Understanding Price Granularity in Prebid: A Practical Guide for Publishers

Managing header bidding on your site means making key choices that impact both ad operations and revenue. One of these foundational choices is how to handle price granularity—essentially, how finely you define bid price buckets for your line items.

Without getting this right, you risk either overwhelming your ad server with unnecessary line items or leaving money on the table through imprecise bid matching. This article breaks down price granularity so you can find the right balance for your business.

What is Price Granularity and Why Does it Matter?

Price granularity is the way you group bid prices from header bidding demand partners into set buckets. This mapping directly affects how many line items you need to create in your ad server and determines how closely winning bid prices match what buyers are actually willing to pay.

How Price Granularity Works in Prebid Auctions

When a user visits your site, Prebid queries multiple demand partners in real time. Each partner sends a bid, sometimes to the penny or even sub-cent. But your ad server must match those bids to pre-defined line items with fixed CPM values.

If your line items are set at $1.00 increments and you get a $1.80 bid, that bid is mapped (rounded down) to the $1.00 bucket on your ad server. Unless there’s an ad server bid between $1.00 and $1.80, the header bidding demand wins. However, if there’s a $1.10 direct campaign set up, you lose revenue even though a higher bid came in. Tighter price buckets (e.g., $0.10 increments) allow more precise competition, but increase the operational workload.

Key Components of Price Granularity

Configuring price granularity involves four parts: increments, cap, range, and buckets. Getting these details right sets up your header bidding for success.

Increment

This is the step size between each price bucket. For example, a $0.10 increment produces buckets like $0.00, $0.10, $0.20, and so on. Smaller increments mean finer competition—but more line items to manage.

Cap

This defines the maximum price tracked by your buckets. If your cap is $5.00 with a $0.10 increment, your last bucket is $5.00. Bids above the cap are rounded down to the cap. For video or high-value inventory, caps often need to be set much higher than standard display.

Range and Multiple Increments

Ranges let you use different increments at different CPM levels. For instance, using $0.05 increments for bids below $5.00 and $0.50 above that can optimize precision where it’s needed while controlling line item sprawl.

Buckets (and the Role of Rounding)

Each bucket represents a line item price. Prebid rounds bids down to the nearest bucket, which the ad server then matches. For example, a $2.75 bid with $0.50 increments is rounded to $2.50. Importantly, rounding doesn’t reduce what you get paid—the winning SSP pays their original, unrounded bid—but it can affect which bid wins the impression if a direct or programmatic deal is priced between buckets.

Prebid’s Built-in and Custom Granularity Options

Prebid.js and Prebid Server offer a selection of predefined granularities, as well as full support for custom configurations. Your choice should reflect both your typical bid distribution and operational constraints.

Built-In Options

Prebid provides low, medium, and high presets:
– Low: $0.50 increments up to $5.00—requiring only 11 line items per bidder, but less precision.
– Medium: $0.10 increments up to $20.00—offers a common compromise, with 201 line items per bidder.
– High: $0.01 increments—ultra-fine, over 2,000 line items for high caps, maximizing price precision at the cost of extreme complexity.
There are also ‘auto’ and ‘dense’ options, which use sliding scales: increments become coarser as bid values increase. For most web display setups, medium or auto provide enough balance.

Custom Granularity

Custom setups are best when you want complete control, such as for high-value video inventory or non-standard currencies. Analyze historical bid data—cluster bids with fine granularity in high-competition CPM ranges, and use broader increments elsewhere. Don’t use more precision than needed below your price floor; it’s just extra work with no revenue gain.

Currency Nuances

Prebid’s defaults are tuned for currencies like USD, EUR, etc. If you work in currencies with smaller units (e.g., JPY, INR), you’ll need to adjust granularities using multipliers or custom settings to prevent buckets from missing valid bids.

Operational Tradeoffs: Finding the Right Balance

Choosing your price granularity is a balancing act between revenue optimization and technical or operational overhead. Too granular, and you overwhelm your ad server and trafficking teams. Too loose, and you leak revenue through imprecise matching.

Practical Scenarios

– High granularity ($0.01 increments): Excellent for maximizing revenue in theory, but can mean thousands of line items—and this multiplies across bidders. For a site with 10 bidders and $20.00 cap, that’s at least 2,000 line items per bidder.
– Low granularity ($1.00 increments): Easy to set up (just a few line items per bidder), but you risk letting lots of higher bids get mapped too far down, leaving revenue gaps.
– Most publishers find medium or auto granularity is the practical starting point. Adjust after reviewing line item performance, especially if you detect significant rounding gaps in auction logs.

Special Cases: Video and Mobile

Video often needs custom, higher caps—since $20.00 bids are common. Mobile in-app setups require price buckets to be set via Prebid Server, as Prebid Mobile SDK does not control granularity on-device.

What this means for publishers

Price granularity directly impacts operational efficiency, auction fairness, and revenue. Underestimating its importance can lead to two main issues: 1) administrative bloat from excessive line items that become unmanageable as you add bidders, and 2) lost revenue if your price mapping is too loose and fails to capture the highest possible value from competing demand. Auditing your price bucket setup regularly and aligning it with how bidders actually price your inventory is crucial for best results.

Practical takeaway

Start with Prebid’s medium or auto granularity presets, as they offer a reasonable balance between revenue precision and operational overhead. Only adopt fine-grained custom buckets if your auction data shows clear revenue left on the table due to rounding—or if you operate in markets where standard increments don’t fit prevailing bid behavior or currency norms.

Work closely with your ad ops and engineering teams to ensure your ad server’s line items align exactly with Prebid’s price buckets to avoid mismatches. Reassess your configuration periodically as your inventory mix or demand landscape evolves, and be proactive about adjusting settings for high-value video or mobile placements. Small changes in granularity can translate to significant shifts in both revenue and workflow complexity.