The daily budget is the amount a campaign spends on average per day. The key word is "average" – advertising systems may exceed the limit on individual days if they expect higher performance, and even it out on other days, while keeping to the monthly cap. This catches firms out especially with short-term campaigns, where no evening-out occurs. When splitting a budget between campaigns, a rule applies: too low an amount on a campaign with automated bidding won't let the system collect enough data, and performance will stay weak – in that case it is better to run fewer campaigns with a higher budget. A budget constraint also shows up in the impression share report, which states what portion of opportunities was lost specifically due to insufficient budget.
See also: Budget pacing, Impression share, Smart bidding (automated bidding).
