How are damages calculated in a professional negligence claim in Ontario?

The general goal is to put you in the position you would have been in if the professional had met the required standard of care - not to punish the professional, and not to give you a windfall. That means the court looks at your actual financial position now, compares it to where you would reasonably have been without the negligence, and awards the difference. This requires proving both that the negligence caused the loss and what the loss is actually worth, which is often the harder half of these cases.

Quantifying that difference usually requires expert evidence - a valuator for a botched appraisal, an accountant for a tax-related loss, another lawyer for a legal malpractice claim - because courts need a reliable, fact-based way to compare the two scenarios rather than speculation. Damages are also reduced for any failure on your part to reasonably mitigate the loss once it became apparent, and further reduced if you're found partly at fault under Ontario's contributory negligence rules. Speculative or unproven losses, and amounts you would have incurred anyway regardless of the negligence, generally aren't recoverable.