Petroleum products are converted to 15 degrees Celsius because it is a convenient temperature for doing business. It is close to the average temperature in most parts of the world, and it is also a temperature at which most petroleum products are in a liquid state. This makes it easier to measure and transport petroleum products.

Additionally, 15 degrees Celsius is the standard temperature for most petroleum industry transactions. This means that when buyers and sellers are negotiating a contract for the sale of petroleum products, they will typically use 15 degrees Celsius as the reference temperature. This helps to ensure that both the buyer and the seller are on the same page about the quantity and quality of the product being sold.

In some cases, petroleum products may be converted to a different temperature if required by the specific application. For example, if a petroleum product is being used in a cold climate, it may be converted to a lower temperature to ensure that it remains in a liquid state.