rounding of decimals

[mandimoo]

when you are rouding up / down decimals (for currency calculations), are we supposed to round "0.5" up or down

[Rollo]

The usual convention is to round 0.5 up.

[sandmaster]

I agree with rollo there. That is the usual convention.

[dzug]

Agree - but a lot depends on who is doing the calculation and why.

For example some cashback cards invariably round down - eg half a percent of �3.99 is 1p

[Buenchico]

Rollo and Sandmaster have both correctly stated the mathematical convention that 'point five' is always rounded up. (The convention is adopted for consistency. For example, 'point five one' and 'point five seven' are rounded up, as is anything else which starts with 'point five', so it's logical that plain old 'point five' should also round up).

However Dzug has wisely pointed out that commercial conventions (or necessities) aren't always consistent with mathematical conventions. These problems arise partly through your use of the words 'supposed to' in your question.

For example, a shop might advertise a 'half price sale'. When they relabel the price of an item which formerly cost 99p, they have to decide whether the new price should be 50p (as the mathematical convention would dictate) or 49p. Since many customers would argue that charging 50p doesn't really match the 'half price' claim, most people would probably say that the shop staff are supposed to round the new price down, rather than up.

Chris

[Rev. Green]

My son always rounds upwards, but when I was as school we rounded towards an even number.

So 6.5 would round to 6; 9.5 would round to 10.
The logic behing this was that the sum of many values would tend to be more accurate - some of the values rounded down might compensate for some rounding up.