What = What  or put completely what does x light source equal in x light source.   Warning: You are about to enter into some math problems.

This is the most common question in the history of lighting.   Unfortunately it's not always an easy answer.  

Factors:

• Lumens per watt of LED
• Existing fixture light loss
• Reflectivity
• CRI value (Color Rendering Index)... like how many megapixels are in a photo.

The above factors can dramatically alter the conversions.  

Examples:

1. If you have an indirect T8 fixture and are converting to LED.   Indirect light fixtures can have light loss of greater than 50%.   So simply stating that you have a 400 watt lamp inside doesn't really tell the whole story of what = what.
2. A 1st generation LED was getting 50 lumens per watt... quite poor.   Most LEDs now are getting greater than 80 lm/w and some are now pushing 130 lm/w.   New generation LEDs can have a lower conversion such as 1 to 1/5
3. A typical 4 lamp T8 or T5 fixture relies on reflectivity to get 50% of it's light to the ground.   LEDs use direct light and do not usually need outside reflectivity to project light in the intended direction.   So in this case you can drop at least 1 lamp going from 4 to 3, or from 4 to 2.   Then you can make the conversion.

Conversions:
HID (Metal Halide or High Pressure Sodium) To LED
1 to 1/4-1/3

Halogen, Incandescent, Mercury Vapor to LED
1 to 1/6

HID to CFL
1 to 1/3 to 1/2

Incandescent to CFL
1 to 1/4

CFL to LED
1 to 1/2 to 2/3

T8 or T5 to LED
1 to 3/5