---
title: percentile
tags: math,intermediate
---

Uses the percentile formula to calculate how many numbers in the given array are less or equal to the given value.

- Use `Array.prototype.reduce()` to calculate how many numbers are below the value and how many are the same value and apply the percentile formula.

```js
const percentile = (arr, val) =>
  (100 * arr.reduce((acc, v) => acc + (v < val ? 1 : 0) + (v === val ? 0.5 : 0), 0)) / arr.length;
```

```js
percentile([1, 2, 3, 4, 5, 6, 7, 8, 9, 10], 6); // 55
```