Vitamin D Loading Dose Calculator
Vitamin D Loading Dose Calculator
Work out a vitamin D loading schedule from a published regimen — the UK’s 300,000 IU total over six to ten weeks, or the Endocrine Society’s 50,000 IU weekly for eight — and see the dose per administration, the total and the daily equivalent side by side.
Vitamin D loading schedule
Regimen + weeks + frequencyThe Royal Osteoporosis Society’s loading regimen: a total of 300,000 IU, given once weekly over six weeks
One total, three ways of looking at it
Dose per administration = total loading dose ÷ (weeks × doses per week)
The total and the course length come from the published regimen; the frequency only changes how the same total is packaged.
- UK loading total
- “a loading regimen to provide a total of approximately 300,000 IU vitamin D, given either as separate weekly or daily doses over six to ten weeks” — Royal Osteoporosis Society (National Osteoporosis Society), Vitamin D and Bone Health, 2018
- US loading regimen
- “50,000 IU of vitamin D2 or vitamin D3 once a week for 8 wk”, or 6,000 IU daily, then maintenance of “1500–2000 IU/d” — Endocrine Society 2011. The totals in the selector are that rate multiplied by that duration, and are derived here rather than published there
- Maintenance
- “800–2,000 IU daily (occasionally up to a maximum of 4,000 IU daily)” in the UK guideline; “1500–2000 IU/d” in the Endocrine Society’s, rising to “3000–6000 IU/d” after the high-dose regimen. Loading without a maintenance plan simply recreates the deficiency
- The upper intake level
- 4,000 IU a day for ages 9 and over, 3,000 at 4 to 8 years and 2,500 at 1 to 3. It describes safe continuous intake, which is why a loading regimen is allowed to exceed it for a defined period and a maintenance dose is not
Worked example
The Royal Osteoporosis Society's loading regimen: a total of 300,000 IU, given once weekly over six weeks
Total loading dose from the selected regimen = 300,000 IU
Doses = 6 weeks × 1 per week = 6 doses
Dose per administration = 300,000 ÷ 6 = 50,000 IU weekly — which is the guideline's own first published schedule, reproduced exactly
Daily equivalent = 300,000 ÷ (6 × 7) = 300,000 ÷ 42 = 7,143 IU/day
That sits above the 4,000 IU/day tolerable upper intake level and below 10,000, which is where a loading regimen is supposed to sit — deliberately above what is safe as a continuous intake, for a defined six weeks
Now check the same arithmetic against another of the guideline's schedules: 280,000 IU over ten weeks given daily is 280,000 ÷ 70 = 4,000 IU a day, which is its "four 1,000 IU tablets daily" and its "five 800 IU capsules daily" — the same dose in two tablet sizes
Afterwards: step down to 800–2,000 IU a day, and check an albumin-adjusted calcium one month after the course finishes in case primary hyperparathyroidism has been unmasked
The published regimens, side by side
| Regimen | Loading | Total | Maintenance afterwards |
|---|---|---|---|
| UK — Royal Osteoporosis Society, 2018 | “approximately 300,000 IU … over six to ten weeks”. Published schedules: 50,000 IU weekly × 6; 40,000 IU weekly × 7; 4 × 1,000 IU daily × 10 weeks; 5 × 800 IU daily × 10 weeks | 300,000 or 280,000 IU | “800–2,000 IU daily (occasionally up to a maximum of 4,000 IU daily)” |
| US — Endocrine Society, 2011, adults | “50,000 IU of vitamin D2 or vitamin D3 once a week for 8 wk”, or “6000 IU … daily” | 400,000 IU (weekly route) or 336,000 IU (daily route) — derived, not published | “1500–2000 IU/d” |
| US — Endocrine Society, 2011, children 1–18 | “2000 IU/d … for at least 6 wk” or “50,000 IU of vitamin D2 once a week for at least 6 wk” | 84,000 or 300,000 IU — derived | “600-1000 IU/d” |
| US — Endocrine Society, 2011, obesity, malabsorption, enzyme-inducing drugs | “two to three times higher”, “at least 6000–10,000 IU/d” | 336,000–560,000 IU over 8 weeks — derived | “3000–6000 IU/d” |
Vitamin D2 and vitamin D3 are not equipotent
| Source | Finding |
|---|---|
