Chelsea signed the youngster from Exeter City last summer, but after the two clubs failed to compromise on a fee, it soon became clear that the financial details of the transfer would be agreed by a tribunal.
Ampadu - who does not turn 18 until September - has already made seven first-team appearances for the Blues, but the West Londoners will have to pay just £2.5m to finalise the deal.
A statement read: "Chelsea football club should pay Exeter City an initial net fee of £850,000. In addition to the initial payment a fee of £450,000 is also payable.
"This fee incorporates an amount for the player signing a first professional contract, making five first-team appearances and obtaining a senior international cap."
Exeter chairman Julian Tagg has said that he is disappointed with the decision, suggesting that the club have not been rewarded for developing Ampadu from the age of eight.