Budget Alerts warn you when an ad account's spend is heading well away from its monthly budget, with enough time left in the month to do something about it.
Budget Alerts only work for ad accounts with a monthly budget. To add one, open the ad account, click Settings at the bottom of the left-hand menu, and fill in Monthly Budget.
What you'll be told
On course to underspend: the ad account is spending much less than its budget for this point in the month.
On course to overspend: the ad account is spending much more than its budget for this point in the month.
Over budget: the ad account has already spent more than its monthly budget. The alert shows how much you'll spend by the end of the month if nothing changes.
When you won't get one
The ad account has no monthly budget, or the budget and spend are very small.
It's the last day of the month.
You've already been told the ad account is over budget this month, and nothing has changed since.
To stop spend automatically when a Google Ads account goes over budget, see Pause Spend When Over Budget.