NO, a mg. is not equal to a ml.
BUT, a cc is equal to a ml.
1 teaspoon is = 5 cc (or 5 ml).
REMEMBER, syringes are cc or ml (same thing), they are NOT mg.
If your medicine is 1/8 to 1/4 teaspoon to equal 400 to 800 mg, then that would be:
.625 cc / ml for 400 mg. (1/8 tsp)...