Can you tell exactly how much revenue email generated last month? Not "approximately" — exactly?

Most eCommerce brands say "email marketing works" but cannot put a specific number on it. They know campaigns went out. They know open rates were decent. They have a vague sense that email contributes to revenue. But when someone asks "how much revenue did email generate last month?" the answer is either a guess or a number pulled from one platform that contradicts what another platform says.

This is a measurement problem, not a performance problem. Klaviyo shows one revenue number. GA4 shows another. Shopify shows a third. Each uses different attribution windows, different models, and different definitions of "email revenue." Without understanding these differences and picking a consistent measurement framework, you cannot make informed decisions about email investment.

This guide shows you how to set up email ROI tracking that gives you one trustworthy number. What tools to use, which attribution model to follow, and how to calculate true email ROI including all costs — not just the easy-to-see platform fees.