Churn rate formula and how to calculate it correctly

The churn rate formula is `(customers lost during the period ÷ customers at the start of the period) × 100`. In plain English, you count how many customers left during a defined time window, divide that by the customers you started with, and express the result as a percentage. Use that formula when you want standard customer churn for a report, dashboard, or investor update. If you change the time window or the customer base, you are no longer measuring the same thing, so the result won’t compare cleanly across periods.

The churn rate formula

Use this formula:

churn rate = (customers lost during the period ÷ customers at the start of the period) × 100

The numerator is the number of customers who left during the time window you chose. The denominator is the customer count at the start of that same window. Multiplying by 100 turns the result into a percentage, which makes it easier to report and compare.

For example, if you start the month with 200 customers and lose 8 by the end of the month:

8 ÷ 200 = 0.04

0.04 × 100 = 4%

So your monthly churn rate is 4%. That means 4 out of every 100 starting customers left during that month.

What to define before you calculate it

Before you calculate anything, define two things clearly: the time window and the customer base.

The time window can be a month, quarter, or year, but it has to be fixed. You should not mix customers lost in one period with a customer count from another.

The customer base is usually the customers you had at the start of that period. That is the cleanest way to keep the formula consistent.

If you use a different base, say so explicitly. Otherwise, the number may look like churn but mean something else.

Customer churn is not the same as revenue churn. Customer churn counts how many customers leave. Revenue churn looks at lost recurring revenue, so a few large accounts can create high revenue churn even if customer churn is low.

The mistakes that make churn look wrong

The most common mistake is changing the denominator to include new customers added during the period. That changes the metric and usually makes churn look lower than it really is.

Another trap is using inconsistent time windows, such as counting customers lost over 30 days but dividing by a customer base taken from mid-month or quarter-end. The numerator and denominator need to describe the same period.

A third mistake is using the wrong denominator for the question you are trying to answer. If you want churn among starting customers, use starting customers. If you want a different metric, define it separately instead of bending the churn formula to fit it.

Where the formula does not apply

This formula works for standard customer churn, where the question is how many of the starting customers left during the period.

It does not apply cleanly if you want to measure retained revenue, expansion, or net revenue change. Those need different definitions and different formulas.

It also stops being comparable if teams calculate it with different customer bases or different time windows. A monthly churn rate and a quarterly churn rate are both valid, but they are not interchangeable.