The England Under-21 central defender will undergo the examination with the Barclays Premier League newcomers before sealing his move to St Andrews.
Villa boss Martin O'Neill confirmed: "It should all go to plan if he passes a medical tomorrow."
Blues boss Steve Bruce has been trying to sign Ridgewell since the start of the year.
Birmingham had a £2million bid rejected in January and also failed with an attempt to take Ridgewell on loan before the transfer window closed after the departure of Matthew Upson to West Ham.
Bruce has continued to monitor Ridgewell's situation and a fee of around £2.5million is finally set to persuade Villa manager O'Neill to part with the player.