Vitamin D Calculator

Vitamin D deficiency (< 20 ng/mL) symptoms: fatigue and general weakness, bone pain or aches (especially back), frequent illness/infections, depression or mood changes, muscle weakness, hair loss, slow wound healing, and bone loss. Severe deficiency causes rickets in children (soft bones, bowed legs) and osteomalacia in adults. An estimated 1 billion people worldwide are deficient — it's one of the most common nutritional deficiencies. Many people are deficient with no symptoms, so testing is important if you have risk factors.

Sun exposure for vitamin D depends on skin tone, time of day, season, and latitude. Generally, 10–30 minutes of midday sun (10am–3pm) exposing arms, legs, and face 3–4 times per week is sufficient for light-skinned people in summer at mid-latitudes. Dark-skinned people need 5–10 times more sun exposure for the same vitamin D production. Sunscreen (SPF 30) reduces vitamin D synthesis by 95%. In winter above 37°N latitude, UVB rays are too weak for meaningful vitamin D production regardless of time spent outside.

Vitamin D toxicity (hypervitaminosis D) is rare but serious — it causes hypercalcemia (excess calcium) leading to nausea, weakness, confusion, kidney damage, and heart rhythm problems. The tolerable upper intake level (UL) is 4,000 IU/day for most adults per the IOM, though the Endocrine Society places it at 10,000 IU/day for short-term treatment under medical supervision. Toxicity almost never occurs from sun exposure (skin has a built-in protective mechanism) — it's always from excessive supplementation. Most studies show 2,000–4,000 IU/day is safe for long-term supplementation.