Why does percentage format specifier multiply by 100?

You normally work with decimal percents inside of code, like 0.5 and 1.0, but the user likes to see nice whole numbers with a percent sign tacked on the end.

Percent means "out of 100" and clearly your decimal percents are out of 1. Therefore, .ToString("p") multiplies your number by 100 and then appends a percent sign.

It's just the definition.

If you don't want to multiply the value by 100 , just want the percentage % besides the value entered , i.e. VALUE.00 % if value entered otherwise default 0.00%, then the answer is

PERCENTAGE_FORMAT = "#0.00\%";

It definitely worked for me !!

As to the why, "percent" literally means "out of one hundred", so 50% is mathematically equivalent to 0.50. As to formatting, why not just add a percent sign?

value + "%"

... or something like this:

value.ToString("#.00\\%")