How many lumens are in a fluorescent light bulb?

The amount of lumens that are emitted from a fluorescent lamp is given approximately by the luminance (cd/m^2) times the length (m) times the diameter (m) times pi squared. Unfortunately, we don't usually know the luminance.

The number of lumens varies greatly from bulb to bulb, across the various types of fluorescent technologies. One rule of thumb, that won't apply to every type of fluorescent lamp, is to multiply the rated wattage by 80 (lm/W) to get the number of lumens.

If so, a 40 watt fluorescent lamp will output about 3200 lumens, but, as you will find elsewhere, on Wikipedia:-

100 W tungsten incandescent (220 V): 1380 lm
100 W tungsten incandescent (120 V): 1750 lm.

So the above information does not seem quite right for a 40 watt fluorescent bulb.

Furthermore, for the analysis to be accurate, one has to realize that light can be either focused as in a laser or dispersive as in an incandescent bulb or fluorescent tube. Lumen ratings given with a square meter to distance ratio would help a lot but are not often given.

Technical information is sometimes printed on the boxes which fluorescent tubes come in from the factory. For instance, on one box of compacts fluorescents it states that a 100 watt compact fluorescent will put out 1700 lumens. Dispersive of course. For further information see the Related links shown below.