Does an earn-out calculation get checked by an independent accountant, or do buyer and seller work it out themselves?

It depends entirely on what the purchase agreement's dispute-resolution mechanism actually says, since there is no automatic requirement for an accountant to be involved. Many Ontario purchase agreements provide that if the buyer and seller cannot agree on the earn-out calculation, or on the underlying financial figures it is based on, within a defined negotiation period, either party can refer the specific dispute to an independent or mutually agreed accounting firm, whose determination on the accounting question is often treated as final and binding.

This kind of mechanism tends to work well for genuinely financial or accounting disagreements, such as how a particular expense should be classified, while broader legal disputes, for example over whether the buyer breached an operating covenant that affected the numbers, are usually left for negotiation, arbitration, or court under a separate part of the agreement's dispute-resolution clause. If the agreement is silent on any of this, the parties are left to negotiate the calculation directly, and ultimately to litigate it if they cannot agree, which is a materially less predictable position for both sides.