Why does the ceil(5.0) function in PHP return 5 instead of a larger integer?

M66 2025-06-13

In PHP, the ceil() function is used to round up a floating-point number to the smallest integer greater than or equal to that number. Intuitively, ceil(5.3) would return 6, while ceil(5.0) results in 5. Many beginners are puzzled as to why ceil(5.0) doesn’t return a larger integer like 6, and instead returns 5. This actually involves the design logic of the function and the nature of floating-point numbers.

1. Definition of the ceil() function

The official definition of the ceil() function is:

Returns the smallest integer greater than or equal to the parameter value.

The key point here is the condition "greater than or equal to." If the given number is already an integer (or a floating-point number equivalent to an integer), the function will return that integer.

For example:

<?php
echo ceil(5.3);  // outputs 6
echo "\n";
echo ceil(5.0);  // outputs 5
?>

5.0 is a floating-point number, but its value is equal to the integer 5, so the result is 5.

2. Relationship Between Floating-point Numbers and Integers

5.0 represents a floating-point number, but its value happens to be an integer. The ceil() function checks the numeric value, not the data type, so:

If the value is 5.0, the rounded-up result is 5;
If the value is 5.00001, the rounded-up result is 6.

You can test this with the following code:

<?php
var_dump(ceil(5.0));      // float(5)
var_dump(ceil(5.00001));  // float(6)
?>

3. Why not a larger integer?

The behavior of the ceil() function follows the mathematical definition of "rounding up," which means "returning the smallest integer greater than or equal to the parameter," not simply returning the next integer greater than the parameter.

So:

ceil(5.0) is 5 because 5 itself is the smallest integer greater than or equal to 5.0;
If it returned 6, it would violate the principle of returning the smallest integer.

4. Related Applications and Considerations

Sometimes, you may want to ensure the result is an integer; you can force the result to an integer type:

<?php
$result = (int) ceil(5.0);
var_dump($result);  // int(5)
?>

Also, note that the ceil() function returns a floating-point number, even if the result is an integer.

5. Conclusion

ceil(5.0) returns 5 instead of a larger integer, which fully complies with the function's design and mathematical definition. The core principle is "return the smallest integer greater than or equal to the given value," rather than "return the next integer greater than the given value."

ceil