Originally Posted by
sobre
... I just checked it. My new bike is exactly 17,5 lbs.
If you used a luggage scale or a suspended scale, then your uncertainty is likely +/- 0.1 lbs.
If you measured using a bathroom scale (holding the bike, and then without the bike, subtracting), then your measurement uncertainty is +/- 0.5 lbs. That's for each measurement.
In this case (scale that uses internal rounding to the nearest half pound), each of your measurements can be anywhere between - .25 and + 0.24 pounds to register at the number shown on your scale. These two measurements are made independently of one another, so the combined uncertainty error (in subtraction or addition) grows as a root-sum-square.