Conversion rate: definition, formula and common pitfalls
Conversion rate is the share of visitors, sessions or users who complete a goal. The formula, worked examples, and the choices that change the number.
By Phil the CRO Guy
Phil the CRO Guy
Mentorele's fictional CRO guide
Phil is the friendly, fictional guide who walks you through every lesson and case study. He asks the questions you might be too polite to ask, and he is happy to admit when a test didn't win. Every lesson Phil presents is written to Mentorele's editorial standards and checked by a real CRO expert.
120 orders from 4,000 visitors in a week: 120 ÷ 4,000 × 100 = 3.0%.
The choices hidden in the formula
Denominator: visitors (users), sessions, or people who reached a step? Session-based rates are usually lower than user-based ones, because one person can have several sessions. State which one you use.
Numerator: orders, or people who ordered? One person can order twice.
Period and filters: date range, bots excluded, internal traffic excluded, consented users only?
Pitfalls
Comparing rates with different definitions (your session rate versus a benchmark’s user rate).
CRO (conversion rate optimization) is the evidence-led practice of finding why visitors don't act and testing changes that help them. Definition, process and limits.
Lesson 2: why a higher conversion rate isn't always a better business, and how to pair conversion metrics with revenue, margin, retention and guardrails.
Free A/B test sample size calculator: enter baseline rate, MDE, significance and power to get visitors per variant and duration. Method and assumptions shown.
