formulas can be found on this web page:

(This web site also lists the above formula and uses a constant of 3.16 as shown above)

5) If you have more than one fan, they should be mounted side-by-side rather than inline if you want to add their different CFM ratings.

For inline fans, use the lowest airflow rating of all fans in the path. A fan on the inlet and a fan on the exhaust of the box are considered inline fans. Fans inside the box should not be counted for airflow but must be included in wattage. A standard computer fan is normally right around 30 CFM (50 CMH).

The two lookup charts solve this equation for common lights. Make sure you get the proper one (English or metric). For those of you who are wondering if you did this right, here are a few numbers in English units :


Note: a 30cfm computer fan is trying to cool a 1000w HID bulb, in the 3rd from the last row, as an extreme example

If you are adding any carbon scrubbers or extensive ductwork, this is where you add to the fan size to account for air pressure losses. You have to move this many CFM, or the numbers don't come out right. The deciding factor for these items depends on your exact configuration and is beyond this discussion.

6) When your box is built, buy a thermometer and measure the air blowing out of the box (temp probe or thermometer should be in the air stream just after the fan, outside of the box enclosure) and the temp of the air entering the box (again, from outside the box perimeter). Make sure there is no direct light shining on the thermometers to ruin the measurement. DON'T MEASURE THE TEMP INSIDE THE BOX YET!!!! It's best to do this with 2 thermometers or a single thermometer with a remote probe. Cheap thermometers don't work well because they aren't very accurate. If you only have cheap thermometers, use the same one for all measurements to avoid accuracy issues.

7) Subtract your measured inlet from measured outlet temp. Compare to Tdiff from above. Is your measured difference as good or better than your estimated from step 2? If not, go find out why. Your problems are probably:

A. Heat source you didn't account for (the ballast?)
B. Your fan is overrated
C. You have blocked airflow
D. Your temperature measurement was inaccurate
E. Air leaks into the box (especially around the fan!) that ruin efficiency.

8) Once you get your measured temp difference equal to step 2, measure temps inside the box. Don't let the light shine right on the sensor, it will give faulty readings!! Use a light shield made from a tin can or something. If temps inside the box are higher than your exhaust temp at a reasonable distance from the bulb, you have air circulation problems inside the box. Get some kind of fan to stir up the air in there or look for airflow paths that allow air to travel from inlet to exhaust without spending any time in the box.

9) Always monitor the temperature difference between inlet and outlet temps every time you water the plants. If it varies much more than a degree or two, find out why. I use digital indoor/outdoor thermometer. It tracks high and low for both locations, outdoor probe is on a long wire, $14 at Kmart. No part of the thermometer is inside the box, just in the measuring air blowing in and out from the outside.

Please note that conversion values have been slightly rounded off to make this easy. Using the metric and english formulas will yield slightly different answers if compared. The difference should be less than one percent and can be ignored.

You can use the two load graphs attached if you prefer to do calculations visually rather than using the formulas listed above. Find the line for your light wattage and ignore all others. Each axis is logarithmic, make sure you count along each axis properly. The formulas listed in step 4 were used to make the graphs.