# Churn Cohort Retention Calculator

> Track one sign-up cohort over time: retention against the starting size, churn in each period, cumulative churn and the average churn rate per period.

Category: Statistics & probability · Applies to: Worldwide · Last updated: 2026-10-29
Interactive calculator: https://calculators.geekmidst.com/calculators/churn-cohort-retention-calculator

A cohort is a group that started together, such as all customers who joined in January. Following how many are still active each period gives a retention curve that new sign-ups cannot inflate.

Also called: Cohort analysis calculator, Retention curve calculator.

## Inputs
- Cohort size at the start (default 1000)
- Still active at the end of each period (default 800, 680, 612)

## Results
- Retention at the last period
- Cumulative churn
- Average churn per period (compound)
- First-period churn
- Still active
- Lost

## Formula

```
Retention_t = A_t ÷ C;  Churn_t = (A_{t−1} − A_t) ÷ A_{t−1};  Average churn = 1 − (A_k ÷ C)^(1 ÷ k)
```

Where:
- C: cohort size at the start (A₀)
- A_t: members still active after period t
- k: number of periods

## Worked example
1. Retention: 800, 680 and 612 of 1,000 = 80%, 68%, 61.2%.
2. Period churn: 200 ÷ 1,000 = 20%, 120 ÷ 800 = 15%, 68 ÷ 680 = 10%.
3. Average churn = 1 − 0.612^(1/3) ≈ 15.10% per period.

**Result:** After three periods 61.2% of the cohort remains, an average churn of about 15.1% a period.

## What the result means

Falling period churn, as here, means the cohort is settling into a loyal core. A curve that flattens is a strong sign of product fit; one that keeps falling at the same rate is not.

## Assumptions and limits
- The cohort is closed: reactivated and new members are excluded.
- Periods are equal in length.
- Covers one cohort; compare several cohorts to see whether retention is improving.
- Revenue retention (expansion and downgrades) is a different measure.

## Questions

### Why use a compound average churn?

Churn applies to a shrinking base. The compound rate is the constant churn that would leave the same number active after k periods, so it reproduces the result exactly.

### How is this different from a retention-rate calculation?

A single-period retention rate starts from the customer count at the start and subtracts new customers. A cohort follows the same people across many periods.

## Related calculators
- [Customer Retention Rate Calculator](https://calculators.geekmidst.com/calculators/customer-retention-rate-calculator)
- [Subscription Churn Calculator](https://calculators.geekmidst.com/calculators/subscription-churn-calculator)
- [Time Series Growth Calculator](https://calculators.geekmidst.com/calculators/time-series-growth-calculator)

Source: GM¹X CalcVerse — https://calculators.geekmidst.com/calculators/churn-cohort-retention-calculator