How to Round a Number Down in Python? 6 Easy Ways

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Problem Formulation: Given a float number. How to round the float down in Python?

Here are some examples of what you want to accomplish:

  • 42.52 --> 42
  • 21.99999 --> 22
  • -0.1 --> -1
  • -2 --> -2

Solution: If you have little time, here’s the most straightforward answer:

To round a positive or negative number x down in Python, apply integer division // to x and divide by 1. Specifically, the expression x//1 will first perform normal float division and then throw away the remainder—effectively “rounding x down”.

In general, there are multiple ways to round a float number x down in Python: πŸ‘‡

  1. Vanilla Python: The expression x//1 will first perform normal division and then skip the remainder—effectively “rounding x down”.
  2. Round down: The math.floor(x) function rounds number x down to the next full integer.
  3. Round down (float representation): Alternatively, numpy.floor(x) rounds down and returns a float representation of the next full integer (e.g., 2.0 instead of 2).
  4. Round up: The math.ceil(x) function rounds number x up to the next full integer.
  5. Round up and down: The Python built-in round(x) function rounds x up and down to the closest full integer.

Let’s dive into each of those and more options in the remaining article. I guarantee you’ll get out of it having learned at least a few new Python tricks in the process!

Method 1: Integer Division (x//1)

The most straightforward way to round a positive or negative number x down in Python is to use integer division // by 1. The expression x//1 will first perform normal division and then skip the remainder—effectively “rounding x down”.

For example:

  • 42.52//1 == 42
  • 21.99//1 == 21
  • -0.1//1 == -1
  • -2//1 == -2

This trick works for positive and negative numbers—beautiful isn’t it? 🌻

Here are a couple of Python code examples:

def round_down(x):
    return x//1

# 42

# 21

# -1

# -2

πŸŽ“ Info: The double-backslash // operator performs integer division and the single-backslash / operator performs float division. An example for integer division is 40//11 = 3. An example for float division is 40/11 = 3.6363636363636362.

Feel free to watch the following video for some repetition or learning:

Python Division Deep Dive

Method 2: math.floor()

To round a number down in Python, import the math library with import math, and call math.floor(number).

The function returns the floor of the specified number that is defined as the largest integer less than or equal to number.

πŸ’‘ Note: The math.floor() function correctly rounds down floats to the next-smaller full integer for positive and negative integers.

Here’s a code example that rounds our five numbers down to the next-smaller full integer:

import math

# 42

# 21

# -1

# -2

The following video shows the math.floor() as well as the math.ceil() functions — feel free to watch it to gain a deeper understanding:

Python Math floor(), ceil(), trunc(), and modf()

Method 3: np.floor()

To round a number down in Python, import the NumPy library with import numpy as np, and call np.floor(number).

βœ… Recommended: How to Install NumPy?

The function returns the floor of the specified number that is defined as the largest integer less than or equal to number.

Here’s an example:

import numpy as np

# 42.0

# 21.0

# -1.0

# -2.0

Both math.floor() and np.floor() round down to the next full integer. The difference between math.floor() and np.floor() is that the former returns an integer and the latter returns a float value.

Method 4: int(x)

Use the int(x) function to round a positive number x>0 down to the next integer. For example, int(42.99) rounds 42.99 down to the answer 42.

Here’s an example for positive numbers where int() will round down:

# 42

# 21

However, if the number is negative, the function int() will round up! Here’s an example for negative numbers:

# 0

# -2

Before I show you how to overcome this limitation for negative numbers, feel free to watch my explainer video on this function here:

Python int() Function

Method 5: int(x) – bool(x%1)

You can also use the following vanilla Python snippet to round a number x down to the next full integer:

  • If x is positive, round down by calling int(x).
  • If x is negative, round up by calling int(x) - bool(x%1).

Explanation: Any non-zero expression passed into the bool() function will yield True which is represented by integer 1.

The modulo expression x%1 returns the decimal part of x.

  • If it is non-zero, we subtract bool(x%1) == 1, i.e., we round down.
  • If it is zero (for whole numbers), we subtract bool(x%1) == 0, i.e., we’re already done.

Here’s what this looks like in a simple Python function:

def round_down(x):
    if x<0:
        return int(x) - bool(x%1)
    return int(x)

# 42

# 21

# -1

# -2

Alternatively, you can use the following slight variation of the function definition:

def round_down(x):
    if x<0:
        return int(x) - int(x)!=x
    return int(x)

Method 6: round()

πŸ’‘ This method is probably not exactly what you want because it rounds a number up and down, depending on whether the number is closer to the smaller or larger next full integer. However, I’ll still mention it for comprehensibility.

Python’s built-in round() function takes two input arguments:

  • a number and
  • an optional precision in decimal digits.

It rounds the number to the given precision and returns the result. The return value has the same type as the input numberβ€”or integer if the precision argument is omitted.

Per default, the precision is set to 0 digits, so round(3.14) results in 3.

Here are three examples using the round() function—that show that it doesn’t exactly solve our problem.

import math

# 42

# 21

# 0

Again, we have a video on the round() function — feel free to watch for maximum learning!

Python round() β€” A Helpful Interactive Guide

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