Different ways to round numbers in Python

Last updated : Jul 30, 2023 12:00 AM

There are several ways to round numbers in Python. We can use the built-in round function to handle simple use cases, the Math module, or the NumPy library to handle anything more sophisticated.

Using Python's round function

Python's round function uses the "round half to even" strategy. That means if the number being rounded is precisely halfway between two rounded numbers, the rounding is done to the nearest even number.

Round function

round(2.5) #would result in 3
round(3.5) #would result in 4

Here are some examples using the round() function

1. Rounding to a specific number of decimal places

decimal places

num = 3.141592653589793 
rounded_num = round(num, 2)
print(rounded_num) # Output: 3.14

2. Rounding to the nearest number

nearest number

num = 3.7 
rounded_num = round(num) 
print(rounded_num) # Output: 4

num = 3.3 
rounded_num = round(num) 
print(rounded_num) # Output: 3

3. Rounding to the nearest multiple of a given value

nearest multiple

base = 5
num = 17
print(base * round(num/base)) # Output: 15

Rounding with Python's Math module

Python's math module offers several functions for rounding that offer more flexibility over the round function.

1. Rounding up with the ceil() function

Rounding up

import math
num = 3.1
rounded_num = math.ceil(num)
print(rounded_num)  # Output: 4

2. Rounding down with the floor() function

Rounding down

import math
num = 3.9
rounded_num = math.floor(num)
print(rounded_num)  # Output: 3

3. Rounding to the nearest integer with the trunc() function

The trunc() function truncates the decimal points of the number

Nearest integer

import math
num = 3.7
rounded_num = math.trunc(num)
print(rounded_num)  # Output: 3

4. Rounding to the nearest multiple of a given value using the ceil() and floor() functions

Nearest multiple

import math
num = 17
base = 5
rounded_num = math.floor(num / base) * base  # round down to nearest base
print(rounded_num)  # Output: 15

rounded_num = math.ceil(num / base) * base  # round up to nearest base
print(rounded_num)  # Output: 20

Rounding with Python's Numpy library

The numpy library offers several methods to round numbers. The numpy doesn't come with the default Python installation. I used the below command to install it.

Install numpy

pip3 install numpy

1. Rounding to a specific number of decimal places using the round() function

Specific number of decimal places

import numpy as np
num = 3.141592653589793
rounded_num = np.round(num, 3)  # round to 3 decimal places
print(rounded_num)  # Output: 3.142

2. Rounding to the nearest integer using the rint() function

Nearest integer

import numpy as np
num = 3.7
rounded_num = np.rint(num)
print(rounded_num)  # Output: 4.0

num = 3.5
rounded_num = np.rint(num)
print(rounded_num)  # Output: 4.0

num = 3.4
rounded_num = np.rint(num)
print(rounded_num)  # Output: 3.0

Note that the rint() function uses the "half-to-even" rounding approach.

3. Rounding up to the nearest integer using the ceil() function

Nearest integer

import numpy as np
num = 3.01
rounded_num = np.ceil(num)
print(rounded_num)  # Output: 4.0

4. Rounding down to the nearest integer using the floor() function

Nearest integer

import numpy as np
num = 3.99
rounded_num = np.floor(num)
print(rounded_num)  # Output: 3.0

5. Rounding to the nearest multiple of a given value using the around() function

Nearest multiple

import numpy as np
num = 17
base = 5
rounded_num = np.around(num / base) * base
print(rounded_num)  # Output: 15.0

num = 28
base = 5
rounded_num = np.around(num / base) * base
print(rounded_num)  # Output: 30.0

Different ways to round numbers in Python

Using Python's round function

Rounding with Python's Math module

Rounding with Python's Numpy library

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By: Lance