Calculate your optimal
vitamin D3 dose.

A full tank takes you further. To therapeutically raise your vitamin D level, we have to distinguish two phases: the filling of the deficit (setup phase) and the daily maintenance (maintenance). Use this calculator for your precise treatment plan.

Dr. Ackermann · Vitamin D Calculator · Herzogenaurach D₃

1 · Your body data

Obesity (BMI above 30) doubles the requirement – a notice appears automatically.
For automatic obesity detection and dose adjustment.

2 · Your current level

ng/ml
No lab value? 15–20 ng/ml is typical in winter without sun.
Recommendation: 20–30 days. High daily doses (above 10,000 IU) trigger a warning.

3 · Your target value

DGE standard
20 ng/ml
Sufficient for bone health according to DACH reference values.
Clinical standard
30 ng/ml
Recommended for general health (Endocrine Society 2011).
Functional optimum
50 ng/ml
Higher range, discussed by parts of the literature.
Not confirmed by RCTs

Custom value possible: ng/ml — overrides the selection above.

⚠ Notice: high daily dose

The calculated setup dose is above 10,000 IU/day. Extend the filling span to 30–60 days to lower the daily dose. Doses above 10,000 IU/day should only be taken under medical supervision.

⚠ Obesity notice

Your BMI is above 30. Fat tissue stores vitamin D and reduces its biological availability. The Endocrine Society recommends 2–3 times the standard dose for obesity. The calculation accounts for this – have your level checked after 3 months.

PHASE 1
Setup phase
— IU / day

Daily for — days.

Total requirement: — IU
PHASE 2
Maintenance dose
— IU / day

Daily after the setup phase is complete, to maintain the level.

Note: lab check recommended after 3 months.

Have your level reliably measured – in Herzogenaurach

Online calculators are based on statistics. Your metabolism is individual. A real deficiency can only be confirmed unequivocally in a blood test.

Book a vitamin D check → Dr. med. Thomas Ackermann · Specialist in Anesthesiology · Herzogenaurach
Medical notice & disclaimer This calculator is for guidance information only and is not a medical device (EU MDR 2017/745). The results do not replace a medical diagnosis or consultation. Individual factors (illnesses, medications, absorption) can cause considerable deviations. Doses above 4,000 IU/day (EFSA tolerance limit) should only be taken with medical supervision. Dr. med. Thomas Ackermann accepts no liability for decisions made without medical consultation.

Vitamin D: more than a bone vitamin

Vitamin D is actually a prohormone that is active in almost every body tissue – its significance extends far beyond bone health.

About 60% of the German population does not reach the DGE minimum value of 20 ng/ml. In winter, the majority lies significantly below that. Vitamin D influences:

  • 01.
    Immune system: Activates and regulates innate and adaptive immune responses. Observational studies show an association with susceptibility to infection and autoimmune risk.
  • 02.
    Muscle strength & fall prevention: Vitamin D receptors in muscle cells are well documented. Supplementation demonstrably reduces fall risk in older adults – one of the few effects confirmed by RCTs.
  • 03.
    Mood & cognitive function: Observational studies show associations between low levels and depressive symptoms; RCTs have not yet shown a consistent picture.
  • 04.
    Skin & hair: Vitamin D receptors in hair follicles and keratinocytes. Low levels are associated with diffuse hair loss (telogen effluvium).

At the practice of Dr. Ackermann we view vitamin D as part of integrative basic diagnostics – not as a lifestyle supplement.

Which target value is the right one?

The guidelines differ considerably. Here an honest classification.

≥ 20 ng/ml DGE / DACH 2025
Considered sufficient for bone health. The DGE recommends 800 IU/day in the absence of endogenous synthesis. No screening of healthy individuals recommended.
≥ 30 ng/ml Endocrine Society 2011
Still common in clinical practice. The Endocrine Society 2024 withdrew this target value for healthy individuals – no serum-level targets are recommended anymore.
40–60 ng/ml Functional optimum
Advocated by Holick and others. Large RCTs (VITAL, n=25,871) could not show measurable advantages over 30 ng/ml.
Not supported by randomized trials
Dr. Ackermann's conclusion: For most patients, a level of 30–40 ng/ml is a pragmatically sensible target – above the confirmed minimum, below the scientifically unsupported maximum range. What matters is a measured baseline value, not the statistics of an online calculator.

The hours of sunshine matter

Germany lies between 47 and 55 degrees northern latitude. From October to March the UVB intensity is not sufficient for relevant vitamin D synthesis.

Endogenous production in Germany – what is realistic

Summer (May–September)10–30 minutes of midday sun on face, arms and legs are enough for 10,000–20,000 IU – if the UV index is ≥ 3 (approx. 11:00–15:00).
Winter (October–March)UV index permanently below 3 throughout Germany. No relevant vitamin D synthesis possible – regardless of sun exposure duration.
Sunscreen from SPF 15Reduces vitamin D synthesis by up to 95%. In everyday life, the actual impairment is smaller because application is often incomplete.
Melanin-rich skin typeDarker skin requires up to 6 times longer sun exposure for the same synthesis. Higher deficiency risk.

