,000 IU is 25 µg, 4,000 IU is 100 µg.

ng/ml or nmol/l?

Labs print either. 1 ng/ml is about 2.5 nmol/l (exactly 2.496). The calculator converts for you.

Sources

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