**Percentages** are often useful in programs. With the numbers one and two, we can get a percentage of 50%. We display and process percentages with doubles. We solve an annoying rounding problem.

**Example.** First, we see some code that uses string.Format to display two numbers or ratio as a percentage. The following code shows four methods. The first is Main. And the last three are custom methods for displaying percentages.

C# program that calculates percentsusing System; class Program { static void Main() {// Display percentage of visits that resulted in purchases.int purchases = 10; int visitors = 120; DisplayPercentage((double)purchases / visitors);// Display 50 percent with overloaded method.DisplayPercentage(1, 2);// Write percentage string of nine tenths.Console.WriteLine(GetPercentageString((double)9 / 10)); }/// <summary> /// This method writes the percentage form of a double to the console. /// </summary>static voidDisplayPercentage(double ratio) { string percentage = string.Format("Percentage is {0:0.0%}", ratio); Console.WriteLine(percentage); }/// <summary> /// This method writes the percentage of the top number to the bottom number. /// </summary>static voidDisplayPercentage(int top, int bottom) { DisplayPercentage((double)top / bottom); }/// <summary> /// This method returns the percentage-formatted string. /// </summary>static stringGetPercentageString(double ratio) { return ratio.ToString("0.0%"); } }OutputPercentage is 8.3% Percentage is 50.0% 90.0%

**DisplayPercentage** accepts a double that is a ratio, usually between 0 and 1. It uses a custom formatting string to display the percentage to the Console. The {0:0.0%} substitution indicates you want a percentage with one decimal place.

**Also:** The second DisplayPercentage accepts two parameters and then passes the ratio of them to the other method. It casts to double.

**Finally:** GetPercentageString accepts a double containing a ratio and returns a percentage string using ToString().

**Example 2.** Here we convert two integers into a percentage manually with division and multiplication. Sometimes you can need raw percentages when you have percentages in the database stored in different formats.

C# program that converts ratiosusing System; class Program { static void Main() {// We want to have 92.9% from these two numbers.int valid = 92; int total = 99;// First multiply top by 100 then divide.double percent = (double)(valid * 100) / total;// <-- Use cast // This is the percent number.Console.WriteLine(percent); Console.WriteLine(Math.Floor(percent)); Console.WriteLine(Math.Ceiling(percent)); Console.WriteLine(Math.Round(percent, 1)); } }Output92.9292929292929 92 93 92.9

**Casting to double.** The double must be assigned to a value cast to double. If you omit the cast, your value will be rounded and probably useless. When casting to double, you do not need to surround the entire expression with parentheses.

**The final four statements** in the program display different forms of the percentage. Math.Floor rounds down to the nearest integer. Math.Ceiling rounds up to the nearest integer. And Math.Round rounds to a single decimal place.

**Modulo.** The percentage sign in the C# language has a use as the modulo operator. This forms an expression that will return the remainder of a division of the two operands. You can find more about the modulo operator.

**Tip:** With modulo division, we can run an operation every N times. This has uses in many programs.

**Summary.** We saw two examples of using percentages in the C# language. First we saw how to format ratios as percentages with three different methods. Second, we saw how to get a percentage value directly with math, and then round it.