# Churn Rate Calculator

> Computes the customer churn rate for a period from customers at the start and customers lost, the retention rate, the ending customer count and net growth including customers gained, the annualised churn rate and the implied average customer lifetime (1 / churn).

- Calculator id: `churn-rate` · Category: Business & Marketing (`business`) · Tool name: `calculate_churn_rate`
- Canonical page: https://tttkmbb.com/business/churn-rate · This document: https://tttkmbb.com/business/churn-rate.md · JSON definition: https://tttkmbb.com/business/churn-rate.json

## Purpose

Computes the customer churn rate for a period from customers at the start and customers lost, the retention rate, the ending customer count and net growth including customers gained, the annualised churn rate and the implied average customer lifetime (1 / churn).

**Use when:** You track subscribers, members or accounts over a month, quarter or year and need churn, retention or expected customer lifetime.

**Do not use when:** You need revenue churn in currency terms or MRR projections (use mrr-growth) or the full acquisition economics (use cac-ltv).

## Input

| Parameter | Type | Unit | Required | Description |
| --- | --- | --- | --- | --- |
| `customers_at_start` | integer |  | required | Active customers on the first day of the period. (> 0, max 1000000000000) |
| `customers_lost` | integer |  | required | Customers who cancelled during the period (only those present at the start). (min 0, max 1000000000000) |
| `customers_gained` | integer |  | optional, default 0 | New customers added during the period (excluded from the churn denominator). (min 0, max 1000000000000) |
| `period` | enum: month \| quarter \| year |  | optional, default "month" | Length of the measurement period; used to express lifetime in months and to annualise churn. |

## Output

| Field | Type | Unit | Description |
| --- | --- | --- | --- |
| `churn_rate_percent` | number | % | customers_lost / customers_at_start × 100. |
| `retention_rate_percent` | number | % | 100 − churn rate. |
| `customers_at_end` | integer |  | customers_at_start − customers_lost + customers_gained. |
| `net_growth_percent` | number | % | (customers_at_end − customers_at_start) / customers_at_start × 100. |
| `annualized_churn_percent` | number | % | 100 × (1 − (1 − churn)^(12 / months in period)). |
| `average_customer_lifetime_periods` | number | periods | 1 / churn rate, in periods (only when churn > 0). |
| `average_customer_lifetime_months` | number | months | Lifetime converted to months (only when churn > 0). |

## Formula

`churn = customers_lost / customers_at_start; retention = 1 − churn; end = start − lost + gained; annualised churn = 1 − (1 − churn)^(12/m) with m = months per period; lifetime = 1 / churn`

Churn is measured against the customers present at the start of the period, so customers gained during the period are excluded from the denominator (the most common convention). Annualisation compounds retention.

## Data Sources

- Wikipedia – Churn rate — https://en.wikipedia.org/wiki/Churn_rate (reference, retrieved 2026-09-24)
- Wikipedia – Customer retention — https://en.wikipedia.org/wiki/Customer_retention (reference, retrieved 2026-09-24)
- Andreessen Horowitz – 16 Startup Metrics — https://a16z.com/16-startup-metrics/ (reference, retrieved 2026-09-24)

Data freshness: `static`. Deterministic formula with fixed constants; results never go stale. Inputs supplied by the caller determine the output.

## API

- `GET https://tttkmbb.com/api/v1/calculate/churn-rate?customers_at_start=…&customers_lost=…`
- `POST https://tttkmbb.com/api/v1/calculate/churn-rate` with JSON body `{"inputs": {…}}`
- Response: unified envelope (`success`, `request`, `result.values`, `result.units`, `sources`, `freshness`, `timestamp`, `next_actions`, `links`); see https://tttkmbb.com/docs/response-format.md
- Schema: https://tttkmbb.com/api/v1/calculators/churn-rate · OpenAPI operationId `calculate_churn_rate` in https://tttkmbb.com/openapi.json
- Authentication: none. Rate limit: fair use, see https://tttkmbb.com/docs/rate-limits.md.

## MCP

- Server: `https://tttkmbb.com/mcp` (Streamable HTTP, JSON-RPC 2.0, no auth)
- Tool:  `run_calculator` with `{"calculator_id": "churn-rate", "inputs": {…}}`

## Example

- 500 start, 25 lost, 60 gained (month): inputs `{"customers_at_start":500,"customers_lost":25,"customers_gained":60,"period":"month"}` → `{"churn_rate_percent":5,"retention_rate_percent":95,"customers_at_end":535,"net_growth_percent":7,"annualized_churn_percent":45.96,"average_customer_lifetime_periods":20,"average_customer_lifetime_months":20}`
- 1,200 start, 90 lost (quarter): inputs `{"customers_at_start":1200,"customers_lost":90,"period":"quarter"}` → `{"churn_rate_percent":7.5,"retention_rate_percent":92.5,"customers_at_end":1110,"net_growth_percent":-7.5,"annualized_churn_percent":26.79,"average_customer_lifetime_periods":13.33,"average_customer_lifetime_months":40}`

```
GET https://tttkmbb.com/api/v1/calculate/churn-rate?customers_at_start=500&customers_lost=25&customers_gained=60&period=month
```

## Limitations

You need revenue churn in currency terms or MRR projections (use mrr-growth) or the full acquisition economics (use cac-ltv). Churn is measured against the customers present at the start of the period, so customers gained during the period are excluded from the denominator (the most common convention). Annualisation compounds retention. All values are computed from the formula above; no measurement or live data is involved.

## FAQ

**Do new customers who leave in the same period count as churn?**

Not in this convention: churn counts only customers who existed at the start. Some companies use the average customer count as the denominator, which gives slightly different rates.

**Why is annualised churn not 12 × monthly churn?**

Because the base shrinks each month. 5 % monthly churn leaves 0.95^12 = 54 % of customers after a year, i.e. 46 % annual churn, not 60 %.

## Related

- [CAC and LTV Calculator](https://tttkmbb.com/business/cac-ltv.md) — Turn churn into customer lifetime value.
- [MRR Growth Calculator](https://tttkmbb.com/business/mrr-growth.md) — Revenue-based growth including churned MRR.
- [Percentage Change Calculator](https://tttkmbb.com/math/percentage-change.md) — Plain percentage change between two counts.