What is the formula for calculating how many amps my motor draws?

V=IR
Voltage=Amps x Resistance

Or if you have access to a clampmeter maybe that would work.

I do not know thew formula, but this can help with estimates.
http://scriptasylum.com/rc_speed/_power.html

I've never seen a formula that takes any of the real world factors that affect amp draw into account. Get an onboard data recorder like an eagle tree and measure the amp draw under running conditions if you have to have something very close. Or find someone who is running the same setup thats already done it and can tell you.

Yep, there's a lot more to it than just V/R. If you're looking to find out how much current your setup is drawing it's best to get the EagleTree datalogger.

actually there is a simple formula for finding out the amps your motor draws at a specific rpm, but the problem is that you need to know the inductance of your motor and that is hard/expensive to measure. if you dont feel like (re)learning college physics then it is simpler and probably cheaper to just by the eagle tree data logger, but if you are up to the task you should be able to find the information on wikipedia if you look into electric motors and inductance and current.

A motor under load draws more amps...

There is a formula to get a rough idea

Average amps used

Batttery amp hours /(run time in seconds /3600) Burst is usually 6 to 8 times the average amps.

Example my 3s pack is 1800mah, it last about 13 min.

Velineon 380 out of my MERV

1.8/(780/3600) = average amps

1.8/.2166666666666667 = 8.308

8.308 average amps used

amps times voltage = power

3s 11.1v x 8.08 = 89.688 watts

burst = average amps x 6

8.08 x 6 = 48.48 estimated burst and thats 538.112 watts or .72hp

if you look at the specs of the system it falls right in line

Velineon 380
max 50amp burst

Their is no real formula..as your voltage is always dropping and your resistance varies from turn to straight to jumps..Get a watt logger and run that then download and figure it out.. you might be vary surprised at how much LESS its pulling than you thought.. I E R is not really useful in racing..

hi i am planning to design a quadcopter for agricultural purpose like pesticide spraying,taking RF images etc could anyone help in selecting required motors,batteries and why they are selected( theoritical explaination )....by the way i am a beginner in this area please help me...

Unfortunately posting to a thread this old that is not really related to what you are doing is not going to yield many results. I happened to see your post, and would recommend that for this type of question you post to the forum below. this is the best forum going for the cars, but for the aircraft, and particularly a specialized application such as commercial use you will have much better luck there.

https://www.rcgroups.com/commercial-uav-fpv-981/