You must enter the base font you are using. The browser's default is 16px. If you have configured or set your base font to other values (18px or 20px), you must input it here so the equivalent, especially in REM and EM converters, are precise.
Enter the your desired pixels per inch (ex: 300ppi). For web-related works, pixels per inch is equal to 96px.
After you have entered the value you want to be converted (and Base or PPI values if necessary), then click on the convert button to start the operation.