double x = 1493.1987;
string s1 = x.ToString("F3");
string s2 = string.Format("Your total is {0:F3}, have a nice day.", x);
// s1 is "1493.199"
// s2 is "Your total is 1493.199, have a nice day."

double y = 1493;
string s3 = y.ToString("F3");
// s3 is "1493.000"

double pi = 3.1415927;
string output = pi.ToString("#.000");

float a = 12.3578f;
double b = 12.3578d;
Console.WriteLine("The tolerance specs are: {0:F4} and: {1:F3}", a,b);