Candlepower

Quick answer: They don't. These are two completely different measurements.

Do a search for more info.

Lumens is a measure of the total light output from the source, but it doesn't tell you how much of that light is aimed in any one particular direction. The configuration and position of the LED, the type of reflector or optics, and other factors can influence the spread or concentration and therefore the relative brightness of the beam.

Candlepower measures the amount of light hitting one particular spot, but it doesn't tell you how much total light was produced and available or how much of that total might have been 'wasted' away by an inefficient reflector or optic.

Measuring lumens takes fairly expensive equipment. Candlepower can be measured cheaply. But neither gives the whole story, and both can be deceiving. At best, these measurements are somewhat helpful in comparing lights, but it could be a mistake to assume that the larger number is always 'better'. Often it's a matter of comparing apples to oranges, as in the case of a maker who might simply use an LED manufacturer's data sheet without regard to optics as opposed to another who states 'out the front' lumens. Another point is that higher lumens are generally produced at the expense of runtime, and that is not always a wise trade-off.

There are an awful lot of really nice lights out there that don't have the most impressive numbers. I've reached the point of almost ignoring these quantitative comparisons, choosing instead to focus on the quality and features of the light, the quality of the beam, and its adequacy for the intended use.

In any case, browsing around the CPF and comparing all the lights is fun and addicting.

Have you read through the Welcome Mat?

Yours is a general question which is not specific to LED Flashlights, so I'm moving it.