Not a test at all - but an EZ way to measure your existing light without leaving the bathroom!! It will not be exact, but likely is "close enough"!
TP sheets are generally 4" - 4.5" wide, and lights vary in size by about 4". So take a length of Toilet paper and hold up to your light - if a little less than 5 sheets wide you have an 18"- 20" light. The common 4 bulb light is 24" and around 6 sheets wide.
5 sheets = 20" 3 bulb
6 sheets = 24" 4 bulb
7 sheets = 30" 5 bulb
9 sheets = 36" 6 bulb
Again, if you have an odd fixture that does not look like one of these here, send in a pic to us and the approximate measure! We have probably seen it all before. Be sure to put your pants on before taking that photo though! 🤭
