Rather than using a formula, why not just use common sense?

Example: Assume you are using a 3000 mah battery and you only want to use 80% of its capacity. Then 80% X 3000 = 2400 mah would be the maximum you want to consume.
If I fly my airplane for 5 minutes (the way I normally fly it all the time) and it only consumes 1200 mah as noted on my charger when I recharge the battery, then it is safe to assume that you can fly the airplane for approximately 10 minutes at which time you would consume approximately the intended 2400 mah maximum. Hope this makes sense.

This estimate will most likely change as the batteries become old and loose some of its original capacity.

That much makes sense and can figure that out. I was wanting to know how to figure what size pack in mah to use for an Eflite 60. For example: To get 7-10 min. flying time would I use a 3000, 5000 etc. mah pack. This is what I'm trying to learn about.
I know the voltage, cells, watts etc that the motor uses but how about mah. for run time. How many mah will give me 8 min +/- a few of flying time. I dont want to by an 18v 2000mah pack and find out I can take off, do a loop and then have to land right away.
That wouldnt be any fun.

Formula for Run Time

run time in minutes = (Battery Capacity in Ah/Motor Current in A) x 60

This is just a rough guide, and it requires that you know approximately how much current your power system draws. A wattmeter will tell you. The formula also needs your battery capacity in Amp Hours (Ah). Divide mAh by 1000 to get Ah.

The simplest method is to measure your current at full throttle and use that number in the formula. In reality, you probably won't fly at full throttle all the time, so your average current usage probably will be less than the full-throttle value. Also, a power system will often draw less current in flight than it does during ground testing.

So if you use the full-throttle current in the formula, it will generally give you a worst case flight time. Actual flight time will usually be greater.

I've included some examples below.

- Jeff


Example 1:
1500 mAh battery
power system draws 12A

1500 mAh = 1.5 Ah

1.5/12 x 60 = 7.5 minutes


Example 2:
1500 mAh battery
power system draws 18A

1500 mAh = 1.5 Ah

1.5/18 x 60 = 5 minutes


Example 3:
4000 mAh battery
power system draws 25A

4000 mAh = 4.0 Ah

4.0/25 x 60 = 9.6 minutes


Example 4:
5000 mAh battery
power system draws 25A

5000 mAh = 5.0 Ah

5.0/25 x 60 = 12 minutes

Thanks Jeff. Thats what I was looking for.
Appreciate the help!