Vitamin D Calculator
Whether the NHS 10 µg (400 IU)10 µg (400 IU) daily guideline applies to you — by season and sun exposure, counting what you already take. Not medical advice.
Methodology
This is the NHS/SACN decision treeUK NHS/SACN decision tree (shown alongside US NIH reference values), applied honestly rather than a formula: adults should consider 10 µg (400 IU) of vitamin D a day from October to March, and all year round when sunlight exposure is limited — little time outdoors, skin usually covered, or darker skin tones, which synthesise less vitamin D from the same sunlight. Anything you already take (multivitamins, cod liver oil) counts toward the 10 µg, so the tool only tops you up. The adult safe upper limit is 100 µg (4,000 IU) a day; totals above it are flagged rather than personalised. Conversion: 1 µg = 40 IU.
Why 10 µg (400 IU) is the UK number
The SACN 2016 review set a Reference Nutrient Intake of 10 µg/day for everyone aged four and over, and NHS guidance turns that into practical advice: consider a daily 10 µg supplement from October to March, when UK sunlight is too weak for your skin to make vitamin D. It's deliberately a population-level figure — enough to protect bone and muscle health for nearly everyone without needing a blood test first.
Who should take it all year round?
If you're rarely outdoors in daylight (indoor work, night shifts, housebound), usually covered when outside, or have a darker skin tone — which synthesises less vitamin D from the same sun — the guidance applies in summer too. The calculator asks exactly those questions, keeps the answers on your device, and counts anything you already take (multivitamins, cod liver oil) so it only tops you up.
More is not better
Vitamin D is fat-soluble and accumulates. Above 100 µg (4,000 IU) a day, sustained intake can push calcium to harmful levels — so the tool flags totals over the limit instead of personalising them. If a clinician has prescribed you a high-dose course, follow them, not a calculator.
Sources
- NHS — Vitamin D: consider 10 µg/day October–March (year-round for at-risk groups); do not exceed 100 µg/day. link NIH Office of Dietary Supplements — Vitamin D fact sheet: adult RDA 600–800 IU/day, upper limit 4,000 IU/day. link
- SACN — Vitamin D and Health report (2016): RNI 10 µg/day for the UK population aged 4+. link
Frequently asked questions
How much vitamin D should I take? How much vitamin D should I take?
UK guidance (NHS, based on the SACN 2016 report) is 10 micrograms (400 IU) a day for adults through autumn and winter, and year-round if you get little sun, usually cover your skin outdoors, or have a darker skin tone. US guidance (NIH) sets an adult RDA of 600–800 IU (15–20 µg) a day. This tool shows the UK 10 µg (400 IU) supplement guideline; both sit well inside the 4,000 IU/day upper limit.
Do I need a supplement in summer? Do I need a supplement in summer?
Most people make enough vitamin D from everyday sunlight between about April and September. If you are rarely outdoors, usually covered, or have darker skin, the 10 µg/day guidance applies all year. Most people make enough vitamin D from everyday sunlight between about April and September. If you are rarely outdoors, usually covered, or have darker skin, the 10 µg/day guidance applies all year.
What is the difference between µg and IU? What is the difference between µg and IU?
Micrograms (µg) and international units (IU) measure the same thing at a fixed ratio: 1 µg = 40 IU. So 10 µg = 400 IU and the 100 µg adult upper limit = 4,000 IU. Micrograms (µg) and international units (IU) measure the same thing at a fixed ratio: 1 µg = 40 IU. So 10 µg = 400 IU and the 100 µg adult upper limit = 4,000 IU.
Can I take too much vitamin D? Can I take too much vitamin D?
Yes. Vitamin D builds up in the body, and more than 100 µg (4,000 IU) a day over time can raise calcium to harmful levels. Count everything — multivitamins, cod liver oil and fortified foods — toward your daily total. Yes. Vitamin D builds up in the body, and more than 100 µg (4,000 IU) a day over time can raise calcium to harmful levels. Count everything — multivitamins, cod liver oil and fortified foods — toward your daily total.
Does my multivitamin count? Does my multivitamin count?
Yes. Most multivitamins contain 5–10 µg of vitamin D. Enter what you already take and the calculator only tops you up to the guideline rather than doubling it. Yes. Most multivitamins contain 5–10 µg of vitamin D. Enter what you already take and the calculator only tops you up to the guideline rather than doubling it.
Should I get a blood test first? Should I get a blood test first?
You do not need a test to follow the general 10 µg/day guidance. If you have symptoms, a condition affecting absorption, or take interacting medication, ask your GP whether testing makes sense. You do not need a test to follow general supplement guidance. If you have symptoms, a condition affecting absorption, or take interacting medication, ask your physician whether testing makes sense.
Is this medical advice? Is this medical advice?
No. It is an educational summary of public-health guidance. If you have kidney disease, a history of high calcium, sarcoidosis, or have been given individual advice, follow your clinician. No. It is an educational summary of public-health guidance. If you have kidney disease, a history of high calcium, sarcoidosis, or have been given individual advice, follow your clinician.