The short answer is, you can't. There is no universal method of measuring flashlight output, not even when the ratings are in the same units. A lumen rating is the total unfocused visible light output of either the bare bulb or the entire flashlight (depending on the manufacturer). A candlepower rating used by a flashlight manufacturer will usually be "peak beam" candlepower. This is a calculated value based on measuring the hot spot in the focused beam and multiplying by a number found by squaring the distance between the photocell used for the measurement and the flashlight. What this number gives is the number of "standard candles" that would need to be burning to produce the same amount of illumination as the hot spot in the beam at the same distance as the measurement is taken. Because peak beam candlepower is a measure of focused light and lumens are used to measure unfocused light, it is impossible to convert the two.