A responsive search ad (RSA) is the standard ad format in Google Ads Search campaigns. Instead of one fixed ad, the advertiser supplies a pool of headlines and descriptions, and Google assembles a combination for each search.
That turns ad writing into a pool design problem. General copywriting structure is covered in How to Write Ad Copy. This guide covers the mechanics specific to RSAs.
RSA Limits
According to Google Ads’ responsive search ads documentation:
| Field | Maximum number | Character limit |
|---|---|---|
| Headline | 15 | 30 |
| Description | 4 | 90 |
| Display URL path | 2 | 15 each |
What Shows When the Ad Serves
Headline position 1 and description position 1 always appear. Headline 3 and description 2 appear only when Google predicts they will help and when there is room on the screen, which is less often on mobile.
The combination math
With 15 headlines and 4 descriptions, the number of ordered combinations using three headlines and two descriptions is 15 × 14 × 13 × 4 × 3 = 32,760. Google doesn’t test them evenly. It predicts which assets fit which searches and favors those.
The math also shows why variety matters more than volume. Fifteen near-synonyms produce thousands of combinations on paper that all say the same thing.
What Pinning Actually Costs
Pinning locks an asset to a position. Assets pinned to headline 1, headline 2, or description 1 always show. Assets pinned to headline 3 or description 2 are not guaranteed to show.
Pinning a single headline to position 1 leaves 14 headlines for the other two slots. Ordered combinations drop to 14 × 13 × 4 × 3 = 2,184, about fifteen times fewer than an unpinned pool.
When Pinning Is Really Necessary
Pinning should be a constraint, not a preference. Use it when an asset must appear in every impression:
- Legal disclosures or licensed entity names that must show for every ad in regulated fields
- Brand guidelines that require the brand name in every ad impression
- Time-sensitive offers with an expiration date that the campaign runs through
Pinning in regulated industries
Financial services, healthcare, and legal advertisers in markets such as the US and UK often need a disclosure or a licensed entity name in every ad. Pinning is unavoidable there, but it doesn’t have to mean a single locked line.
Pinning three differently worded headlines that each carry the required element to the same position keeps compliance and restores some variation. Position 1 then has three options, and the remaining slots draw from 12 headlines. That gives 3 × 12 × 11 × 4 × 3 = 4,752 combinations.
The 30-Character Limit and Headline Structure
English copy runs close to the 30-character limit faster than it appears. Benefit-driven headlines need a verb and often a number or concrete detail. A headline like “Process Payroll in 5 Minutes” is already 29 characters.
This limit affects how headlines pair. Google displays headlines 1 and 2 on the same line, separated by a vertical line. Two headlines that read as a single message when shown together—like “Payroll Software” and “For Growing Startups”—need to work independently if they appear separately. Without pinning, each headline must stand alone.
The tight limit also means some concepts need to split across headlines or skip certain details. A full phrase like “Automatic Tax Filing Included” requires all 29 characters. “Automated Compliance” uses only 20, freeing space for another angle.
Ad Strength Is a Checklist, Not a Score
Ad Strength rates how complete and varied an RSA’s asset pool is and how relevant it is to the keywords. Google’s documentation says advertisers who improve Ad Strength from “Poor” to “Excellent” see 15% more clicks and conversions on average. It also recommends at least two RSAs rated “Good” or “Excellent” per ad group.
Ad Strength measures inputs, not outcomes. A pool padded with keyword-stuffed variants can reach “Excellent” and still underperform a “Good” ad with sharper messages. Decisions about which assets to keep should come from the asset and combinations reports.
Building a Varied Headline Pool
Grouping headlines by job is the simplest way to force variety. The examples below are for payroll software.
| Headline job | Example | Characters |
|---|---|---|
| Keyword | Payroll Software for Startups | 29 |
| Benefit | Run Payroll in Under 5 Minutes | 30 |
| Proof | Rated 4.8 by 3,000 Reviewers | 28 |
| Offer | 30-Day Free Trial, No Card | 26 |
| Differentiator | Automatic Tax Filing Included | 29 |
English copy runs close to the 30-character limit faster than it seems, because benefit-led headlines need a verb and a number. Two or three headlines per job fill the pool without repeating a message. The review figure above is illustrative. Real ads need claims that can be substantiated.
Common Mistakes
Pinning every asset to a fixed position turns the RSA into a static ad and removes the system’s room to learn.
Replacing a “Low” rated asset after a few days is another. Ratings based on too few impressions are unreliable.
A third is putting an offer in a headline that the landing page doesn’t repeat. That gap hurts both Quality Score and conversion rate, as covered in What Is Landing Page Optimization?.
Summary
A responsive search ad builds a combination for each search from a pool of up to 15 headlines and 4 descriptions. Headline 1, headline 2, and description 1 always show. Pinning guarantees placement at the cost of most combinations. Pinning several variants to the same position is a better compromise for regulated copy. Ad Strength checks the pool’s completeness, while real decisions come from asset performance data.