Each month applies churn to opening customers, then adds the entered acquisitions. This is a bounded constant-rate scenario, not a cohort forecast.
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Calculation details
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What does this calculator estimate?
Churn calculator: measure how subscription churn cuts monthly recurring revenue and lifetime value, and what retention gains are worth. Free, no signup.
How this calculator works
Formula
Each month, churned customers = opening customers × monthly churn; ending customers = opening customers − churned customers + new customers. Monthly revenue = ending customers × ARPU.
Worked example
1,000 customers, 5% monthly churn, no acquisition, and $20 ARPU leave about 540 customers and $10,807 monthly revenue after 12 modeled months.
Assumptions to verify
- Churn, acquisition, and ARPU remain constant throughout the horizon.
- Churn is applied before new customers are added each month.
- Cohorts, expansion revenue, reactivation, seasonality, failed payments, and acquisition lag are excluded.
Frequently asked questions
When is churn applied?
To opening customers before the entered new customers are added.
What is replacement need?
Month-one customers expected to churn at the entered rate; rounding up gives a whole-customer planning target.
Is this a retention forecast?
No. It is a bounded constant-rate scenario without cohort behavior.
Cite this tool
BoringToolsKit. “Subscription Churn Impact Calculator.” boringtoolskit.com/subscription-churn-impact-calculator/ (reviewed July 2026). Free to reference in articles, syllabi, and answer posts with a link.
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Worked example: 1,000 customers at 5% monthly churn
Take 1,000 customers, 5% monthly churn, 100 new customers per month, $50 ARPU, over 3 months. Churn converts to 0.05, and the month-one replacement need is 1,000 x 0.05 = 50 customers. Above 50 the base grows; below it, the base shrinks.
Month 1: lost = 1,000 x 0.05 = 50, customers = 1,000 - 50 + 100 = 1,050, revenue = 1,050 x $50 = $52,500. Month 2: lost = 1,050 x 0.05 = 52.5, customers = 1,097.5, revenue = $54,875. Month 3: lost = 1,097.5 x 0.05 = 54.875, customers = 1,142.625, revenue = $57,131.25. Ending customers display as 1,143, total modeled churn is 50 + 52.5 + 54.875 = 157.375 (shown as 157), and cumulative revenue is $52,500 + $54,875 + $57,131.25 = $164,506.25.
Watch the replacement need climb: 50, then 52.5, then 54.875, because churn applies to a base that inflow keeps raising. The path levels toward the fixed point where outflow equals inflow, 100 / 0.05 = 2,000 customers, short of that target after 3 months.
Boundary cases: 0% churn, the 100% cap, and rejected inputs
At 0% churn the loop becomes pure addition: 1,000 starting, 100 new per month, 12 months gives 1,000 + 100 x 12 = 2,200 ending customers, total churn of 0, and a replacement need of 0.
At the 100% cap every customer leaves before arrivals land. With 1,000 starting and 100 new per month, month 1 loses all 1,000 and ends at 100; months 2 through 12 each lose 100, so 12-month total churn is 1,000 + 11 x 100 = 2,100 while ending customers stay pinned at 100 and revenue sits at 100 x ARPU. Revenue is recorded after arrivals, so month one already includes them.
Out-of-range inputs are rejected, not clamped: negative customers, churn outside 0% to 100%, negative inflow or ARPU, and horizons below 1 or above 120 months return the validation message instead of a number. Fractional horizons floor, so 12.9 months models 12 rows.