By introducing the new Average Daily Budget feature, Facebook changed the way daily budgets of campaigns are treated.
How does it affect campaign spend?
Prior to this change, when advertisers have chosen to create campaigns (or ad sets) with daily budgets, Facebook was attempting to spend exactly that daily budget amount each day. For example, if you set a daily budget of $10, Facebook was trying to spend exactly $10 every day — even though the opportunities to show ads to people on Facebook who’ll be the most valuable to the advertisers will vary from day to day.
With this new feature, Facebook will try to get you roughly your daily budget's worth of the result you optimized for every day. However, there may be certain days that are great opportunities to get you lots of that result. On those days, Facebook may spend up to 25% over your daily budget.
For example, if your daily budget is $10, Facebook may spend up to $12.50 on a given day.
However, your charges will average out over a calendar week. (A calendar week is Sunday to Saturday.) If your campaign spends more on a day with lots of opportunities for results, Facebook will make it spend less on a day with fewer.
To continue our example from above, Facebook wouldn't go over $70 in one calendar week even if, on some of those days, they spent over $10. (But again, they wouldn't spend more than $12.50 in a single day).
What happens when I change my daily budget in the middle of the week?
If you change your budget in the middle of the week, FB calculates a new amount to spend for the rest of that week, based on the new budget and the amount of time remaining for that week. For example, if you run an ad set Sunday to Tuesday with a daily budget of $10, then switch it to $20 at 12am on Wednesday, we'll switch from aiming to spend $70 over 7 days, to $80 over 4 days (the amount of days left in a calendar week when you changed your budget). Note that the actual amount spent from Sunday to Tuesday does not affect what FB'll aim to spend Wednesday through Saturday. Though the two time periods fall within one calendar week, they're independent of each other when it comes to spending your budget because of the change.
What happens when I change my daily budget in the middle of the day?
If you change your budget in the middle of the day, FB aims to spend a prorated amount for the rest of that day. For example, if you increased your budget from $100 to $200 at noon, FB'd aim to spend $100 for the rest of that day. This is because the day is half over, so we divide the $200 by 2. If, however, you decreased your budget from $100 to $50 at noon, FB'd aim to spend $25 for the rest of that day. This is because the day is half over, so we divide $50 by 2.
Note that this prorated spend is what FB aims for, but isn't guaranteed. The only guarantee on the day of the change is that FB won't spend more than 125% of the highest daily budget in place for your ad set that day. For the rest of the week after the change, FB won't spend more than your new budget multiplied by the number of days remaining in the week, regardless of how much gets spent on the day of the change. For example, if you change your budget from $100 to $50 at noon on a Wednesday, FB won't spend more than $175 between noon on Wednesday and Saturday ($50 multiplied by 3.5 days).
How does billing work when I change my daily budget?
An amount spent before you change your budget settings (ex: pausing a campaign, changing the budget amount, changing the delivery schedule) is billable if it's within 125% of what you had set for your daily budget, even if it's not within 125% after the change.
Please remember that this feature cannot be switched off.