Treating millilitres and grams as the same

A millilitre and a gram are the same thing.

A millilitre is a volume, a gram a mass. They are numerically equal only for water: 1 ml of water weighs about 1 g at 4 °C. A millilitre of cooking oil weighs about 0.92 g, a millilitre of honey about 1.4 g.

In the kitchen the two are used interchangeably for water, milk and stock, because their density sits close to 1 g/ml. For oil, honey, flour or sugar it deviates clearly, and the mix-up turns into wrong quantities.

Relate volume figures in a recipe to the ingredient, not to a weight. If you use a scale, multiply the volume by the density of the ingredient: 200 ml of oil is about 184 g, 200 ml of honey about 280 g.