What to look for when choosing

Dietary supplements in Germany do not require authorization. The quality varies considerably.

Quality criteria for vitamin D3 preparations

Active ingredient: cholecalciferol (D3), not ergocalciferol (D2)D3 is biologically more active and raises serum levels more effectively. D2 has a shorter half-life.
Combination with vitamin K2 (MK-7)K2 regulates where calcium is deposited. Useful at higher D3 dosages. Prefer K2 as MK-7.
GMP certification & independent lab analysisGood Manufacturing Practice ensures manufacturing quality. Third-party lab analyses document purity and the actual active-ingredient content.
Lipid-based formulationVitamin D is fat-soluble – oil drops or soft capsules with a fatty carrier improve absorption considerably compared to dry tablets.
Transparency note: Dr. Ackermann does not recommend specific manufacturers or brands on this website. Naming product recommendations by physicians is prohibited under §27 (3) MBO-Ä professional regulations. The criteria above are product-neutral.

Two phases, one strategy

A vitamin D protocol has two clearly separated phases – those who do not distinguish between them fail at one or both.

Phase 1: fill the deficit

A level of 15 ng/ml cannot be raised to 30 ng/ml within a reasonable time by 1,000 IU/day. A targeted setup phase with higher daily doses is the scientifically based route.

Important: Take vitamin K2 at the same time. Ensure sufficient magnesium (cofactor for vitamin D metabolism). Check the level after 8–12 weeks.

Phase 2: maintain the level

After replenishment, a permanently sufficient maintenance dose is needed. Most people underestimate this – and fall below their target value again after a few months.

Typical: 1,000–2,000 IU/day for individuals of normal weight without regular sun exposure; up to 3,000–4,000 IU/day with obesity or a high target value.

Only in Herzogenaurach

Calculation is good. Knowing is certain.

Online calculators are based on statistics. Your metabolism is individual. A real deficiency can only be confirmed unequivocally in a blood test.

  • ✓ Low-pain blood draw directly at the practice
  • ✓ Result usually within 24 h by email
  • ✓ Medical treatment consultation in case of deficiency
The 24h check incl. lab & medical service
Duration: only 5 minutes Book an appointment *Billing according to GOÄ (e.g. codes 250, 3560).
Background & safety
For those interested in detail

This calculator uses a weight-adjusted loading formula based on van Groningen et al. (2010, European Journal of Endocrinology, PMID: 20139241). Formula: Deficit (IU) = (target − actual) × body weight × factor (~100).

The factor ~100 corresponds to the distribution volume empirically determined by van Groningen. The maintenance dose is based on a daily replacement rate of ~4.5% (derived from the serum half-life of 25-OH-D3 of ~15 days; Jones 2008, AJCN).

Limitations: individual absorption, liver function, vitamin D binding protein (DBP) genotype and magnesium status can considerably influence the response. A follow-up lab check after 8–12 weeks is essential.

The new Endocrine Society guideline (Demay et al., JCEM 2024) is a paradigm shift. It defines no serum-level targets anymore for healthy adults under 75 and recommends no routine screening.

Large studies like VITAL (25,871 participants, 2,000 IU/day, 5 years) could not show a reduction in cardiovascular events or cancer incidence. Exceptions: Children, pregnant women, adults ≥75 years, individuals with elevated prediabetes risk.

For practical medicine: existing deficiencies should still be corrected. The controversial question is whether screening of asymptomatic individuals is sensible.

EFSA/BfR tolerable limit: 4,000 IU/day for adults. Clinical toxicity (hypercalcemia) typically begins at serum levels above 150 ng/ml and usually requires sustained doses above 10,000 IU/day over months.

Symptoms: nausea, vomiting, weakness, frequent urination, kidney stones, cardiac arrhythmias.

Risk groups: Sarcoidosis, primary hyperparathyroidism, kidney insufficiency, lymphomas. Medical supervision is mandatory here.

Vitamin D receptors (VDR) are detected in keratinocytes and hair follicles. VDR knockout mice develop complete alopecia – which shows the biological relevance.

In humans, several cross-sectional studies show lower 25-OH-D levels in women with diffuse hair loss. A causal connection is plausible but not yet conclusively supported.

Practical note: In case of hair loss, also always investigate ferritin, thyroid values and hormone status in parallel – these are more often the actual cause.

With meals: Vitamin D is fat-soluble – always take it with a fat-rich meal. Intake on an empty stomach considerably reduces absorption.

Do not forget magnesium: Magnesium is a cofactor in converting vitamin D to its active form. With magnesium deficiency, vitamin D cannot fully work even with a sufficiently high serum level.

Combine with vitamin K2: At doses above 2,000 IU/day, K2 (MK-7) is useful to regulate calcium distribution.

When to measure: First follow-up value at the earliest 8 weeks after starting supplementation.

All information on this page serves general orientation. This calculator is not a medical device under EU MDR 2017/745. Health data is processed exclusively on the client side. If you suspect a vitamin D deficiency or before starting a high-dose supplementation, a medical examination is required. · Practice of Dr. med. Thomas Ackermann · Niederndorfer Hauptstraße 41 · 91074 Herzogenaurach · thomas-ackermann.com