We're not sure how to calculate it but we did some crappy math and it got us around 6.2 lumens per watt. Again, we're not sure if this is correct. You can use the following datasheet if you'd like to calculate it yourself.
http://www.adafruit.com/datasheets/FLR-100WAS-RGB.pdfOverall, it's decently bright but not as bright as a normal light bulb.