There are two ways to calculate your per diem allowance- standard daily rate and city by city. Each has its advantages. Domestically, either method might yield the best result however if you fly a lot of international pairings, city by city is clearly the best option.
The Watson CPA Group has developed this free per diem calculator. While this is generally used for taxpayers in the aviation or airline industry, all travelers might find it beneficial. If you need per diem rates including lodging, we suggest reviewing our Per Diem Methods and Rates page.