For the mathematically minded chef, Michael Ruhlman’s Ratio app is a must-have tool in the kitchen. Ruhlman’s app, inspired by his book by the same name, breaks down dishes into simple formulas, rather than detailed recipes. Ratio is basically an ingredient calculator. For instance, if you only have one egg and your recipe calls for three, Ratio will do the ingredient math so you can still make your dish, just less of it. The app is available for iPad, iPhone, and Android for just $4.99.