Adding Up Rounded Percentages to Equal 100%

When working with rounded percentages, it's actually not that easy to make sure the percentages add up to 100%. Take the following example:

If you use basic rounding, you'll get 41%, 21%, and 39%... which adds up to 101%!

So, what can we do?

The Solution

The logic behind the solution is fairly straightforward and is just a spin-off of the largest remainder method:

  1. Instead of rounding to the closest integer, round everything down
  2. Take the sum of those numbers, and subtract it from 100 to determine the difference
  3. Re-allocate the difference by adding 1 to the numbers with the highest decimal

Going back to our example:

  1. We round the numbers to 40, 20, and 38
  2. We subtract the sum of those (98) from 100, which equals 2
  3. We add 1 to the top 2 highest decimals, which gives us 40, 21, and 39... the sum of which equals 100!

That was the easy part. The real trick is duplicating that in code. Thankfully, it's already been done! Read the next section for a class that automatically rounds these numbers, while still preserving their original order.

The Code

To see the full gist, click here.

~ posted in professional on 08.17.2017