[kchamplewski]

As I understand it the reasoning given tends to be that ad auctions happen extremely quickly, and therefore too quickly to check some single source of truth for your current budget.

Therefore, the budget is only "eventually consistent" - i.e. at the end of the campaign or after a day or so, the number you're being charged is accurate. However during the campaign itself it's not possible to guarantee that things won't go slightly over budget as each individual ad auction cannot feasibly check the central budget.

That said, it definitely feels like there should be a way to implement this such that it backs off as the budget is approached, so that overshoots are likely to be minor, rather than 100% of the budget as apparently happens on a regular basis.

[jakobegger]

I'm pretty sure fixing bugs that make money is on the very bottom of the list of priorities....

[hnick]

They don't need a bug fix as a stopgap. The fair thing to do would be refund the excess. If I set a limit and they display ads and exceed it, why should it be my problem?

[ganeshkrishnan]

Excess is calculated at end of month not daily

[lrem]

Back when I was in the ads org, there was no month without a big important future launch, that was supposed to make us make less money in the short term.

[danpalmer]

I suspect a factor that makes this trickier is cost per click. You're bidding for a click not a particular spend, so if your cost per click is ~$0.01 then overrunning by 100 clicks only costs $1 over budget. If you're more like $1-5 a click (not uncommon) then you can easily get to $100 over budget.

My guess is that this is worse the lower your budget, and $100 a day is a low budget for Google ads when you're considering the whole industry.

I would expect that with a budget of, say, $5000 a day, you might still be ~$100 out.

[heavenlyblue]

No. They overshoot because there’s a small-letter asterisk within the budged definition that allows then to overshoot at twice the amount.