Bid cap is a Meta bid strategy that sets the maximum amount you will bid in each auction. It is a control on bidding, not on cost: Meta states a bid cap does not reflect the cost you actually end up paying, and that bid controls are not directly related to your costs.
What it controls, and what it doesn't
A bid cap caps how aggressively Meta can bid, so the ad set can only win auctions where a conversion is predicted at or under that bid. Set it below what the market clears at and Meta loses most auctions, the ad set underdelivers, and, starved of conversions, it stays learning limited. The cap does not steer your cost per acquisition; it throttles delivery.
When to use it
Bid cap suits advertisers who can calculate the exact bid from a known conversion rate and margin. If your goal is a cost target rather than auction-level control, the right tool is a cost cap, which targets the result. We walk through the trap in a Meta bid cap doesn't control your CAC.