Tolerated amount of weight exceeding upper limit for hold luggage

As stated in other answers, this really depends on the airline and, above all, on the particular agent that is checking in your luggage. While they usually give some leeway, exactly for practical reasons of scales not being properly calibrate and so on, in theory, they do not have to.

As an example, several years ago I was flying with Aeroflot from London to Moscow. The allowance was 20 kg. When I put my suitcase on the scale, it was 19.8 kg. I then said, half jokingly, "ok, I can put my newspaper in there" - and proceeded to put into the side pocket a couple of British newspapers that my friend in Moscow asked me to bring. The scales went up to 20.1 kg. The agent said, "no, this is over the limit".

I didn't quite believe that she was serious, so questioned this - she insisted that this was no joke and that I would have to pay for the extra weight. I shrugged and took one of the newspapers out and into my carry on backpack, leaving exactly 20.0 kg on the scale. She happily checked the suitcase in.

For practical reasons, the agents have to have some latitude in determining this. Because:

1. The scale might not be perfectly calibrated.
2. The bags position on the scale can affect the reading.
3. 0.9kg might be under their tolerance.

This will not be published anywhere. I've had it happen with two bags, one under, one slightly over and the response was "close enough".

Bags are tagged Heavy not just for revenue, but for weight/balance and to warn the handlers.

This may be country and airline specific. As a thumb rule, there is no extra tolerated amount of extra weight. I have experienced discomfort for being .5 lbs over the limit - where the agent behind the counter was bent on me and my partner reorganizing 6 large suitcases (paid for since over the limit) - but this was in the US though.

The amount of exceedence that is acceptable varies. In the past, I regularly got away with an extra 5lbs (2.5kg) on US flights. Now days, they're tightening up, so those easy days are gone.

I've flown a particular airline (Tiger Airlines) which was strict about their overages. At the ticket counter, you could see countless people opening their bags and desperately trying to fit to the weight limit.

These days I'm generally under - well under - but it all adds up.

The general rule of thumb is that if you want to try to get away with really overweight baggage, check in early. The later it gets, the more the weight restrictions apply.

This being said, I've gotten away with 50+kg on a 20kg ticket several times over the years.

With overweight baggage do the following: check in with a person, not the machines; be nice and friendly; be early - but not ridiculously so; and don't do it all the time, because airlines monitor these things. If you're under most of the time, you can get away with an insanely overweight bag occasionally.