The folks at McCormick & Company say for every tablespoon of dark rum called for in a recipe, you should use 2 tablespoons of rum extract. For every 5 tablespoons of light rum, use 1 tablespoon of rum extract.

So if the recipe calls for 1/2-cup of light rum, you would use 1 tablespoon and 2 teaspoons rum extract for the equivalent flavor (and depending on the recipe, you would probably need to add 3/8-cup of water to keep the ratio of liquid ingredients right).

If the recipe calls for 1/2-cup of dark rum, it probably makes sense to go out and buy a bottle of rum, since a cup of rum extract will cost a fortune and the extra half-cup of liquid would sink many recipes.