# Math::Round Method

**.NET Framework (current version)**

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Rounds a value to the nearest integer or to the specified number of fractional digits.

**Namespace:**System

**Assembly:**mscorlib (in mscorlib.dll)

Name | Description | |
---|---|---|

Round(Decimal) | Rounds a decimal value to the nearest integral value. | |

Round(Decimal, Int32) | Rounds a decimal value to a specified number of fractional digits. | |

Round(Decimal, Int32, MidpointRounding) | Rounds a decimal value to a specified number of fractional digits. A parameter specifies how to round the value if it is midway between two numbers. | |

Round(Decimal, MidpointRounding) | Rounds a decimal value to the nearest integer. A parameter specifies how to round the value if it is midway between two numbers. | |

Round(Double) | Rounds a double-precision floating-point value to the nearest integral value. | |

Round(Double, Int32) | Rounds a double-precision floating-point value to a specified number of fractional digits. | |

Round(Double, Int32, MidpointRounding) | Rounds a double-precision floating-point value to a specified number of fractional digits. A parameter specifies how to round the value if it is midway between two numbers. | |

Round(Double, MidpointRounding) | Rounds a double-precision floating-point value to the nearest integer. A parameter specifies how to round the value if it is midway between two numbers. |

In this section:

Overloaded method syntax

Parameters

Return value

Exceptions

Which method do I call?

Midpoint values and rounding conventions

Rounding and precision

Rounding and single-precision floating-point values

Examples:

Math.Round(Decimal)

Math.Round(Double)

Math.Round(Decimal, Int32)

Math.Round(Decimal, MidpointRounding)

Math.Round(Double, Int32)

Math.Round(Double, MidpointRounding)

Math.Round(Decimal, Int32, MidpointRounding)

Math.Round(Double, Int32, MidpointRounding)

Version information

For additional guidance on choosing an overload, see Which method do I call?

- Decimal Math.Round(Decimal
*d*) Rounds a decimal value to the nearest integer, and rounds midpoint values to the nearest even number (example).

- Double Math.Round(Double
*a*) Rounds a double-precision floating-point value to the nearest integer, and rounds midpoint values to the nearest even number (example).

- Decimal Math.Round(Decimal
*d*, Int32*decimals*) Rounds a decimal value to a specified number of fractional digits, and rounds midpoint values to the nearest even number (example).

- Double Math.Round(Double
*value*, Int32*digits*) Rounds a double-precision floating-point value to a specified number of fractional digits, and rounds midpoint values to the nearest even number (example).

- Decimal Math.Round(Decimal
*d*, MidpointRounding*mode*) Rounds a decimal value to the nearest integer, and uses the specified rounding convention for midpoint values (example).

- Double Math.Round(Double
*value*, MidpointRounding*mode*) Rounds a double-precision floating-point value to the nearest integer, and uses the specified rounding convention for midpoint values (example).

- Decimal Math.Round(Decimal
*d*, Int32*decimals*, MidpointRounding*mode*) Rounds a decimal value to a specified number of fractional digits, and uses the specified rounding convention for midpoint values (example).

- Double Math.Round(Double
*value*, Int32*digits*, MidpointRounding*mode*) Rounds a double-precision floating-point value to a specified number of fractional digits, and uses the specified rounding convention for midpoint values (example).

This is a complete list of parameters for the Round method. For the parameters used by each overload, see the overload syntax above.

Parameter | Type | Description |
---|---|---|

| A number to be rounded. | |

-or-
| A double-precision floating-point number to be rounded. | |

-or-
| The number of fractional digits in the return value. For Decimal values, it can range from 0 to 28. For Double values, it can range from 0 to 15. Note that 15 is the maximum number of integral and fractional digits supported by the Double type. Default value: 0, which rounds to an integer | |

| A flag that specifies the convention to use for rounding midpoint values. See Midpoint values and rounding conventions. Default value: MidpointRounding::ToEven |

Type: Decimal or Double, depending on the type of the value to be rounded.

A numeric value of the same type as *d*, *a*, or *value*, but rounded either to an integer or to the specified number of decimal digits. See Midpoint values and rounding conventions for information about rounding numbers with midpoint values.

Warning |
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When rounding midpoint values, the rounding algorithm performs an equality test. Because of problems of binary representation and precision in the floating-point format, the value returned by the method can be unexpected. For more information, see Rounding and precision. |

If an overload with a Double parameter is called with Double::NaN, the method returns Double::NaN. If it is called with Double::PositiveInfinity or Double::NegativeInfinity, the method returns Double::PositiveInfinity or Double::NegativeInfinity, respectively.

