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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).

When to use

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).

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.

Inputs

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

Outputs

Output	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).

Example

500 start, 25 lost, 60 gained (month): {"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): {"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

Machine access

API: GET https://tttkmbb.com/api/v1/calculate/churn-rate (query parameters) or POST with a JSON body {"inputs": {...}}
Schema: https://tttkmbb.com/api/v1/calculators/churn-rate · Markdown: https://tttkmbb.com/business/churn-rate.md · JSON definition: https://tttkmbb.com/business/churn-rate.json
MCP: server https://tttkmbb.com/mcp, tool run_calculator with calculator_id="churn-rate"
OpenAPI operationId: calculate_churn_rate
Freshness: static. Authentication: none. Rate limit: fair use (see rate limits).

Sources

Wikipedia – Churn rate (reference)
Wikipedia – Customer retention (reference)
Andreessen Horowitz – 16 Startup Metrics (reference)

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 calculators

CAC and LTV Calculator — Turn churn into customer lifetime value.
MRR Growth Calculator — Revenue-based growth including churned MRR.
Percentage Change Calculator — Plain percentage change between two counts.