Vitamin D Calculator

Estimate vitamin D from sun exposure and get personalized supplement dosing based on your location, skin tone, and blood level.

Sun Exposure Inputs

Latitude (° N) 0° = equator, 90° = North Pole. Your city's lat.

Season

Skin Tone (Fitzpatrick)

Daily Sun Exposure (minutes) Arms + legs exposed (no sunscreen)

Body Surface Area Exposed (%) Face+arms = ~10%, arms+legs = ~25%, swimsuit = ~40%

Supplement & Blood Level

Current Blood Level (ng/mL) 25(OH)D test result. Optimal: 40–60 ng/mL

Current Supplement (IU/day)

Risk Factors

Your Vitamin D Estimate

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IU/day from sunlight

Blood Level Status —

Sun IU/day —

Supp IU/day —

Recommended Dose —

Total IU/day —

Time to Optimal —

Upper Safe Limit 4,000 IU/day

IU = base × lat_factor × season × skin × BSA × minutes Dose gap = (target − current) × 40 IU per 1 ng/mL

Personalized Recommendations

Latitude Reference

City	Latitude	Summer IU (20 min)	Winter IU (20 min)

D2 vs D3 — Supplement Comparison

Feature	Vitamin D2 (Ergocalciferol)	Vitamin D3 (Cholecalciferol)
Source	Plant / fungal (mushrooms, yeast)	Animal (lanolin, fish oil, lichen-based vegan)
Potency	Lower — shorter half-life	Higher — 87% more effective at raising 25(OH)D
Half-life	~15 days	~25 days
Prescription?	Yes (50,000 IU capsules)	OTC & prescription
Vegan option?	Yes (always plant-derived)	Lichen-based D3 is vegan
Cost	Slightly cheaper	Very affordable
Best for	Short-term Rx loading doses	Daily maintenance (preferred form)

Recommended Daily Dosage by Age & Weight

Group	Age / Weight	RDA (IU/day)	Endocrine Society (IU/day)	Upper Limit (IU/day)
Infants	0–12 months	400	400–1,000	1,000–1,500
Children	1–8 years	600	600–1,000	2,500–3,000
Teens	9–18 years	600	600–1,000	4,000
Adults	19–70 years	600	1,500–2,000	4,000
Elderly	71+ years	800	1,500–2,000	4,000
Pregnancy	All trimesters	600	1,500–2,000	4,000
Obesity	BMI ≥ 30	600	3,000–6,000	10,000

Top Food Sources of Vitamin D

1,360 IU

Cod liver oil (1 tbsp)

570 IU

Salmon, cooked (3 oz)

570 IU

Swordfish, cooked (3 oz)

366 IU

UV mushrooms (1/2 cup)

120 IU

Fortified milk (1 cup)

100 IU

Fortified OJ (1 cup)

80 IU

Fortified cereal (1 srv)

46 IU

Sardines, canned (2 pcs)

44 IU

Egg yolk (1 large)

40 IU

Tuna, canned (3 oz)

36 IU

Cheddar cheese (1 oz)

8 IU

Beef liver (3 oz)

Absorption & Timing Tips

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Take With Fat

Vitamin D is fat-soluble. Taking it with a meal containing fat (avocado, olive oil, nuts) boosts absorption by up to 50%.

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Morning or Lunch

Some evidence suggests vitamin D may interfere with melatonin production. Take it earlier in the day when possible.

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D3 Over D2

Choose cholecalciferol (D3) over ergocalciferol (D2). D3 is 87% more potent at raising and maintaining blood levels.

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Pair With K2

Vitamin K2 (MK-7) helps direct calcium to bones rather than arteries. Many experts recommend D3+K2 combo supplements.

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Magnesium Matters

Magnesium is a cofactor for vitamin D metabolism. Deficiency in magnesium can impair vitamin D activation in the body.

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Test Every 3 Months

When starting supplementation, retest 25(OH)D after 3 months to confirm your dose is working. Adjust as needed.