Exception | Condition | Thrown by |
---|---|---|

The result is outside the range of the Decimal type. | ||

-or-
| ||

| Round(Decimal, MidpointRounding) Round(Double, MidpointRounding) |

To | Call |
---|---|

Round a number to an integer by using the rounding to nearest convention. | -or- |

Round a number to an integer by using a specified rounding convention. | Round(Decimal, MidpointRounding) -or- |

Round a number to a specified number of fractional digits by using the rounding to nearest convention. | -or- |

Round a number to a specified number of fractional digits by using a specified rounding convention. | Round(Decimal, Int32, MidpointRounding) -or- |

Round a Single value to a specified number of fractional digits by using a specified rounding convention and minimizing the loss of precision. | Convert the Single to a Decimal and call Round(Decimal, Int32, MidpointRounding). |

Round a number to a specified number of fractional digits while minimizing problems of precision in rounding midpoint values. | Call a rounding method that implements a "greater than or approximately equal to" comparison. See Rounding and precision. |

Round a fractional value to an integer that is greater than the fractional value. For example, round 3.1 to 4. | |

Round a fractional value to an integer that is less than the fractional value. For example, round 3.9 to 3. |

Rounding involves converting a numeric value with a specified precision to the nearest value with less precision. For example, you can use the Round(Double) method to round a value of 3.4 to 3.0, and the Round(Double, Int32) method to round a value of 3.579 to 3.58.

In a midpoint value, the value after the least significant digit in the result is precisely half way between two numbers. For example, 3.47500 is a midpoint value if it is to be rounded two decimal places, and 7.500 is a midpoint value if it is to be rounded to an integer. In these cases, the nearest value can't be easily identified without a rounding convention.

The Round method supports two rounding conventions for handling midpoint values:

- Rounding away from zero
Midpoint values are rounded to the next number away from zero. For example, 3.75 rounds to 3.8, 3.85 rounds to 3.9, -3.75 rounds to -3.8, and -3.85 rounds to -3.9. This form of rounding is represented by the MidpointRounding::AwayFromZero enumeration member.

Rounding away from zero is the most widely known form of rounding.

- Rounding to nearest, or banker's rounding
Midpoint values are rounded to the nearest even number. For example, both 3.75 and 3.85 round to 3.8, and both -3.75 and -3.85 round to -3.8. This form of rounding is represented by the MidpointRounding::ToEven enumeration member.

Rounding to nearest is the standard form of rounding used in financial and statistical operations. It conforms to IEEE Standard 754, section 4. When used in multiple rounding operations, it reduces the rounding error that is caused by consistently rounding midpoint values in a single direction. In some cases, this rounding error can be significant.

The following example illustrates the bias that can result from consistently rounding midpoint values in a single direction. The example computes the true mean of an array of Decimal values, and then computes the mean when the values in the array are rounded by using the two conventions. In this example, the true mean and the mean that results when rounding to nearest are the same. However, the mean that results when rounding away from zero differs by .05 (or by 3.6%) from the true mean.

By default, the Round method uses the rounding to nearest convention. The following table lists the overloads of the Round method and the rounding convention that each uses.

Overload | Rounding convention |
---|---|

Determined by | |

Determined by | |

Determined by | |

Determined by |

In order to determine whether a rounding operation involves a midpoint value, the Round method multiplies the original value to be rounded by 10n, where *n* is the desired number of fractional digits in the return value, and then determines whether the remaining fractional portion of the value is greater than or equal to .5. This is a slight variation on a test for equality, and as discussed in the "Testing for Equality" section of the Double reference topic, tests for equality with floating-point values are problematic because of the floating-point format's issues with binary representation and precision. This means that any fractional portion of a number that is slightly less than .5 (because of a loss of precision) will not be rounded upward.

The following example illustrates the problem. It repeatedly adds .1 to 11.0 and rounds the result to the nearest integer. Regardless of the rounding convention, 11.5 should round to 12. However, as the output from the example shows, it does not. The example uses the "R" standard numeric format string to display the floating point value's full precision, and shows that the value to be rounded has lost precision during repeated additions, and its value is actually 11.499999999999998. Because .499999999999998 is less than .5, the value is not rounded to the next highest integer. As the example also shows, this problem does not occur if we simply assign the constant value 11.5 to a Double variable.

Problems of precision in rounding midpoint values are most likely to arise in the following conditions:

When a fractional value cannot be expressed precisely in the floating-point type's binary format.

When the value to be rounded is calculated from one or more floating-point operations.

When the value to be rounded is a Single rather than a Double or Decimal. For more information, see the next section, Rounding and single-precision floating-point values.

In cases where the lack of precision in rounding operations is problematic, you can do the following:

If the rounding operation calls an overload that rounds a Double value, you can change the Double to a Decimal value and call an overload that rounds a Decimal value instead. Although the Decimal data type also has problems of representation and loss of precision, these issues are far less common.

