How do you write one third in percentage format?

It's a repeating decimal.

It's technically 33.3333333333333333% with infinite 3's.

You can write it out by rounding to two decimal places (33.33), or four decimal places (33.3333), or any number you want, but it's more accurate to write 33.3 with a dot above the last 3 (this means repeating).

If you have a repeating decimal such as 1.72727272727272, where the 7 and 2 both repeat, you can round to 1.73, or write 1.72 with a dot above both the 7 and 2.

To calculate a percentage from a fraction, divide the top of the fraction by the bottom. In 1/3 you would calculate 1 divided by 3. You will get a decimal (0.33333...). To put that into a percentage, simply multiply by 100.