Projection Settings

Latitude (°)

Supplement Dose (IU/day)

Starting 25(OH)D (ng/mL)

Skin Type (Fitzpatrick)

Projected 25(OH)D Levels by Month

Optimal range (30–100 ng/mL) Projected level

Monthly Breakdown

Month	Sun IU/day	Supp IU/day	Est. 25(OH)D	Status

How to Use This Calculator

Enter your latitude — use the reference table or search your city. Latitude determines UV intensity.

Select season and skin tone — these dramatically affect vitamin D synthesis. Dark skin can need 5–10x more exposure.

Enter your blood test result — the 25(OH)D test is the standard measure. Aim for 40–60 ng/mL.

Review recommendations — the calculator suggests supplement doses and alerts for risk factors.

Use the Seasonal Projector — see how your levels change month-by-month based on latitude and supplements.

Frequently Asked Questions

How long to get vitamin D from the sun?

It varies by latitude, season, skin tone, and exposed area. In summer at mid-latitudes, fair-skinned people can produce 10,000–20,000 IU in 15–20 minutes in a swimsuit. Dark skin may need 5–10x longer.

Can you get vitamin D through glass?

No. Standard window glass blocks virtually all UVB radiation, which is required for vitamin D synthesis. You need direct outdoor sun exposure.

What is the optimal blood level?

Most experts recommend 40–60 ng/mL (100–150 nmol/L). The Endocrine Society defines deficiency as <20 ng/mL and insufficiency as 20–29 ng/mL. Levels above 100 ng/mL can cause toxicity.

How much does 1,000 IU raise blood levels?

On average, 1,000 IU/day of supplemental vitamin D3 raises 25(OH)D by about 10 ng/mL over 2–3 months, though individual responses vary based on baseline levels, obesity, and genetics.

D2 vs D3 — which is better?

Vitamin D3 (cholecalciferol) is more effective at raising blood levels than D2 (ergocalciferol). Most supplements and recommendations use D3. D2 is plant-derived and used in some vegan supplements.

Can you get too much vitamin D from the sun?

No — the skin has a self-regulating mechanism that degrades excess previtamin D when UVB exposure is prolonged. Toxicity only occurs from excessive supplementation (typically >10,000 IU/day for months).

Should I take vitamin D with food?

Yes. Vitamin D is fat-soluble, so absorption is significantly better when taken with a meal containing fat. Taking it with dinner or lunch improves bioavailability.

How often should I test my vitamin D?

If supplementing or at risk of deficiency, test every 3–6 months until optimal levels are reached, then annually. A simple 25-hydroxyvitamin D blood test is all that is needed.

Does sunscreen block vitamin D synthesis?

SPF 30 sunscreen theoretically blocks 97% of UVB, which would dramatically reduce vitamin D synthesis. However, in practice, people rarely apply enough sunscreen to cover all skin, so the real-world impact is less.

Why do I need more vitamin D in winter?

Above 35°N latitude, the sun's angle in winter is too low for UVB to reach the Earth's surface effectively. From October to March, virtually no vitamin D is produced through sun exposure in much of the northern US, Canada, and Europe.

Is vitamin D important for immunity?

Yes. Vitamin D receptors are found on immune cells. Deficiency is associated with higher susceptibility to respiratory infections. Multiple studies link adequate vitamin D levels with better immune function and reduced inflammation.

What is the RDA for vitamin D?

The US RDA is 600 IU/day for adults 1–70 and 800 IU/day for those 71+. Many researchers argue these are too low — the Endocrine Society recommends 1,500–2,000 IU/day to maintain levels above 30 ng/mL.

Do supplements work as well as sunlight?

For raising blood levels, vitamin D3 supplements are highly effective and predictable. Sun exposure has additional benefits (nitric oxide release, circadian rhythm regulation) but is harder to dose and carries skin cancer risk with prolonged exposure.

Why does obesity reduce vitamin D levels?

Vitamin D is fat-soluble and gets sequestered in adipose (fat) tissue, reducing its availability in the bloodstream. People with obesity typically need 2–3x more vitamin D to achieve the same blood level as normal-weight individuals.

Can I get enough vitamin D from diet alone?

It is very difficult. Few foods naturally contain significant vitamin D. Even eating fatty fish several times a week typically provides only 1,000–2,000 IU/day. Most people need sun exposure or supplements to reach optimal levels.