Significant digits

A good rule of thumb: never claim a level of precision you cannot measure.

  • I am really certain I can measure 1 kg. 
  • I am pretty certain I can measure 1.1 kg.
  • I am fairly certain I can measure 1.11 kg.
  • I am sure I can find a scale somewhere that can tell me 1.111 kg with some certainty after calibration.
  • I know of a couple of labs where I can get them to tell me if something has a mass of 1.1111 kg.
  • Those same labs can probably tell me 1.11111 kg.
  • I am not sure I would trust anyone telling me something is 1.1111111 kg unless they came with the receipts.
  • I am skeptical of anyone claiming to measure 1.111111111 kg.

All this to say, three significant figures is usually fine in most cases, though there might be a few situations where four or more are necessary. You’ll need to justify to yourself and to those you are giving the numbers all of those digits are needed. We want numbers we can trust, not merely more of them.