| Tripkovic 2012, meta-analysis | Vitamin D3 raised 25(OH)D more than D2, weighted mean difference 15.23 nmol/L (95% CI 6.12–24.34; P = 0.001). The effect was significant with bolus dosing (P = 0.0002) and “the effect was lost with daily supplementation” |
| Balachandar 2021, meta-analysis | Mean difference 15.69 nmol/L (95% CI 9.46–21.93) favouring D3; D3 also regulated PTH better. Some studies show a fall in 25(OH)D3 after D2, suggesting degradation |
| NIH Office of Dietary Supplements | “Vitamin D3 increases serum 25(OH)D levels to a greater extent and maintains these higher levels longer than vitamin D2, even though both forms are well absorbed in the gut” |
| Royal Osteoporosis Society 2018 | “vitamin D3 is recommended as the vitamin D preparation of choice”; D2 for those who cannot take D3 “for cultural, dietary or religious reasons” |
| Endocrine Society 2011 | Recommendation 3.1 treats “either vitamin D2 or vitamin D3” as interchangeable for treatment and prevention |
A schedule, not a decision — and why this page does not ask for a 25(OH)D
This page deliberately does not take a 25-hydroxyvitamin D value, and the omission is the most important thing about it. The thresholds most people would enter one against — deficient below 20 ng/mL, insufficient 20 to 30, sufficient above 30 — come from the Endocrine Society’s 2011 guideline, and in 2024 the Endocrine Society withdrew them. Its words are unambiguous: it “no longer endorses specific 25(OH)D levels to define vitamin D sufficiency, insufficiency, and deficiency”, and it suggests against routine 25(OH)D testing in every population it considered. A calculator that opened with “enter the 25(OH)D” and returned a dose would be asserting a decision rule that the body which created it has retired. The site’s vitamin D status interpreter sets out the whole argument, including the Institute of Medicine’s genuinely different position, and it is the page to read before this one. What this page does is narrower and safer: given that a decision to treat has already been made, on whatever grounds, it works out the schedule.
The two national approaches differ in shape. The UK guideline — the National Osteoporosis Society’s, now the Royal Osteoporosis Society’s — fixes a total and lets you distribute it: “a loading regimen to provide a total of approximately 300,000 IU vitamin D, given either as separate weekly or daily doses over six to ten weeks”, with worked schedules of 50,000 IU weekly for six weeks, 40,000 IU weekly for seven, and four 1,000 IU tablets or five 800 IU capsules daily for ten weeks. The Endocrine Society’s 2011 guideline fixes a rate and a duration instead: “50,000 IU of vitamin D2 or vitamin D3 once a week for 8 wk” or 6,000 IU daily, with two to three times that — “at least 6000–10,000 IU/d” — in obesity, malabsorption syndromes and patients on anticonvulsants, glucocorticoids, antifungals or HIV medication. That structural difference is why the totals shown for the Endocrine Society regimens on this page are marked as derived: they are the published rate multiplied by the published duration, and the arithmetic is shown in the selector so it can be checked rather than trusted.
Both traditions agree on the shape of what follows, and it is the part most often dropped. Loading fills a deficit; it does not keep it filled. Maintenance is 800 to 2,000 IU a day in the UK guideline, occasionally up to 4,000, and 1,500 to 2,000 IU a day in the Endocrine Society’s, rising to 3,000 to 6,000 after the high-dose regimen. A loading course given without a maintenance plan simply recreates the deficiency over the following year. The band notes on this page are anchored on those numbers and on the tolerable upper intake level of 4,000 IU a day for anyone aged 9 or over — lower in young children, at 3,000 IU from 4 to 8 years and 2,500 IU from 1 to 3. Note what that upper level describes: safe continuous intake. A loading regimen is allowed to exceed it, on purpose, for a defined six to ten weeks, which is exactly why the 50,000-IU-weekly schedule works out at about 7,143 IU a day. A maintenance dose is not allowed to exceed it.
