Is the level low?
The NIH ranges help answer this. Below 12 ng/mL is associated with deficiency. From 12 to below 20 ng/mL is generally inadequate. A level of 20 ng/mL or more is generally adequate for most people.
Vitamin D tool
What does your vitamin D result mean?
Enter the 25(OH)D number from your blood test. The calculator converts ng/mL and nmol/L and shows where the result sits in the NIH range. It does not choose a supplement dose.
Result calculator
Check the number on your lab report
Practical Tool
What does my number mean?
Enter the 25(OH)D result from your blood test. The calculator converts the units and shows where the result sits in the NIH range.
The ranges come from the NIH Office of Dietary Supplements. The 2024 Endocrine Society guideline explains why one blood number is not a universal treatment target.
How to read it
One number, three different questions
Did your clinician set a target?
Some clinicians use 40 to 60 ng/mL for a particular condition or treatment plan. Enter that target only if it was set for you. It is not a universal “optimal” range or a proven target for clearing brain fog.
Does the dose need changing?
The result alone cannot answer this. Treatment also depends on why the level is low, current intake, body weight, absorption, kidney and calcium problems, medicines, and the dose you're already taking.
Why treatment doses vary
A published formula is not a personal prescription
A 2010 study by van Groningen and colleagues tested weight-based loading doses in 208 adults with confirmed deficiency. It helps explain why a clinician may consider the starting level and body weight. The study doesn't tell this page enough about you to choose a dose.
For general use, the NIH adult upper limit is 4,000 IU a day. A clinician may use more for a diagnosed deficiency, with a clear reason and follow-up. The 2024 Endocrine Society guideline did not set one blood-level target for disease prevention in generally healthy adults.
Vitamin D, K2 and magnesium
Why people take them together
Vitamin K2
Vitamin K helps activate proteins used in blood clotting and bone health. This is why K2 is often sold with vitamin D, but it is not an automatic requirement. Vitamin K can interact seriously with warfarin and similar blood thinners.
Magnesium
Magnesium is used in vitamin D metabolism. That explains the connection, but taking vitamin D doesn't prove you're low in magnesium or need a magnesium supplement.
Food
Vitamin D is fat-soluble. Taking it with a meal that contains some fat can improve absorption. An ordinary meal is enough.
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