A bid simulator is a Google Ads tool that estimates what would have happened if you had used a different bid, target or budget. It replays recent auctions with the alternative setting and shows the likely change in impressions, clicks, cost and conversions, so you can test a decision before spending money on it. Think of it as a quick, free sense check rather than a forecast you can bank on.
How the bid simulator works
Simulators appear at keyword, ad group and campaign level, and for bidding strategies such as Target CPA and Target ROAS. Budget simulators work in the same way for daily budgets. Where one is available, a small graph icon appears next to the bid, target or budget, and clicking it opens a curve of estimated results.
Google builds the estimate by re-running recent auctions with the alternative value, taking into account the competition, Quality Score and the other ads that took part. At the time of writing (October 2026), Google’s help pages describe simulations as based on roughly the last week of auction data. The results are estimates of the recent past, not forecasts of the future.
A simulator only appears when there is enough data. New campaigns, low-volume keywords, recently changed settings and some campaign types may show no simulation at all.
Why it matters
Bidding decisions are hard to reverse cheaply. Doubling a target CPA to see what happens can cost a month’s budget before you learn anything. A simulator gives you a rough sense of the trade-off first: perhaps raising a bid from £2 to £3 brings 15% more clicks but costs 60% more. That is the shape of diminishing returns, and seeing it before acting saves money.
It is also a useful check on enthusiasm. If an agency or a Google recommendation suggests raising targets, the simulator shows whether the extra conversions are worth their cost at the margin, which connects directly to marginal ROAS.
Budget simulators answer a related question. If a campaign is limited by budget, the simulator can show roughly how many more conversions a bigger budget might buy and at what cost. That is useful when you need to justify, or argue against, extra spend to a finance director or business partner.
Common mistakes
- Treating the estimate as a promise. Competitors change their bids, seasons shift and the simulator cannot see either coming.
- Reading a simulation from an unusual week, such as Black Friday or a bank holiday, as typical.
- Looking only at extra conversions, not the extra cost of each one.
- Assuming a missing simulator means a setting has no effect. Usually it means too little data.
- Making a large change on the strength of a simulation and not checking actual results afterwards.
How to act on it
Before any significant bid, target or budget change, open the simulator and look at the curve, not just one point on it. Work out the cost of the extra conversions: if £300 more spend brings five more, each costs £60. Compare that with what a customer is worth to you.
Then make the change in moderate steps and compare real results with the estimate after two to four weeks. Where you need more certainty, a campaign experiment tests the change against a control. Reading simulators against real margins is part of how I approach PPC management.