To calculate the minimum re-raise first calculate how much the initial raise was than the big blind. The minimum re-raise must be double the amount of the raise. For example, if initial player raised the minimum to 200 when the big blind was 100. The initial raise size is 100. The minimum re-raise amount must then be double the original raise size of 100, or 200. Therefore at big blind 100, when an initial raise was to 200, a minimum re-raise would be to 300.