How many lumens in a 40 watt hallogen?

That will vary on the manufacture and quality of the bulb. There is no set amount of lumens for a 40 watt halogen or any kind of bulb for that matter. To find out the lumen output you need to look at the box it came in as it should be stated. If you don't have the box it may be tricky to find out. You will have to see if you can find the manufacture somewhere on the bulb then google it and check the specs. It's probably safe to say its going to be around 500 lumens on average.