Definition
Checkout abandonment rate is the share of shoppers who begin checkout but don't finish. It's narrower than cart abandonment: these shoppers had already decided to buy and something in checkout stopped them.
Formula
Checkout abandonment rate = (checkouts started − orders completed) ÷ checkouts started × 100.
Where to look
Break checkout into steps (contact details, shipping, payment, review) and track each as an event. The step with the biggest drop is the one to fix first. Look at payment failures separately: a declined card is a different problem from a shopper who changed their mind.