Have you been on a DAFNE course? The explaination below is an example of how the DAFNE course works.
I assume you mean "the sugar is 18.1g" Carbohydrate of which Sugar is 18.1g?
10g of carbohydrate or 1 CP (Carbohydrate Portion) raises the BG (Blood Glucose) by 2-3mmol/l
The 18.1g would be rounded up to 2CP which would raise your BG up by 4-6mmol/l, so if you were following the DAFNE programme you would inject 2 units of QA (Quick Acting) insulin to cover the Carbohydrates. You do need to test aswell.