Vitamin D Dosage Calculator

The UK sits between 50°N and 60°N latitude. From October through March, the sun never climbs high enough for UVB rays to reach ground level. Your skin cannot produce vitamin D during those months, no matter how long you spend outside. The government's own advisory body, SACN, confirmed this in 2016 and recommended everyone in the UK take at least 400 IU (10 mcg) daily.

That 400 IU figure is a population-wide minimum — the dose needed to prevent outright deficiency in 97.5% of people. It is not an optimal target. The Endocrine Society's clinical practice guideline suggests 1,000–2,000 IU daily for adults, and many UK GPs now prescribe 800–4,000 IU depending on blood test results. The gap between "preventing rickets" and "reaching healthy blood levels" is large, which is exactly why a personalised estimate matters.

Around 1 in 5 adults in the UK has vitamin D levels below 25 nmol/L — the clinical threshold for deficiency. Among certain groups (people who cover most of their skin, those who rarely go outdoors, people with darker skin tones), the rate is much higher. This is not a niche problem. It is one of the most common nutrient gaps in Britain.

Children are particularly vulnerable to vitamin D deficiency. Their bones are still growing, and inadequate vitamin D during childhood can cause rickets — a condition where bones become soft and deformed. The UK saw a measurable increase in rickets diagnoses in the 2000s, which prompted clearer supplementation guidance from the NHS.

Babies from birth to 12 months. The NHS recommends all breastfed babies receive 8.5–10 mcg (340–400 IU) of vitamin D daily from birth, given as drops. Breast milk, even from a mother with good vitamin D levels, contains very little of the vitamin. Formula-fed babies who drink 500ml or more per day do not need a supplement, because UK infant formula is fortified by law. Babies on less than 500ml of formula, or those who are mix-fed, should still receive drops.

Children aged 1–4 years. SACN recommends 10 mcg (400 IU) per day year-round for all children in this age group. Heavier or darker-skinned toddlers may benefit from slightly more, but should stay well under the upper limit of 2,500 IU for this age range. Vitamin D drops or chewable gummies are the easiest delivery method.

Children aged 5–17 years. The same 400 IU minimum applies, but larger children — especially those approaching adult body weight — often need more. The calculator above adjusts for weight and skin tone using age-appropriate multipliers and enforces stricter upper limits than the adult version. The upper limit rises from 3,000 IU (ages 4–8) to 4,000 IU (ages 9–17).

Families on a low income can access free vitamin D drops through the NHS Healthy Start scheme, available for all children under 4. Some local authorities also provide free vitamins through children's centres and health visiting services.

The algorithm uses four inputs to produce a single IU estimate. Here is the logic behind each one.

Base dose (600 IU for ages 1+ / 400 IU for infants). For adults and children over 1, we start higher than the SACN minimum of 400 IU because most clinical research and the Endocrine Society recommend at least 600–1,000 IU as a sensible baseline. For infants under 12 months, the NHS-recommended 400 IU is used directly — there is strong consensus on this number and no reason to go higher without medical guidance.

Body weight. Vitamin D is fat-soluble. It gets stored in adipose tissue rather than circulating freely in the blood. Heavier individuals — particularly those with higher body fat — need proportionally more. The calculator uses age-appropriate IU-per-kilogram multipliers: 10 IU/kg for 6–12 month olds (conservative, reflecting limited clinical data in infants), scaling up to 15, 20, and 25 IU/kg for older children and adults. These sit within the 10–30 IU/kg range used in clinical dosing studies.

Skin tone. Melanin absorbs UVB radiation. This is useful protection against sunburn, but it means darker-skinned individuals synthesise less vitamin D per hour of sun exposure. The Fitzpatrick Scale provides a rough proxy: very light skin gets a x1.0 multiplier, while dark skin gets x2.0. This mirrors real-world findings that people of African and South Asian descent in the UK have substantially lower vitamin D levels on average.

Season and activity. Between May and September, someone who works or plays outdoors in the UK can synthesise meaningful amounts of vitamin D from sunlight, so the multiplier drops to ×0.5. An indoor worker or school-age child indoors gets ×0.8 — some benefit from incidental exposure, but limited. From October to April, UVB barely reaches the UK at this latitude. Those who still spend time outdoors get a slight edge (×1.3), but people who commute or attend school in the dark need the full ×1.5.

Age-appropriate clamping. For adults, the final number is clamped between 400 IU and 10,000 IU (the Endocrine Society's upper safety limit with medical supervision). For children, stricter upper limits apply: 1,000 IU for babies under 6 months, 1,500 IU for 6–12 months, 2,500 IU for ages 1–3, 3,000 IU for ages 4–8, and 4,000 IU for ages 9–17. These match the SACN and EFSA Tolerable Upper Intake Levels for each age group.

All results are rounded to the nearest 100 IU for practical dosing.

You can get some vitamin D from food, but not much. Oily fish (salmon, mackerel, sardines) is the best dietary source — a 100g serving of wild salmon provides roughly 600–1,000 IU. Farmed salmon typically delivers less, around 250 IU. Egg yolks contain about 40 IU each. Fortified foods (some cereals, orange juice, plant milks) add small amounts, usually 40–100 IU per serving.

For most people in the UK, food alone falls well short of even the 400 IU minimum. That is why SACN recommended universal supplementation — you cannot reliably eat your way to adequate vitamin D levels during the British winter. A supplement that provides 1,000–4,000 IU in a single capsule closes the gap in seconds, at a cost of 3-10p per day.

Looking for a specific product? Browse our vitamin D supplement reviews with real scores and price comparisons.

Vitamin D deficiency develops slowly, and symptoms are easy to dismiss or blame on something else. The most common complaints include persistent tiredness that sleep does not fix, aching bones and joints (especially in the lower back and legs), muscle weakness, and frequent colds or infections. Some people notice low mood during winter months — while this overlaps with seasonal affective disorder, low vitamin D is often a contributing factor.

In children, signs of deficiency include slow growth, delayed walking, irritability, and soft or bowed legs. Severe deficiency causes rickets. If you notice any of these in your child, ask your GP for a 25(OH)D blood test.

If any of this sounds familiar, ask your GP for a 25(OH)D blood test. It costs the NHS nothing and gives you a clear number to work with. A level below 25 nmol/L means deficiency. Between 25–50 nmol/L is insufficient. Most experts consider 75–125 nmol/L the sweet spot.