Definition
Incremental conversion rate isolates the true impact of an email campaign by subtracting conversions that would have happened anyway — even without the email. It is calculated using a holdout group or control group that does not receive the campaign.
For example, if 100 recipients convert but a control group shows that 80 would have purchased anyway, the incremental conversion rate is based on 20 additional conversions. This metric prevents over-attributing organic behaviour to email.
Formula
Incremental Conversion Rate = ((Campaign Conversions − Baseline Organic Conversions) / Delivered) × 100
Why It Matters
Standard conversion rate double-counts natural purchases that just happened to occur during a campaign window. Incremental conversion rate reveals how many sales email genuinely drove. This is particularly important for high-frequency senders and brands with strong organic demand.
Related Glossary Terms
A/B Testing
A/B testing in email marketing is the practice of sending two variations of an email to a small sample of your list to determine which version performs better before sending the winner to the remaining subscribers.
Abandoned Cart Email
An abandoned cart email is an automated message sent to customers who added items to their online shopping cart but left without completing the purchase. It is one of the highest-converting email types in ecommerce.
ARPU (Average Revenue Per User)
ARPU (Average Revenue Per User) is a metric that measures the average revenue generated per email subscriber over a specific period, used to evaluate list value and campaign effectiveness.
Attention Rate
Attention rate is the percentage of email opens that last longer than 5 seconds, distinguishing genuine reads from passive opens, preview-pane views, or Apple MPP auto-loads.
Bounce Rate Threshold
A bounce rate threshold is the percentage of hard or soft bounces at which email platforms and ISPs begin to penalise sender reputation, typically around 2-5%.
Bounce Rate
Email bounce rate is the percentage of emails that were rejected by the receiving server before reaching the recipient. It is a key indicator of list health and data quality.