The 40•30•30 iPhone App lets you quickly calculate the percentage of calories in a food from the three macronutrient groups: carbohydrates, protein, and fat.
Several popular diets recommend a 40%-30%-30% ratio of these macronutrients, but this can be a chore to figure out when reading a Nutrition Facts label. Using this app you can quickly enter the grams of carbohydrates, protein, and fat and instantly see the fraction of calories from each of these sources.
Of this app will work as well with any other balance of macronutrients, so you're by no means limited to a 40-30-30 program.
As always, consult your doctor before beginning any new diet.
Also please check out the full version of this app, 40•30•30+.