I was going to guess around 10 lumens or slightly higher, it is impressively bright with fresh batteries. I know it drops dramatically early on if running on max, from the runtime chart I've seen. Though I'm guessing that if you have it running at a more reasonable level of 4-6 lumens, it will be able to maintain close to that output for quite awhile, especially if you're just using the light for fairly short periods of time as most people probably do with this kind of light. And it has a great interface and can go extremely low. I've tried a CR2032 in one of mine, didn't work, haven't tried it in the other - it's apparently hit and miss with the CR2032 working. I passed on the light initially about 3 years ago when I saw it for about $16, thinking it was a bit much for a button cell light. Then I found it for under $10 and think it's an awesome light. The cheap knockoff 'fauxton' lights from DX are also a great bargain and work fine if you don't care about output adjustment or a waterproof case. I've been using a DX 'fauxton' with the Nichia GS & a single CR2032 on a keychain for over a year and even that puts out a decent amount of light with an insane runtime for a button cell.