How many 40 mg to ml?

Assuming you are talking about water, answer is 40. mL.

Here's why:

The specific gravity of water is 1. And 1 gal of water weighs 8.34 lbs.

Since density = SPG x 8.34lbs/gal then the density of water is 8.34 lbs/gal.

Since there are 3.785 liters in a gallon. And 454 grams in a lb, then:

8.34 lbs/gal x gal/3.785 L x 454 g/ lb = 1000 grams/liter

1000 grams/liter = 1Kg/1L = 1g/1mL

Now using ratiOS we can solve your problem:

? mL/40 grams = 1mL/1g

gives 40. mL