NextConstraint
How it works

Free · no sign-up

SaaS Funnel Leak Calculator

Type in one month of numbers, and your usual rates if you know them. The calculator puts a dollar figure on three leaks in your funnel (fewer sign-ups, fewer of them paying, more cancellations), ranks them, and names the one to fix first. It runs in your browser; nothing you type is sent anywhere.

This month

Your most recent full month.

From your analytics: Vercel Analytics, Plausible, PostHog or Google Analytics.

New accounts, from your database or auth provider.

Customers who started paying, from Stripe or Dodo.

Customers on a paid plan, from Stripe or Dodo.

Paying customers who cancelled, from Stripe or Dodo.

Your MRR ÷ paying customers, from Stripe or Dodo.

Your usual rates · optional

For example, the average of your last 3 months. Leave them blank to skip the comparison.

Sign-ups ÷ visitors.

New paying customers ÷ sign-ups.

Cancellations ÷ paying customers.

Don’t have these numbers handy? NextConstraint pulls these automatically from Stripe (beta) or Dodo Payments, Vercel Analytics, Plausible or PostHog, and Supabase Auth.

Sample result

$ a month

Worked out from the grey sample numbers. Type yours to see your own.

Your likely constraint is the churn leak, at about $147 a month.

  1. Your likely constraint

    Churn leak

    $147/mo

    Churn 6.0% this month, usual 4.0%: about 3 more cancellations than usual.

  2. Sign-up leak

    $98/mo

    Sign-up rate 2.0% this month, usual 2.5%: about 20 fewer sign-ups, so about 2 fewer paying customers.

  3. Conversion leak

    $78/mo

    Sign-up → paid 10.0% this month, usual 12.0%: about 2 fewer paying customers.

This is a rough estimate from the numbers you typed. NextConstraint works from your real data: it checks whether each gap is real or just noise, gives you one fix for the biggest leak, and re-measures it 30 days after you ship.

Find my biggest leak

How it’s worked out

Each leak is a month of revenue. A step that did as well as usual, or better, counts as $0: a good month in one step never cancels out a leak in another. The most expensive leak is your likely constraint. When none is worse than usual, the constraint is churn or traffic.

Sign-up leak

Visitors × (usual sign-up rate − this month’s) × this month’s sign-up → paid rate × price.

The sign-ups you would have had at your usual rate, times the share of sign-ups that pay.

Conversion leak

Sign-ups × (usual sign-up → paid rate − this month’s) × price.

The paying customers your sign-ups would have produced at your usual rate.

Churn leak

Paying customers × (this month’s churn rate − usual) × price.

The cancellations above your usual rate, each one a month of revenue.

Churn, otherwise

Cancellations × price, when cancellations are at least new paying customers.

When no step is worse than usual, or you leave the usual rates blank, and you’re losing about as many customers as you gain.

Traffic, otherwise

1,000 × this month’s sign-up rate × sign-up → paid rate × price.

Your rates are holding and you gain more customers than you lose, so growth is limited by how many people find you: this is what each extra 1,000 visitors adds a month.

Nothing is ranked when a number is too small to read. The floors are NextConstraint’s own: 300 visitors, and 20 sign-ups or paying customers. Lower rates need more, because each number also has to pass NextConstraint’s noise check: losing the whole rate, or doubling churn, has to stand out from chance by two standard errors. At a 2.5% sign-up rate that’s 157 visitors; at 4% churn, 97 paying customers.

A customer you didn’t win, or lost, goes on costing that amount every month after, so the real cost of a leak that stays open keeps adding up.