How to Use This Tool
Paste your values, one per line. You get percentages that sum to exactly 100, with the naive version shown alongside so you can see which entries were adjusted and by how much.
Why the total comes out wrong
Round each percentage on its own and you make a small error each time. Those errors are independent, so they do not cancel — sometimes they all point the same way. Three equal parts each round to 33.3%, and 33.3 × 3 is 99.9.
Nothing is wrong with any individual figure. The problem is that the rounded values are being asked to do something the unrounded ones did automatically: add up.
Largest remainder allocation
The fix is to treat it as an allocation problem rather than a rounding problem:
- Round every share down to the chosen precision.
- Count how many units are left over from the total.
- Hand those units, one at a time, to whichever entries had the largest fractional part discarded.
The result sums exactly, and every value is within one unit of its true share — which is the best any set of rounded numbers can do. This is the same method used to allocate parliamentary seats to parties from vote shares, where the requirement that the total be exact is not negotiable.
When the tie is genuinely arbitrary
If several entries have identical remainders, as in the example above, something has to break the tie and any rule is a convention. Order of appearance is used here. That means one entry gets a hundredth of a percent more than an identical one, which is a real if tiny unfairness — and it is unavoidable, because the total must be exact and the extra unit cannot be split.
This is why seat allocation methods are politically contested: with seats instead of hundredths, the same arbitrary choice decides who governs.
When not to force the total
Two cases where 99.9% is the honest answer:
- The categories overlap or are incomplete. If someone can be counted in two rows, or an "other" bucket is missing, the parts genuinely do not make a whole and forcing them to sum to 100 asserts something false.
- The figures are estimates with real uncertainty. Adjusting a hundredth to make a column tidy implies a precision that is not there.
Where the parts really are a partition of a whole — a budget, traffic sources, a vote — forcing the total is right, because the underlying quantity does sum exactly and only the display was losing it.
