Share of Voice and Market Share: The Relationship Every Marketer Must Understand

The Most Important Equation in Marketing

In 1990, researchers at the Institute of Practitioners in Advertising (IPA) established one of the most commercially significant findings in marketing science: brands that maintain a share of voice (SOV) greater than their share of market (SOM) consistently grow their market share over time. Brands that allow their SOV to fall below their SOM consistently lose market share.

This relationship — known as excess share of voice, or ESOV — is not a theory. It is an empirical observation drawn from decades of data across hundreds of brands and categories. It is one of the few genuinely predictive relationships in marketing, and it provides one of the strongest evidence-based arguments for sustained brand investment.

At the Northern School of Marketing, we teach ESOV as a cornerstone of the RAMMS framework's Business Value phase. Understanding it is essential for any marketer who needs to build a commercial case for brand investment.

What Is Share of Voice?

Share of voice (SOV) is your brand's proportion of total advertising or marketing spend in your category. If the total marketing spend in your category is £10 million per year and your brand spends £2 million, your share of voice is 20%.

In the digital era, SOV can be measured across multiple dimensions:

SOV Dimension	What It Measures
Paid media SOV	Your share of total paid advertising spend in the category
Organic search SOV	Your share of organic search impressions for category keywords
Social media SOV	Your share of social media mentions and engagement in the category
Share of search	Your brand's proportion of total search volume for brand terms in the category

Share of search — the proportion of total branded search queries that go to your brand — has emerged as a particularly useful proxy for brand equity and future market share. Research by Les Binet and others has shown that share of search is a leading indicator of market share change.

What Is Share of Market?

Share of market (SOM) is your brand's proportion of total category revenue or volume. If total category revenue is £100 million and your brand generates £20 million, your market share is 20%.

The ESOV Relationship

The excess share of voice (ESOV) model states that:

If SOV > SOM → market share will grow If SOV = SOM → market share will be stable If SOV < SOM → market share will decline

The rate of market share change is proportional to the size of the ESOV gap. Brands with significantly higher SOV than SOM grow faster; brands with significantly lower SOV than SOM decline faster.

Research by the IPA and Thinkbox suggests that for every 10 percentage points of ESOV, a brand can expect to grow its market share by approximately 0.5 percentage points per year. This is a modest but consistent effect that compounds significantly over time.

Why ESOV Works

The mechanism behind ESOV is straightforward: marketing activity builds mental availability — the probability that a customer will think of your brand when they are in a buying situation. Brands with higher SOV are more present in customers' minds, which translates into higher consideration, preference, and ultimately purchase.

This is the core insight of the Ehrenberg-Bass Institute's work on how brands grow: market share is primarily determined by mental availability (how easily customers think of you) and physical availability (how easily they can buy you). SOV is the primary driver of mental availability.

Calculating Your ESOV Position

To calculate your ESOV position:

1. Estimate total category advertising spend (industry reports, competitive intelligence tools, or media agency data)
2. Calculate your brand's share of that total spend
3. Calculate your brand's current market share
4. Subtract SOM from SOV to get ESOV

A positive ESOV means you are investing above your market share and can expect to grow. A negative ESOV means you are under-investing relative to your market position and are at risk of decline.

Using ESOV to Build the Business Case for Brand Investment

The ESOV model is one of the most powerful tools available for building the business case for brand investment, because it:

Quantifies the cost of under-investment. If your SOV is 5 percentage points below your SOM, you can model the expected market share decline and translate it into revenue at risk. This makes the cost of cutting brand investment concrete and financially meaningful.

Sets evidence-based investment targets. Rather than negotiating budget based on historical precedent or gut feel, you can use the ESOV model to calculate the investment required to maintain or grow market share.

Provides a long-term framework. The ESOV relationship operates over years, not quarters. This helps boards understand why brand investment requires sustained commitment rather than tactical on-off spending.

Connects marketing spend to revenue outcomes. The ESOV model provides a credible, evidence-based link between marketing investment and commercial outcomes — exactly what boards need to see.

The Limitations of ESOV

The ESOV model is powerful but not infallible. Its limitations include:

Category variation. The relationship between ESOV and market share growth varies by category. In some categories, the effect is stronger; in others, it is weaker. The 0.5 percentage point per 10 ESOV points is an average, not a universal rule.

Quality of spend. SOV measures quantity of spend, not quality. A brand that spends its SOV budget on ineffective creative or poorly targeted media will not see the expected market share growth.

Distribution effects. Physical availability — how easily customers can buy your product — interacts with mental availability. A brand with high SOV but poor distribution will not convert mental availability into market share.

Competitive response. If competitors increase their SOV in response to your investment, the ESOV advantage narrows. The model assumes a relatively stable competitive landscape.

Despite these limitations, ESOV remains one of the most robust and commercially useful frameworks in marketing science.

Applying ESOV Within the RAMMS Framework

The RAMMS Business Value phase requires marketing teams to connect their investment decisions to commercial outcomes. ESOV provides the mechanism for doing this in the context of brand investment:

• Track your SOV and SOM quarterly
• Calculate your ESOV position and model the expected market share impact
• Use this model to set investment targets and justify budget requests
• Report on SOV trends alongside revenue and market share data

This approach transforms brand investment from a discretionary spend into a strategically managed asset — one that can be grown, protected, and measured with the same rigour as any other commercial investment.

For further reading, see our articles on brand equity valuation: measuring the intangible worth of your brand and the business case for marketing: how to present marketing to the board.