To utilise the vast recipe databases on the Internet in intelligent nutritional assistance or recommender systems, it is important to have accurate nutritional data for recipes. Unfortunately, most online recipes have no such data available or have data of suspect quality. In this paper we present a system that automatically calculates the nutritional value of recipes sourced from the Internet. This is a challenging problem for several reasons, including lack of formulaic structure in ingredient descriptions, ingredient synonymy, brand names, and unspecific quantities being assigned. We present a system that exploits linguistic properties of ingredient descriptions and nutritional knowledge modelled as rules to estimate the nutritional content of recipes. We evaluate the system on a large Internet sourced recipe database (23.5k recipes) and examine performance in terms of ability to recognise ingredients and error in nutritional values against values established by human experts. Our results show that our system can match all of the ingredients for 91% of recipes in the collection and generate nutritional values within a 10% error bound from human assessors for calorie, protein and carbohydrate values. We show that the error is less than that between multiple human assessors and also less than the error reported for different standard measures of estimating nutritional intake.