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How to Round Floats to Decimal Places in Python

This guide explains how to round floating-point numbers to a specific number of decimal places in Python. We'll cover standard rounding, rounding up, rounding down, and formatting floats as strings with a fixed number of decimal places. We'll use built-in functions like round(), f-strings, math.floor(), math.ceil(), and briefly cover NumPy for array operations.

Rounding with round()​

The built-in round() function is the standard way to round a float to a specific number of decimal places:

my_float = 6.36789

result = round(my_float, 1)     # Round to 1 decimal place
print(result)                   # Output: 6.4

result = round(my_float, 2)     # Round to 2 decimal places
print(result)                   # Output: 6.37

result = round(my_float, 3)     # Round to 3 decimal places
print(result)                   # Output: 6.368

round(number, ndigits):

  • number: The float (or integer) to round.
  • ndigits: The number of decimal places to round to. If omitted, round() rounds to the nearest integer.

For displaying a float with a specific number of decimal places (without necessarily changing the underlying numerical value), f-strings are the most concise and readable option:

my_float = 6.36789

result = f'{my_float:.1f}'  # Format to 1 decimal place
print(result)               # Output: '6.4' (string)

result = f'{my_float:.2f}'  # Format to 2 decimal places
print(result)               # Output: '6.37' (string)

result = f'{my_float:.3f}'  # Format to 3 decimal places
print(result)               # Output: '6.368' (string)

number_of_decimals = 3
result = f'{my_float:.{number_of_decimals}f}'   # Dynamic precision
print(result)                                   # Output: 6.368
  • f'{my_float:.2f}': This formats my_float as a fixed-point number (f) with 2 decimal places (.2).
  • f-strings (and str.format(), which uses the same formatting syntax) produce strings, not floats. This is ideal for display purposes.
  • The width can also be a variable.

Rounding Down with math.floor()​

To always round down to a specific number of decimal places, use math.floor():

import math

def round_down(num, decimals):
    multiplier = 10 ** decimals
    return math.floor(num * multiplier) / multiplier

print(round_down(4.6789, 2))  # Output: 4.67
print(round_down(4.6789, 3))  # Output: 4.678
print(round_down(2.6789, 1))  # Output: 2.6
  • The number is first multiplied by 10 to the power of the required decimals, then rounded down to the nearest integer using math.floor(), and divided by the same power of 10 to retain the decimals required.

Rounding Up with math.ceil()​

To always round up, use math.ceil():

import math

def round_up(num, decimals):
   multiplier = 10 ** decimals
   return math.ceil(num * multiplier) / multiplier
print(round_up(4.6123, 2))  # Output: 4.62
print(round_up(4.6123, 3))  # Output: 4.613
  • The math.ceil method returns the lowest integer that is greater or equal to the number.

Rounding a List of Floats​

Use a list comprehension for conciseness:

list_of_floats = [1.42342365, 3.438834, 5.5843854]

result = [round(num, 2) for num in list_of_floats]              # Using round()
print(result)                                                   # Output: [1.42, 3.44, 5.58]

result_fstrings = [f'{item:.3f}' for item in list_of_floats]    # Using f-string
print(result_fstrings)                                          # Output: ['1.423', '3.439', '5.584']
  • The list comprehension efficiently applies the operation to each element in the list.

Rounding with NumPy​

If you're working with numerical data and already using NumPy, it provides efficient array-based rounding:

import numpy as np
list_of_floats = [2.4834843583, 4.28384291, 8.47989238]

result = list(np.around(list_of_floats, 2)) # Round using numpy.
print(result)                               # Output: [2.48, 4.28, 8.48]
  • The code converts the numbers into a NumPy array, performs rounding using the around function, and then converts it back to a list.

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