Size of the Bulb
A. Measure the bulb at its widest point and calculate the diameter to the nearest eighth of an inch (like the picture to the left shows).
B. Convert the measurement to a fraction in eighths (i.e. 17/8, 25/8, 56/8)
C. Use the numerator (the top number of the fraction) as the Bulb designation number.
Add this number with the shape letter(s) from step one and you have the Bulb designation code. You should come up with a number like R-40, PAR 30, or T-8.
If you would like to narrow your search further or if you are having trouble finding your Bulb designation code, go to step 3 to identify your Base type.
A. First measure the bulb at its widest diameter. The bulb equals 2 and 1/8 inches.
B. Then convert it to a fraction in eighths. This equals 17/8.
C. The numerator of the fraction is 17. The Bulb designation number is the same.
From Step 1, we can see that this bulb falls in the A shape category. So, our bulb's designation is A-17.