Vitamin D2 and vitamin D3 are not equipotent, and the guidelines do not agree about this. Two independent meta-analyses put vitamin D3 ahead by a weighted mean difference of 15.23 nmol/L and 15.69 nmol/L respectively in raising 25(OH)D — figures that agree to within 3%, which is the reason to believe them — and the NIH Office of Dietary Supplements states plainly that “Vitamin D3 increases serum 25(OH)D levels to a greater extent and maintains these higher levels longer than vitamin D2”. The UK guideline follows that: “vitamin D3 is recommended as the vitamin D preparation of choice”, with D2 for people who cannot take D3 for cultural, dietary or religious reasons, since D3 is usually animal-derived and some capsules contain gelatine. The Endocrine Society’s 2011 recommendation 3.1 treats the two as interchangeable, a position that predates both meta-analyses. One nuance matters for this page specifically: Tripkovic’s meta-analysis found the advantage of D3 with bolus dosing, where “the effect was lost with daily supplementation” — so the form matters most for exactly the weekly and intermittent schedules a loading regimen uses.
Finally, safety, and here the published instruction is more specific than the folk version. Two situations need thought. Granulomatous disease — sarcoidosis, tuberculosis, and the lymphomas — carries a real risk, because “patients with granulomatous disease are at risk of hypercalcaemia because of increased 1α-hydroxylase activity”: the diseased tissue converts the substrate you are giving into active hormone outside the kidney’s feedback control. The UK guideline asks for specialist advice before starting vitamin D in these patients, and the Endocrine Society asks for “serial monitoring of 25(OH)D levels and serum calcium levels during treatment with vitamin D to prevent hypercalcemia” wherever extrarenal production is suspected. Primary hyperparathyroidism is handled differently, and it is worth being precise because it is often misdescribed as a contraindication. The guideline’s instruction is to check the adjusted calcium one month after the loading regimen finishes, because vitamin D treatment “can unmask previously undiagnosed primary hyperparathyroidism” — the timing is the point. Routine 25(OH)D monitoring afterwards is “generally unnecessary” and is reserved for symptomatic deficiency, malabsorption or suspected poor compliance. If the calcium does come back high, the hypercalcaemia severity classifier grades it, and for the dose arithmetic between international units and micrograms the vitamin D IU to microgram converter is the tool.
Frequently asked questions
What is the loading dose of vitamin D for deficiency?
In the UK, the Royal Osteoporosis Society’s guideline specifies “a loading regimen to provide a total of approximately 300,000 IU vitamin D, given either as separate weekly or daily doses over six to ten weeks”, with worked schedules of 50,000 IU weekly for six weeks, 40,000 IU weekly for seven weeks, or four 1,000 IU tablets daily for ten weeks. In the US, the Endocrine Society’s 2011 guideline gives “50,000 IU of vitamin D2 or vitamin D3 once a week for 8 wk” or 6,000 IU daily. Both are followed by maintenance — 800 to 2,000 IU a day in the UK guideline and 1,500 to 2,000 IU a day in the Endocrine Society’s — and the maintenance step is the part most often omitted.
Should vitamin D be checked before or after a loading dose?
