How much does 125 ml of double cream weigh in grams?

To convert from mass to volume you need to know the density of the substance concerned.

Density is the weight per unit volume compared to pure water.

Single cream has an appx. density of 1.01 (slightly heavier per unit volume than water)

Double cream has an appx. density of 0.99 (slightly lighter per unit volume than water)

125ml of Water weighs 125g

125ml of Single cream 125 x1.01 = 126.25g

125ml of Double cream 125 x 0.99 = 123.75g

With such a small amount of cream treat mls as gms! Recipies are not to be followed that slavishly!