Most adults need 600 IU (15 mcg) of vitamin D per day, and adults over 70 need 800 IU (20 mcg). Those are the official recommended amounts from the National Institutes of Health. But the right dose for you depends on your weight, how much sun you get, your skin tone, and your current blood levels.
Recommended Daily Amounts by Age
The recommended dietary allowance covers what most healthy people need from food, supplements, and sunlight combined:
- Infants (0–12 months): 400 IU (10 mcg)
- Children and teens (1–18 years): 600 IU (15 mcg)
- Adults (19–70 years): 600 IU (15 mcg)
- Adults over 70: 800 IU (20 mcg)
- Pregnant or breastfeeding women: 600 IU (15 mcg)
These numbers are designed to get most people to a blood level of at least 20 ng/mL, which is the threshold many health authorities consider adequate for bone health. Some experts set the bar higher, at 30 ng/mL, which often requires more than 600 IU per day to reach.
Why Many People Take More Than the RDA
The 600 IU recommendation is a population-level guideline, and it assumes you’re also getting some vitamin D from sunlight and food. In practice, many people fall short. The percentage of adults with blood levels at or above 30 ng/mL dropped from roughly 60% in the early 1990s to about 30% by the mid-2000s among white Americans, and from about 10% to just 5% among Black Americans during the same period.
The Endocrine Society’s clinical guidelines take a more individualized approach, particularly for people at higher risk of deficiency. Body weight is one of the biggest factors. Research on healthy volunteers found that to reach the same blood level, overweight individuals need about 1.5 times more vitamin D than normal-weight people, and those with obesity need 2 to 3 times more. In concrete terms, hitting a blood level of about 40 ng/mL required roughly 2,000 IU per day for normal-weight adults, around 3,000 IU for overweight adults, and about 5,500 IU for those with obesity.
This is why you’ll see many doctors recommend 1,000 to 2,000 IU daily for the general adult population, and higher doses for people with obesity or confirmed deficiency. These amounts are well within the safe upper limit.
The Safe Upper Limit
The tolerable upper intake level, the maximum daily amount unlikely to cause harm, is 4,000 IU (100 mcg) for anyone age 9 and older, including pregnant and breastfeeding women. For younger children, the ceiling is lower: 1,000 IU for infants under 6 months, 1,500 IU for infants 7 to 12 months, 2,500 IU for children 1 to 3, and 3,000 IU for ages 4 to 8.
Taking more than 4,000 IU daily over a long period without medical supervision isn’t recommended. That said, toxicity is rare and typically occurs at blood levels above 150 ng/mL, which usually requires sustained intake far beyond the upper limit. The lowest blood level ever linked to toxicity in people with normal kidney function is around 80 ng/mL. Symptoms of toxicity come from too much calcium building up in the blood: nausea, vomiting, weakness, confusion, kidney stones, and in severe cases, heart rhythm problems.
Choose D3 Over D2
Vitamin D supplements come in two forms: D3 (cholecalciferol, from animal sources) and D2 (ergocalciferol, from plants and fungi). D3 is significantly more effective at raising blood levels. In one study, D3 raised blood levels roughly twice as much as D2 even when D2 was given at double the dose. Other research has found D2 to be about one-third as potent as D3. If you’re choosing a supplement, D3 is the better option for most people.
What Your Blood Levels Mean
The only way to know your actual vitamin D status is a blood test measuring 25-hydroxyvitamin D. Here’s how to read the results:
- Below 10 ng/mL: Severe deficiency
- Below 20 ng/mL: Deficiency
- Below 30 ng/mL: Insufficiency
- 30–80 ng/mL: Sufficient to optimal range
- Above 150 ng/mL: Toxicity range
If your level is below 20 ng/mL, your doctor will likely prescribe a higher corrective dose for several weeks before stepping down to a maintenance amount. Reaching a target of 30 ng/mL takes roughly two months or more on a standard dose of 800 IU per day. Higher doses, like 2,000 to 4,000 IU, can get you there faster, but even then expect at least a month before levels meaningfully shift.
Sunlight, Food, and Other Sources
Your skin produces vitamin D when exposed to UVB rays from the sun. At latitudes near the equator, as little as 3 to 15 minutes of midday sun on exposed skin can maintain adequate levels, depending on skin tone. People with darker skin need more time because melanin reduces UVB absorption. Above about 35 degrees latitude (roughly the line from Memphis to Los Angeles), UVB rays are too weak during winter months to produce meaningful vitamin D, a period researchers call “vitamin D winter.”
Food sources include fatty fish like salmon and mackerel, egg yolks, and fortified milk and cereals, but diet alone rarely provides enough. Most people in northern climates will need a supplement during fall and winter at minimum.
Nutrients That Affect How Vitamin D Works
Vitamin D is fat-soluble, so taking your supplement with a meal that contains some fat improves absorption. Beyond that, magnesium plays a critical role in how your body processes vitamin D. Magnesium acts as a cofactor in several steps: it helps vitamin D bind to its transport protein in the blood, it’s needed to convert vitamin D into its active form, and it influences how cells respond to vitamin D. If you’re low in magnesium, vitamin D supplementation may be less effective. This connection is especially relevant for women, people with obesity, and Black Americans, groups that tend to have both lower magnesium and lower vitamin D levels.
Vitamin K also works alongside vitamin D, particularly in directing calcium into bones rather than soft tissues like arteries. While research on supplementing vitamin K alongside vitamin D is still developing, some experts suggest that taking high-dose vitamin D without adequate vitamin K could contribute to inappropriate calcium deposits. Eating leafy greens, which are rich in vitamin K, is a reasonable strategy if you’re supplementing with vitamin D.
How to Decide Your Dose
If you’re a healthy-weight adult under 70 with moderate sun exposure, 600 to 1,000 IU daily is a reasonable starting point. If you get very little sun, live in a northern climate, have darker skin, or are overweight, 1,000 to 2,000 IU is more realistic. For people with obesity, 2,000 to 4,000 IU may be needed to reach sufficient blood levels. A blood test is the most reliable way to calibrate your dose, especially if you’re taking more than 2,000 IU per day or have risk factors for deficiency.