The guideline’s instruction is about calcium, not vitamin D, and the timing is after. “Adjusted plasma calcium is recommended to be checked one month after completing the loading regimen or after starting lower dose vitamin D supplementation in case primary hyperparathyroidism has been unmasked.” Routine repeat 25(OH)D measurement is “generally unnecessary” and is reserved for symptomatic deficiency, malabsorption, or where poor compliance is suspected. The exception that does need action beforehand is granulomatous disease — sarcoidosis, tuberculosis, lymphoma — where specialist advice should be sought before starting vitamin D at all, because increased 1α-hydroxylase activity in the diseased tissue risks hypercalcaemia.
Is vitamin D2 as good as vitamin D3?
No, and two meta-analyses agree closely on the size of the difference. Tripkovic and colleagues found vitamin D3 raised 25(OH)D more than D2 by a weighted mean difference of 15.23 nmol/L (95% CI 6.12–24.34), and Balachandar and colleagues found 15.69 nmol/L (95% CI 9.46–21.93). The NIH Office of Dietary Supplements states that “Vitamin D3 increases serum 25(OH)D levels to a greater extent and maintains these higher levels longer than vitamin D2”. The UK guideline recommends D3 as the preparation of choice, with D2 for people who cannot take D3 for cultural, dietary or religious reasons. The Endocrine Society’s 2011 guideline treats the two as equivalent, but that position predates both meta-analyses. The difference shows most with bolus or weekly dosing — with daily dosing Tripkovic found the effect was lost.
Can a vitamin D loading dose cause hypercalcaemia?
It can, and two groups are at particular risk. In granulomatous disease — sarcoidosis, tuberculosis and the lymphomas — the diseased tissue carries increased 1α-hydroxylase activity and converts the substrate into active hormone outside the kidney’s feedback control, so specialist advice should be sought before treating and calcium should be monitored during it. The other is unrecognised primary hyperparathyroidism, which vitamin D treatment “can unmask”; that is why the guideline asks for an albumin-adjusted calcium one month after the loading regimen finishes. If the calcium is high, the hypercalcaemia severity classifier grades it and sets out the PTH branch.
Why does this calculator not ask for a vitamin D level?
Because the thresholds it would have to be read against no longer stand. The 2011 Endocrine Society guideline created the familiar cut-offs — deficient below 20 ng/mL, insufficient 20 to 30, sufficient above 30 — and in 2024 the Endocrine Society withdrew them, stating that it “no longer endorses specific 25(OH)D levels to define vitamin D sufficiency, insufficiency, and deficiency” and suggesting against routine 25(OH)D testing in every population it considered. A dosing page that took a level and returned a decision would be reinstating a rule its author has retired. This page computes the schedule for a decision already taken; the vitamin D status interpreter covers the decision itself and why it is contested.
How much vitamin D is too much per day?
For continuous intake, the tolerable upper intake level is 4,000 IU a day for anyone aged 9 and over, 3,000 IU a day at 4 to 8 years and 2,500 IU a day at 1 to 3 years. A loading regimen deliberately exceeds that for a defined period — the UK’s 50,000 IU weekly for six weeks works out at about 7,143 IU a day — which is acceptable precisely because it is time-limited and is followed by a step down to maintenance. The highest continuing rate published in any of the regimens here is the Endocrine Society’s “at least 6000–10,000 IU/d” for obesity, malabsorption and enzyme-inducing drugs. A daily equivalent above 10,000 IU usually means a course length shorter than the regimen intended.
Related calculators
References
- Francis RM, et al. Vitamin D and Bone Health: A Practical Clinical Guideline for Patient Management. National Osteoporosis Society (now the Royal Osteoporosis Society); December 2018, version 2. “a loading regimen to provide a total of approximately 300,000 IU vitamin D, given either as separate weekly or daily doses over six to ten weeks”, with schedules of 50,000 IU weekly for six weeks, 40,000 IU weekly for seven weeks, four 1,000 IU tablets daily for ten weeks and five 800 IU capsules daily for ten weeks; maintenance “800–2,000 IU daily (occasionally up to a maximum of 4,000 IU daily), given either daily or intermittently”; thresholds “<25 nmol/L is deficient”, “25–50 nmol/L may be inadequate in some people”, “>50 nmol/L is sufficient for almost the whole population”; “patients with granulomatous disease are at risk of hypercalcaemia because of increased 1α-hydroxylase activity” with specialist advice sought before treatment; vitamin D treatment “can unmask previously undiagnosed primary hyperparathyroidism”, so “Adjusted plasma calcium is recommended to be checked one month after completing the loading regimen”; “Routine monitoring of plasma 25(OH)D is generally unnecessary”; and “vitamin D3 is recommended as the vitamin D preparation of choice”.
