Brainwaves by Burak Büyükdemir
Field notes from 26 years of investing in early-stage startups.
March 11, 2026
The 40% rule
· · ·
Sean Ellis asked one question to thousands of users. The answer changed how we measure startups forever.
"How would you feel if you could no longer use this product?"
If 40% or more say "very disappointed," you have product-market fit. Below 40%, you don't.
Simple. Brutal. Honest.
I've used this test with dozens of portfolio companies. The results are always revealing. And often painful.
Most founders think they have product-market fit because:
→ They have users (who never come back)
→ They have revenue (from one big contract)
→ They have growth (from spending on ads)
None of these are product-market fit. They're traction theater.
Real product-market fit feels different:
Customers pull the product out of your hands. Support tickets are about expanding usage, not fixing bugs. Word-of-mouth drives growth without you spending a dollar. Churn is close to zero.
The 40% rule works because it measures something no metric dashboard captures: emotional dependency.
If you're below 40%, don't scale. Don't raise. Don't hire.
Instead:
1) Talk to the users who said "very disappointed." Understand WHY.
2) Find what they have in common. That's your real target customer.
3) Double down on the features they love. Cut everything else.
The fastest path to product-market fit isn't building more features. It's finding the people who already love what you have. And building only for them.
· · ·
|
|
Free Guide — 42 Pages
The Complete Framework for Startup Pitches
Reverse-engineered from 87 YC Demo Day pitches. The 8-line skeleton that gets investors to lean in.
Download Free Guide →
|
Until next time,
Burak
Founder & Solo GP, Startupist Ventures Fund
|
|
Burak Büyükdemir
Founder & Solo GP, Startupist Ventures Fund · Startup Istanbul
Founder of Etohum & Startup Istanbul. Solo GP at Startupist Ventures. 26+ years building startup ecosystems across 170 countries.
|
“Keep away from people who try to belittle your ambitions. Small people always do that, but the really great make you feel that you, too, can become great.”
— Mark Twain
Join 42,000+ founders who read every issue
No fluff. Just the lessons I wish someone had told me 20 years ago.
Fundraising frameworks
Founder psychology
Growth playbooks
Subscribe — It’s Free
Read by founders from YC, Techstars, 500 Global & 90+ countries. Unsubscribe anytime.
ALREADY SUBSCRIBED?
GO DEEPER.
Paid members get the full archive, every PDF I publish, a WhatsApp group where founders share war stories, and monthly meetups where we break things down live.
| ✓ Full post archive |
✓ Founder WhatsApp group |
| ✓ All PDFs & frameworks |
✓ Live meetups & webinars |
Unlock the Full Experience
About this newsletter: You are receiving this email because you subscribed to Burak’s Brainwaves. Every week, I share ideas on fundraising frameworks, founder psychology, and growth playbooks from 20+ years of investing in early-stage startups.
Update your subscription preferences: unsubscribe from all emails, or manage your subscriber profile.
Startup Istanbul · Istanbul, Turkey