The density of MEKP is 1.0-1.15g/cm3 meaning that 1litre would weigh 1.0-1.15kg - as such, the volumes, especially in smaller quantities are so close to 1ml = 1g, that most people simply work on the basis of ml & g being equal.

In reality, working on this basis, you would be slightly under catalysing if the density is ever above 1.0. 5g at 1.15 for example is 4.34ml; but at least you would be under (slower) rather than over (faster).