title_body: Determining the mAh of 4S battery for a 3 inch drone I have a 3" drone frame, flight controller that can take 3S or 4S LiPo, and 1507 4200KV motors. How do I find the right capacity of battery to purchase? Should I purchase 4S 850mAh or 4S 1500mAh? And why?
upvoted_answer: As @JacobB suggested in his comment, first check all the parts of your aircraft that connect directly to the battery can accept 3S or 4S voltages (usually the flight-controller and esc.) This may immediately constrain your choice.
  First you need to calculate the maximum weight your drone can lift; you can work backwards from this question:
  
  Get the thrust per motor for your drone
  Multiply this by the number of motors
  Divide by two
  Weigh the drone without the battery and subtract from number above
  
  This will give you a target maximum weight for your battery. You can then choose the highest capacity battery that falls in this range.
  More expensive batteries will give slightly higher mAh ratings at slightly lower weights - to work out what is good cost/flight time balance in your situation, you can estimate the flight time for a given mAh rating with the following formula:
  $$Endurance = \frac{ 0.8 * Ah }{ AverageAmpsDraw } * 60 $$
  [source]
  Note - if your battery is labelled in mAh, divide by 1000 for Ah