How to Calculate Percentile Using NumPy in Python

Use numpy.percentile(array, percentile_value) to calculate the percentile of data in a NumPy array. The function returns the value below which the given percentage of data falls.

Syntax

The basic syntax of the NumPy percentile function is:

• numpy.percentile(a, q, axis=None, out=None, overwrite_input=False, method='linear', keepdims=False)

Where:

• a is the input data array.
• q is the percentile or sequence of percentiles to compute (0-100).
• axis specifies the axis along which to compute percentiles (default is flattened array).
• Other parameters control output and interpolation method.

python

numpy.percentile(a, q, axis=None, out=None, overwrite_input=False, method='linear', keepdims=False)

Example

This example shows how to calculate the 25th, 50th, and 75th percentiles of a NumPy array.

python

import numpy as np

data = np.array([10, 20, 30, 40, 50])
percentiles = np.percentile(data, [25, 50, 75])
print(percentiles)

Output

[20. 30. 40.]

Common Pitfalls

Common mistakes when using numpy.percentile include:

• Passing percentile values outside the 0-100 range, which causes errors.
• Not specifying the axis when working with multi-dimensional arrays, leading to unexpected results.
• Using deprecated interpolation methods; use the default or specify valid options like 'linear', 'nearest', 'midpoint', or 'lower'.

python

import numpy as np

data = np.array([[10, 20, 30], [40, 50, 60]])

# Wrong: percentile value out of range
# np.percentile(data, 110)  # This will raise an error

# Wrong: no axis specified, but want percentiles per column
percentiles_wrong = np.percentile(data, 50)

# Right: specify axis=0 to get median per column
percentiles_right = np.percentile(data, 50, axis=0)

print('Without axis:', percentiles_wrong)
print('With axis=0:', percentiles_right)

Output

Without axis: 35.0
With axis=0: [25. 35. 45.]

Quick Reference

Parameter	Description
a	Input data array
q	Percentile or list of percentiles (0-100)
axis	Axis along which to compute percentiles (default None)
method	Method to use when the desired percentile lies between two data points (default 'linear')
keepdims	If True, retains reduced dimensions with length 1

Key Takeaways

Use numpy.percentile(array, percentile) to find the value below which the given percent of data falls.

Percentile values must be between 0 and 100 inclusive.

Specify the axis parameter when working with multi-dimensional arrays to get correct results.

Avoid deprecated interpolation methods; use the default or valid options like 'linear' or 'nearest'.

The function works on flattened data by default if axis is not specified.