How can you forecast SaaS revenue when you don’t even have customers yet?

When I started working with early-stage SaaS founders, I kept hearing the same frustration:

“How can I forecast revenue, growth, churn, or migration when I don’t even have paying customers yet?”

It’s a fair question. Most pre-revenue founders I’ve met either rely on gut feeling or messy spreadsheets that don’t really connect the dots between marketing, trials, churn, and actual subscriptions.

Over time, I realized the key is to build a logical flow of how customers move through your funnel — not to guess numbers, but to test assumptions. Once you map how awareness turns into signups, and how signups convert (or churn), you can see your growth path even before revenue starts.

I’ve been helping a few early-stage SaaS teams model this more clearly — turning those assumptions into linked forecasts that actually make sense.

I’m curious how other founders here handle this:
• Do you forecast before you have paying customers?
• If yes, how do you set assumptions for growth and churn?
• If not, what do you use instead when investors ask for projections?

Would love to hear your approach.

If you’re interested, I shared a longer breakdown of the framework here:
How pre-revenue SaaS founders can forecast with confidence