How many visitors do I need before my conversion rate means anything?
Short answer
As a working rule you need enough traffic to produce at least 25 to 30 orders in the period before a conversion rate is stable enough to act on, and closer to 100 before a small change in it is trustworthy. At 200 sessions and a 2% rate you are working with four orders — one extra sale moves the rate by half a percentage point, which is larger than most of the changes people try to explain. Below that volume, judge the count of orders and revenue directly rather than the ratio, and compare months rather than days.
Why small numbers mislead
A conversion rate is a ratio, and the smaller the numerator the more each individual sale moves it. With four orders, gaining or losing one shifts the rate by 25% of itself.
That variation is ordinary randomness, not a change in the business. Chasing it produces confident explanations for events that never happened, which is worse than having no measurement at all.
A rough test you can do in your head
Count the orders in the period, not the sessions. If the number of orders is below about 25, treat the conversion rate as indicative only.
Then ask what the figure would have been if one sale had landed an hour later, on the other side of midnight. If that changes your conclusion, you do not have a conclusion.
What to measure instead until you have the volume
- Orders and revenue as counts, over whole months. Absolute numbers do not have the ratio problem.
- Sessions on their own, which usually have far more volume than orders and so stabilise much sooner.
- Longer windows. A quarter against the previous quarter is often readable when a week against a week is not.
The same limit applies to anomaly detection
Any tool that flags an unusual day compares today against the spread of previous days. At low volume that spread is enormous, so either everything looks normal or everything looks alarming.
A tool reporting "nothing unusual" on six visits a day is not reassuring you — it cannot tell. The honest position at that volume is that the question is not yet answerable, and it is worth knowing which one you are being told.