How Google moves the money
There is no fixed split between the campaigns. Google takes budget a campaign isn't using and reallocates it to campaigns that are limited by budget and have enough traffic to spend it. Google's example is $100 a day across two campaigns: if one spends only $40, the leftover $10 of its half can go to the other. In practice, the campaigns that enter the most auctions end up with the most money.
When to use one
Shared budgets suit campaigns that share the same goal, where a conversion from any of them is equally welcome. Google recommends pairing them with portfolio bid strategies, and reports that Search advertisers using the two together see about 13% more conversions on average. They are available for Search, Shopping, Display and Video campaigns, but not for Performance Max, App or Smart Shopping campaigns, Hotel campaigns with a Commission bid strategy, or campaigns using a total budget. Demand Gen isn't on Google's supported list.
Shared budget vs portfolio bid strategy
They solve different problems. A shared budget pools the money several campaigns can spend. A portfolio bid strategy pools the bidding: one strategy and target, such as a target CPA, applied across several campaigns. You can use either alone, and Google lets you link the two so the same group of campaigns shares both the budget and the bidding.
The catch
The money follows whichever campaign can spend it, not whichever matters most. A campaign that finds fewer auctions can end up with little spend and look like it failed. Keep separate budgets for must-fund campaigns and tests, and merge campaigns that overlap on the same searches. More in whichever campaign finds the auctions first gets the money.
Common questions
Can Performance Max use a shared budget?
No. Shared budgets are available for Search, Shopping, Display and Video campaigns, and aren't compatible with Performance Max, App or Smart Shopping campaigns, Hotel campaigns with a Commission bid strategy, or campaigns using a total budget.
Does a shared budget favor my most important campaign?
No. Google reallocates unused budget to campaigns that are limited by budget and can spend it. It doesn't know which campaign you'd rather fund.
How does Google decide which campaign gets the money?
It moves budget a campaign isn't using to campaigns that are limited by budget and have enough traffic to spend it. There is no fixed split, so the campaigns that find the most auctions take the most.
Is a shared budget the same as a portfolio bid strategy?
No. A shared budget pools money across campaigns; a portfolio bid strategy applies one bid strategy and target across campaigns. Google recommends using them together for campaigns that share the same goal.
Should brand campaigns go on a shared budget?
Usually not. A brand campaign has to be funded no matter what, and in a shared pot it competes with every other campaign for the same money. Give it its own budget.