- Holick MF, Binkley NC, Bischoff-Ferrari HA, et al. Evaluation, treatment, and prevention of vitamin D deficiency: an Endocrine Society clinical practice guideline. J Clin Endocrinol Metab. 2011;96(7):1911–1930. Adults, “50,000 IU of vitamin D2 or vitamin D3 once a week for 8 wk” or “6000 IU of vitamin D2 or vitamin D3 daily”, maintenance “1500–2000 IU/d”; children 1–18, “2000 IU/d of vitamin D2 or vitamin D3 for at least 6 wk” or “50,000 IU of vitamin D2 once a week for at least 6 wk”, maintenance “600-1000 IU/d”; obesity, malabsorption syndromes and medications affecting vitamin D metabolism, “two to three times higher”, “at least 6000–10,000 IU/d” then “3000–6000 IU/d”; recommendation 3.1 treating “either vitamin D2 or vitamin D3” as equivalent; recommendation 3.6 asking for “serial monitoring of 25(OH)D levels and serum calcium levels during treatment with vitamin D to prevent hypercalcemia” where there is extrarenal production of 1,25(OH)2D.
- National Institutes of Health, Office of Dietary Supplements. Vitamin D — Fact Sheet for Health Professionals. Recommended dietary allowance “15 mcg (600 IU)” daily for ages 1–70 and “20 mcg (800 IU)” for over 70; tolerable upper intake level “100 mcg (4,000 IU)” daily for ages 9 and over, “75 mcg (3,000 IU)” at 4–8 years and “63 mcg (2,500 IU)” at 1–3 years; “Vitamin D3 increases serum 25(OH)D levels to a greater extent and maintains these higher levels longer than vitamin D2, even though both forms are well absorbed in the gut”.
- Tripkovic L, Lambert H, Hart K, et al. Comparison of vitamin D2 and vitamin D3 supplementation in raising serum 25-hydroxyvitamin D status: a systematic review and meta-analysis. Am J Clin Nutr. 2012;95(6):1357–1364. Weighted mean difference “15.23 (95% CI: 6.12, 24.34; P = 0.001)” favouring vitamin D3; a significant response for D3 given as a bolus dose (P = 0.0002) while “the effect was lost with daily supplementation”.
- Balachandar R, Pullakhandam R, Kulkarni B, Sachdev HS. Relative Efficacy of Vitamin D2 and Vitamin D3 in Improving Vitamin D Status: Systematic Review and Meta-Analysis. Nutrients. 2021;13(10):3328. “mean difference: 15.69, 95%CI: 9.46 to 21.93 nmol/L” favouring cholecalciferol, which was also more efficacious in regulating PTH.
- Demay MB, Pittas AG, Bikle DD, et al. Vitamin D for the prevention of disease: an Endocrine Society clinical practice guideline. J Clin Endocrinol Metab. 2024. The withdrawal of the 2011 numeric thresholds — the guideline “no longer endorses specific 25(OH)D levels to define vitamin D sufficiency, insufficiency, and deficiency” — and the recommendation against routine 25(OH)D testing. The reason this page takes no 25(OH)D value as an input.
Medical Disclaimer: The tools and content provided here are for educational and reference purposes only. They are not intended to substitute for professional medical advice, diagnosis, or treatment. Clinical decisions should always be based on the comprehensive assessment of a qualified healthcare professional.