How do I convert mg/l to lbs /day

You don't. Milligrams per liter is a measure of density; pounds per day is not.

Ebaines is correct. Now, you can have 500 mg in 1000 ml, however, you still have the same liquid amount... 1000 ml.

Then you are going to have to convert pounds to kilograms in most cases.

My apologies to ebaines. I didn't read very carefully. ebaines is correct.

If you knew liters/day, you could calculate it.