Define a custom rounding algorithm that performs a "nearly equal" test to determine whether the value to be rounded is acceptably close to a midpoint value. The following example defines a RoundApproximate method that examines whether a fractional value is sufficiently near to a midpoint value to be subject to midpoint rounding. As the output from the example shows, it corrects the rounding problem shown in the previous example.

The Round method includes overloads that accept arguments of type Decimal and Double. There are no methods that round values of type Single. If you pass a Single value to one of the overloads of the Round method, it is cast (in C#) or converted (in Visual Basic) to a Double, and the corresponding Round overload with a Double parameter is called. Although this is a widening conversion, it often involves a loss of precision, as the following example illustrates. When a Single value of 16.325 is passed to the Round method and rounded to two decimal places using the rounding to nearest convention, the result is 16.33 and not the expected result of 16.32.

This unexpected result is due to a loss of precision in the conversion of the Single value to a Double. Because the resulting Double value of 16.325000762939453 is not a midpoint value and is greater than 16.325, it is always rounded upward.

In many cases, as the example illustrates, the loss of precision can be minimized or eliminated by casting or converting the Single value to a Decimal. Note that, because this is a narrowing conversion, it requires using a cast operator or calling a conversion method.

The following example demonstrates rounding to the nearest integer value.

The following example rounds double values with two fractional digits to doubles that have a single fractional digit.

The following example displays values returned by the Round(Decimal) method, the Round(Decimal, MidpointRounding) method with a *mode* argument of ToEven, and the Round(Decimal, MidpointRounding) method with a *mode* argument of AwayFromZero.

The following example displays values returned by the Round(Double) method, the Round(Double, MidpointRounding) method with a *mode* argument of ToEven, and the Round(Double, MidpointRounding) method with a *mode* argument of AwayFromZero.

The following example demonstrates how to use the Round method with the MidpointRounding enumeration.

// This example demonstrates the Math.Round() method in conjunction // with the MidpointRounding enumeration. using namespace System; void main() { Decimal result = (Decimal) 0.0; Decimal posValue = (Decimal) 3.45; Decimal negValue = (Decimal) -3.45; // By default, round a positive and a negative value to the nearest // even number. The precision of the result is 1 decimal place. result = Math::Round(posValue, 1); Console::WriteLine("{0,4} = Math.Round({1,5}, 1)", result, posValue); result = Math::Round(negValue, 1); Console::WriteLine("{0,4} = Math.Round({1,5}, 1)", result, negValue); Console::WriteLine(); // Round a positive value to the nearest even number, then to the // nearest number away from zero. The precision of the result is 1 // decimal place. result = Math::Round(posValue, 1, MidpointRounding::ToEven); Console::WriteLine( "{0,4} = Math.Round({1,5}, 1, MidpointRounding.ToEven)", result, posValue); result = Math::Round(posValue, 1, MidpointRounding::AwayFromZero); Console::WriteLine( "{0,4} = Math.Round({1,5}, 1, MidpointRounding.AwayFromZero)", result, posValue); Console::WriteLine(); // Round a negative value to the nearest even number, then to the // nearest number away from zero. The precision of the result is 1 // decimal place. result = Math::Round(negValue, 1, MidpointRounding::ToEven); Console::WriteLine( "{0,4} = Math.Round({1,5}, 1, MidpointRounding.ToEven)", result, negValue); result = Math::Round(negValue, 1, MidpointRounding::AwayFromZero); Console::WriteLine( "{0,4} = Math.Round({1,5}, 1, MidpointRounding.AwayFromZero)", result, negValue); Console::WriteLine(); } /* This code example produces the following results: 3.4 = Math.Round( 3.45, 1) -3.4 = Math.Round(-3.45, 1) 3.4 = Math.Round( 3.45, 1, MidpointRounding.ToEven) 3.5 = Math.Round( 3.45, 1, MidpointRounding.AwayFromZero) -3.4 = Math.Round(-3.45, 1, MidpointRounding.ToEven) -3.5 = Math.Round(-3.45, 1, MidpointRounding.AwayFromZero) */

The following example demonstrates how to use the Round(Double, Int32, MidpointRounding) method with the MidpointRounding enumeration.

- .NET Framework
Overloads without a MidpointRounding parameter are supported in: 4.5, 4, 3.5, 3.0, 2.0, 1.1, 1.0

Overloads with a MidpointRounding parameter are supported in: 4.5, 4, 3.5, 3.0, 2.0- .NET Framework Client Profile
All overloads are supported in: 4, 3.5 SP1

- Portable Class Library
All overloads are supported

- .NET for Windows Store apps
All overloads are supported in: Windows 8