The most important place to start to determine how much D you need is to have your vitamin D storage levels checked.
This test will tell you if you are sufficient, insufficient, deficient, or in the toxic level; but these numbers are a greatly debated subject among various D researchers and doctor's in general.
Finding out where you stand with your current levels will give you a good idea on how much to take, since there are various recommendations for the amount each of us need.
If your insurance doesn't cover this test, it is a relatively inexpensive test to have, and many places are offering in-home "blood spot" testing that is sent to the lab through the mail.
Vitamin D3 is the sunshine vitamin, which you get from sun exposure during peak hours between 11:00 a.m. and 1:00 p.m. without the use of sun screen.
The only problem with this is finding out if where you live is too far north to get adequate D synthesis from the sun.
Other variables that affect D absorption through the skin are age, skin color, or mineral metabolism problems, as well as location and climate conditions.
To find out how much vitamin D3 you can get from the sun, where you live, here's a calculator from the (Norwegian Institute for Air Research ©2005) that will figure it for you after you provide some location information. For help filling out the calculator go to this help link by Monica Reinagel, M.S., LD/N at Nutrition Data.com (Aug., 2009).
When you first look at the calculator it may seem daunting, but it's really not that hard to look up the information and insert it into the calculator.
For help filling in the fields, refer to the calculator and help links above, or use your GPS for the information.
If you don't live in any of the listed cities on the Cities field, of the calculator, then just click the radio button on the longitude and latitude line and input your information there instead. After looking up all my information and inputting it into the calculator, I found out that I need to expose my arms, legs and face every other day (around noon) for 28 minutes to get 25 micrograms of D3 for today's date. If I put in a date in August the results show to expose face, arms and legs every other day for 7 minutes to receive 25 mcgs of D3 equivalent to 1,000 IUs.
See what you get for your locations and let us know what it is. Some areas will show that there are insufficient UV rays to get the recommended amount depending on what time of year you calculate, so you may have to rely on supplemental vitamin D.
Courtesy of Norwegian Institute for Air Research
Remember if you live in the Northern areas of the US, then you may have to take vitamin D to get the recommended amounts in winter and possibly other seasons. If you don't absorb D normally due to age or skin type you'll have to factor that in as well.