# Rounding rules **URL:** \ **Category:** Factual Questions\ **Created:** [September 27, 2005, 3:49pm UTC](https://boards.straightdope.com/t/rounding-rules/323682 "2005-09-27T15:49:40Z")\ **Posts on this page:** 5\ **Page:** 2
**Author:** ![Excalibre](https://avatars.discourse-cdn.com/v4/letter/e/898d66/32.png) [@Excalibre](https://boards.straightdope.com/u/Excalibre)\ **Post date:** [September 27, 2005, 9:49pm UTC](https://boards.straightdope.com/t/rounding-rules/323682/21 "2005-09-27T21:49:36Z")
> [@Jurph](#): > > This makes sense to me. Perhaps this was the approach he was recommending? It is possible – however unlikely – that my chemistry teacher was not a total idiot. And that’s as close to an apology as he’s ever going to get. I bet he learned the basic principle, but never understood it, and screwed it up when he taught it to you. At least that’s what I’d tell myself if I were you. Anyway, _is_ the tenths digit equally likely to be any digit? Can someone tell me if that one thingamajigger principle where the last digit is 1 a disproportionate percent of the time applies here? ---
**Author:** ![borschevsky](https://avatars.discourse-cdn.com/v4/letter/b/97f17d/32.png) [@borschevsky](https://boards.straightdope.com/u/borschevsky)\ **Post date:** [September 27, 2005, 10:06pm UTC](https://boards.straightdope.com/t/rounding-rules/323682/22 "2005-09-27T22:06:31Z")
You’re talking about [Benford’s Law](http://en.wikipedia.org/wiki/Benford%27s_law), and yes the effect will be there for some sets of numbers, although it should be very small for the rounded digit in most cases. The effect is fairly large for the first digit, much smaller for the 2nd, much much smaller for the 3rd, etc. ---
**Author:** ![ouryL](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ouryl/32/6067_2.png) [@ouryL](https://boards.straightdope.com/u/ouryL)\ **Post date:** [September 28, 2005, 5:47am UTC](https://boards.straightdope.com/t/rounding-rules/323682/23 "2005-09-28T05:47:10Z")
The question is why do you wanna round? My mother always rounds down when she does her check book. If I owe someone money, I will round up. ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [September 28, 2005, 8:15am UTC](https://boards.straightdope.com/t/rounding-rules/323682/24 "2005-09-28T08:15:41Z")
> [@ouryL](#): > > The question is why do you wanna round? My mother always rounds down when she does her check book. If I owe someone money, I will round up. Rounding becomes necessary when you apply calculations that result in a greater number of decimal places than you can conveniently work with, for example, when calculating percentage discounts or interest on currency values; you end up with parts of a penny that you must deal with by rounding. ---
**Author:** ![Thudlow\_Boink](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/thudlow_boink/32/320_2.png) [@Thudlow\_Boink](https://boards.straightdope.com/u/Thudlow_Boink)\ **Post date:** [September 28, 2005, 12:52pm UTC](https://boards.straightdope.com/t/rounding-rules/323682/25 "2005-09-28T12:52:00Z")
I think **ouryL** was trying to point out that _how_ you round (i.e. up, down, off) depends on _why_ you’re rounding. The rule depends on the context. As for the 5 thing, 38.5 is exactly halfway between 38 and 39, but 38.5… with anything after the 5 is closer to 39 than to 38. If you’re rounding to the nearest whole number, in the latter case, you should definitely round to 39. In the former case, there are different conventions, and what to do depends on which convention you follow. [Previous page](https://boards.straightdope.com/t/rounding-rules/323682.md?page=1)