How Facebook's Ad Auction Works (And Why Your Bid Strategy Matters)

Facebook doesn't let you "pick" your cost per click.

It's an auction.

Your bid is one factor. But not the only one.

Understanding the auction means understanding how to win cheaply.

How the Auction Works

When your ad is eligible to show:

1. Facebook calculates: Your bid × Estimated conversion rate × Relevance score

2. Whoever scores highest wins the auction

3. You pay the second-place bid (not your bid)

Key insight: You don't pay your bid. You pay what you need to outbid the competition.

Example:

• Your bid: £2 × 30% conversion rate × 8 relevance = £4.80 total score

• Competitor bid: £1.50 × 20% conversion rate × 8 relevance = £2.40 total score

• You win, but only pay £2.50 (what you need to beat competitor)

The Three Factors

1. Your Bid

• Higher bid = higher chance of winning

• But not the only factor

2. Estimated Conversion Rate

• Facebook machine learning predicts: Will this person convert?

• Based on audience + creative quality

• Better creative = higher predicted conversion = lower cost to win

3. Relevance Score

• 1–10 scale

• Based on how relevant your ad is to the audience

• Higher relevance = lower cost per result

The Strategy: Optimise Score, Not Just Bid

Most people think: "I'll just bid higher."

Smart approach: Optimise score while bid stays reasonable.

How to improve relevance score:

1. Better targeting (narrow audience)

2. Better creative (ad quality)

3. Better landing page (users actually convert)

If you improve relevance score from 5 to 8:

• Same bid: Cost drops by 30%

• Better results: Conversion rate improves

Bidding Strategies

Manual bidding:

• You set bid (e.g., £2)

• More control

• Requires monitoring

• Best if: You know your data

Automated bidding:

• Facebook optimizes for you

• Less control

• Set a bid cap, Facebook does rest

• Best if: Starting out, limited data

For most service businesses: Start manual, switch to automated after 2 weeks of data.

The Bid Cap Strategy

Set maximum cost per result you're willing to pay.

Example: "Don't spend more than £50 per lead"

Facebook optimises to get as many leads as possible under £50.

What to Do This Week

Check your current bid strategy.

Are you manual or automated?

If manual: Track your cost per result. If it's trending up, switch to automated.

If automated: Make sure bid cap is set (not unlimited).

Small adjustment, big impact on profitability.