Completion percentage

What it's for

How many pages of the book I've read, how many tasks I've finished, how many kilometres of the stage I've covered, how many houses on the list have been visited: two numbers, done and total, and the percentage that comes out. The bar does something specific: a written percentage is read, a bar is seen, and at a glance it says whether you're halfway, nearly there or barely started.

The arithmetic

Percentage = done ÷ total × 100. The total has to be greater than zero, otherwise the question has no answer and nothing appears here instead of a meaningless number. If the done figure passes the total the percentage goes beyond a hundred, and it's shown as it is, because beating a target is information: the bar, on the other hand, stops when it's full, because a bar longer than its box can't be drawn.

What the number doesn't say

The percentage of things done isn't the percentage of work done. In any project the items don't weigh the same: seven tasks out of ten can be 30% of the effort, if the three left are the hard ones. It's why jobs sit «at ninety per cent» for weeks, and it's a way of fooling yourself that you avoid only by writing next to each item how much time it costs, instead of just counting them.

It works the other way round too, and that's the useful part: if you're reading a book and you're at 40% after three days, the arithmetic says four or five are left, and that's an honest forecast because pages, unlike tasks, nearly all weigh the same.

Nearby tools

If you don't have the total and want to work it out from the percentage there is Percentage calculator. To compare two values, before and after, Percentage change. And if what you're measuring is how long until a date, rather than out of a total, the page is Countdown.