Battery life calculators are normally used to determine how long a specified battery will run a piece of equipment. If it is a disposable battery, then that is also the one time use life of the battery. If it is rechargeable, then it is obviously not, since it can be reused.
These calculators will only give at best an estimate. That may be sufficient for a preliminary value.
The best way is to run your equipment off a new, freshly charged battery with the cell chemistry of your choice, and see how it goes. If the load current is variable with use (for example, a Switch Mode Power Supply, or different modules in your circuit running with different duty cycles) you need to account for that as well. If the battery is rechargeable, you then have to factor in how the cell performs over time, that is, what cell capacity loss can you tolerate over the number of charge/discharge cycles before the minimum specified operating time is reached for your equipment. This can be dependent on the charger you use, the best choice is one designed to charge your cell as specified by the manufacturer.
Cheers,
Richard
