How to Estimate your Vitamin D Level

You can estimate your vitamin D levelIf you haven’t taken a vitamin D blood test, and don’t yet take a vitamin D supplement, you might wish to estimate your vitamin D level.

Even if you do take a vitamin D supplement, it may be useful to work out what your vitamin D level would be without supplementation, as this will help you decide how much to supplement at different seasons of the year.

It’s not as hard as you think, and you won’t need to use differential calculus!

The easiest way is to assess your vitamin D status at the end of summer and then work forward from there. You’ll see what I mean in a moment. First let’s look at this table. Your own vitamin D status will be one of those shown below.

Vitamin D Status25(OH)D Blood Level
Severely Deficient0-10
High but not Toxic66-100164-250
Toxicity PossibleAbove 100Above 250

O.K. but which one?

Vitamin D Status at end of Summer

That depends mainly on how much sun exposure you got during the previous three months or so.

We start by estimating your vitamin D status at the end of summer because that’s when you will most likely have had the most sun exposure, and your vitamin D level will be at its highest. So here is the method…

During the last 3 months of summer

  • If you exposed at least 75% of your body  (wearing bathing costume) to direct sunlight, for 15 minutes or more, without sunscreen, almost every day –  for example as a lifeguard, you might reach the High-but-not-toxic range.
  • If you exposed 55% of your body(for example, stripped to waist) to direct sunlight, for 10 minutes or more, without sunscreen, almost every day, you would likely reach the Optimum range.
  • If you exposed 35% of your body (for example, wearing T-shirt and shorts) to strong sunlight for ten minutes or more, without sunscreen, almost every day, you could reach the Adequate range.
  • How to Estimate Your Vitamin D LevelIf you tend to avoid the sun, usually stay indoors at lunchtime, put on sunscreen when you go out at weekends, but eat a good diet and take a daily multivitamin supplement containing 400 IU of vitamin D, you would probably reach the Insufficient range.
  • If you carefully avoid the sun whenever you can, and do not take even 400 IU per day of vitamin D in a multivitamin or some other supplement, you might reach the Deficient range.

Perhaps you can see yourself in one of these descriptions. If not, choose the one that most closely describes your own summer-time habits.

These end-of-summer vitamin D levels are for people who are young and trim. If you are over 60 years of age, you will most likely only reach one band lower than the expected one. If you are substantially overweight, you may also be one band lower than expected. (If you are over 60 and overweight, then you would expect to be two bands lower.)

That is because overweight people need more vitamin D to raise their blood level, and older people are less efficient at making vitamin D from sunlight.

Vitamin D Levels decline during the cooler months

From the end of summer onwards, you can expect your vitamin D blood level to decline by half every 2 to 3 months. The decline will continue (unless you supplement with an effective dose of vitamin D) until you are able to obtain strong, direct sunlight again.

If you supplement vitamin D 400 IU daily, you should be able to avoid the Severely Deficient band even without any sun exposure.

If  your skin type is light Caucasian, you can use the method without adjustment. For other skin types, all you need to do is to multiply the given sun exposure times (by 2 for Mediterranean, by 3 for Middle Eastern, by 4 for Asian and by 6 for African skin types). Darker skins need more time to make vitamin D.

The method yields a quick estimate of your vitamin D status, assuming you do not already supplement vitamin D. It’s not a blood test. But it is a good starting point. It may also help you to realize why most people are vitamin D deficient for part of the year, and others are vitamin D deficient all year.

You can also see why it is so hard for most people to optimize their vitamin D level by sun exposure alone. Most of us just don’t get nearly enough sun.

Hopefully you can also work out the times of year when YOU are likely to be vitamin D deficient. This can help you decide on a vitamin D supplementation strategy.

Free downloadable vitamin D calculator

We have developed another tool for estimating your vitamin D level at any time of the year, which is more precise. It’s called D-Estimator.

How to estimate your vitamin D level - without D-Estimator

Estimating vitamin D levels without D-Estimator

It takes into account your age, body mass, sun exposure, skin type and geographic latitude, as well as how much vitamin D you take in from food and supplements.

D-Estimator is a useful educational tool. You can use it to predict your own vitamin D blood level, and to calculate a vitamin D dosage tailored to your body type and circumstances.

But because everyone’s vitamin D metabolism is different, your actual blood level may vary from the predicted value. So it does not replace a blood test!

D-Estimator is a small (45kb) protected Excel file which downloads in seconds. It’s also free, with my compliments.

Download D-Estimator

After downloading, save it to your hard drive so you can use it whenever you wish. To use it, just click on the file. It will open in Excel. (You may need to enable editing before you can input your data.) You will also need to know your geographic latitude (see Infoplease) and body mass.

If you use D-Estimator and also take a 25(OH)D blood test, feel free to contact me (Alex St Clair) and let me know how D-Estimator’s prediction compared with your blood test. Several people have already done this, and D-Estimator worked pretty well for them.