How to estimate motor current under load
It's not unusual to find that under near-full-power-out conditions the motor will be about 50% efficient. This is a big motor but it still applies: -



Next, you estimate how much mechanical power you need (2 pi n T) where n is revs per second and T is torque in newton.metres. If your dc motor's data sheet doesn't provide this information (like the graph above) then get one that does.

If you don't know how to calculate your mechanical power requirements you need to learn and start with: -

Force = mass x acceleration courtesy of Isaac Newton

Mass is what you want to move and acceleration is final speed divided by time to get to that final speed (i.e. what you want it to do).

Force x distance = work done and work done per second = power.

Don't forget friction as a loss and if it moves fast then there will be other losses due to moving air around.