When estimating with story points, most teams use a predefined set of values that doesn’t include every possible number.
One of the questions I often get about story points is what to do if you can’t decide whether a particular product backlog item should be an 8 or a 13 (or a 3 and a 5). Part of the answer can be found by thinking about water buckets.
Suppose you have 10 liters of water you need to store. You also have an 8-liter bucket and a 13-liter bucket.
Which bucket would you store the water in?
The 13-liter bucket, right? Ten liters of water doesn’t fit in an 8-liter bucket. The water would overflow and spill out.
Extrapolating further, you’d use the 13-liter bucket for all amounts of water from 9 liters through 13 liters. Once you hit 14 liters, though, you’d once again need a bigger bucket.
And so it is with the values you choose to use when estimating stories. Think of each value as a bucket. A value bucket is used for all stories between that value and the next lower value.
Read the complet blogpost here: https://www.mountaingoatsoftware.com/blog/story-point-estimates-are-best-thought-of-as-ranges