How WPM and accuracy are computed
The timer runs while you type the passage shown on screen, and each keystroke is compared with the passage character by character. One word counts as five keystrokes; spaces count as keystrokes too, so gross WPM is keystrokes typed divided by five, divided by minutes elapsed. Accuracy is the share of keystrokes that matched the passage, and net WPM is gross WPM times accuracy: errors cost you twice.