std::ceil, std::ceilf, std::ceill

Defined in header <cmath>
float       ceil ( float num );
double      ceil ( double num );
long double ceil ( long double num );
(until C++23)
constexpr /* floating-point-type */
            ceil ( /* floating-point-type */ num );
(since C++23)
float       ceilf( float num );
(2) (since C++11)
(constexpr since C++23)
long double ceill( long double num );
(3) (since C++11)
(constexpr since C++23)
Additional overloads (since C++11)
Defined in header <cmath>
template< class Integer >
double      ceil ( Integer num );
(A) (constexpr since C++23)
1-3) Computes the least integer value not less than num. The library provides overloads of std::ceil for all cv-unqualified floating-point types as the type of the parameter. (since C++23)
(since C++11)


num - floating point or integer value

Return value

If no errors occur, the smallest integer value not less than num, that is ⌈num⌉, is returned.

Return value

Error handling

Errors are reported as specified in math_errhandling.

If the implementation supports IEEE floating-point arithmetic (IEC 60559),

  • The current rounding mode has no effect.
  • If num is ±∞, it is returned unmodified
  • If num is ±0, it is returned, unmodified
  • If num is NaN, NaN is returned


FE_INEXACT may be (but is not required to be) raised when rounding a non-integer finite value.

The largest representable floating-point values are exact integers in all standard floating-point formats, so this function never overflows on its own; however the result may overflow any integer type (including std::intmax_t), when stored in an integer variable. It is for this reason that the return type is floating-point not integral.

This function (for double argument) behaves as if (except for the freedom to not raise FE_INEXACT) implemented by the following code:

#include <cfenv>
#include <cmath>
double ceil(double x)
    int save_round = std::fegetround();
    double result = std::rint(x); // or std::nearbyint
    return result;

The additional overloads are not required to be provided exactly as (A). They only need to be sufficient to ensure that for their argument num of integer type, std::ceil(num) has the same effect as std::ceil(static_cast<double>(num)).


#include <cmath>
#include <iostream>
int main()
    std::cout << std::fixed
              << "ceil(+2.4) = " << std::ceil(+2.4) << '\n'
              << "ceil(-2.4) = " << std::ceil(-2.4) << '\n'
              << "ceil(-0.0) = " << std::ceil(-0.0) << '\n'
              << "ceil(-Inf) = " << std::ceil(-INFINITY) << '\n';


ceil(+2.4) = 3.000000
ceil(-2.4) = -2.000000
ceil(-0.0) = -0.000000
ceil(-Inf) = -inf

See also

nearest integer not greater than the given value
nearest integer not greater in magnitude than the given value
nearest integer, rounding away from zero in halfway cases
nearest integer using current rounding mode
nearest integer using current rounding mode with
exception if the result differs
C documentation for ceil
Fast ceiling of an integer division — StackOverflow

© cppreference.com
Licensed under the Creative Commons Attribution-ShareAlike Unported License v3.0.