If you really mean millimeters (which is a measurement) there is 1,000,000 millimeters in a liter. 437 millimeters subtracted from 1,000,000 is 999,563 millimeters left.
If you mean milliliters there is 1,000 milliliters in one liter. So 437 subtracted from 1,000 is 563 milliliters left.