Simplify the following text for a Grade 4 native English-speaking person.
Original: By international agreement , it is defined as on the Kelvin scale and as â ' 273.15 Â C on the Celsius scale . This equates to â ' 459.67 Â F on the Fahrenheit scale .
Simplified: The Celsius and Fahrenheit scales are defined so that absolute zero is â ' 273.15 Â C or â ' 459.67 Â F.