Going on vacation with a discount carrier like RyanAir? Then you better watch out because the penalties for excess luggage are harsh.

Expensive
The penalties for excess luggage for discount carriers like RyanAir, EasyJet and others can often be as much as £5/€8/$10 extra per kilo(!). 

It is worth it to pay some attention to the weight of your luggage before you leave home.

How to do it
Use your normal bathroom weight scale. Weigh yourself first, then pick up the bag while still on the scale, get the difference between the two figures. You know now how much your luggage is weighing.