Why I Built FounderMath: Free Equity Calculators for Every Founder

Every founder eventually faces the same terrifying question: "How much of my company will I actually own after raising money?" The answer is usually worse than they think.

I watched founder after founder make the same expensive mistakes. Accepting SAFE notes without understanding conversion. Negotiating term sheets without modeling dilution. Discovering too late that their 33% had become 12% after three rounds.

The Problem: Equity Math Is Deceptively Hard

Here's a pop quiz. You and two cofounders each own 33.3% of your startup. You raise a seed round at a $12M pre-money, selling 20% to investors. The investors also require a 10% option pool created pre-money. What do you own now?

Most founders get this wrong. They'll do a quick calculation ("33% minus 20% for investors... so I have 26.4%?") without accounting for the option pool, anti-dilution protections, or how multiple rounds compound.

The Existing Tools Don't Serve Founders

When I looked for tools to help founders understand equity, I found two categories:

• Enterprise cap table platforms (Carta, Pulley) — $100+/month, designed for companies managing existing cap tables, not for founders trying to understand what-if scenarios.
• Basic online calculators — Single-round, no visualization, no saving, no export. You enter numbers, get a percentage, and forget it.

Nobody built a tool for the individual founder who needs to understand "what happens to MY equity" across multiple scenarios. So I did.

FounderMath: Six Calculators, Zero Barriers

I built FounderMath to be the tool I wished I had. It's free, requires no signup, runs entirely in your browser (no data is ever sent to a server), and covers the six calculations founders need most:

A Real Example: The Three-Founder Trap

Here's the scenario that motivated me to build this. Three cofounders start with 40/35/25 split. They raise:

After a pre-seed SAFE, a seed round, a Series A, and a Series B — with option pools at each stage — Alice's 40% becomes roughly 14%. That's a 65% reduction. And she didn't model any of it beforehand.

Try this scenario yourself →

Technical Decisions: Privacy First

One decision I'm particularly proud of: everything runs client-side. Your equity data never leaves your browser. No accounts, no databases, no tracking sensitive financial information. This isn't just a privacy feature — it's a trust feature. Founders shouldn't have to upload their cap table to a third-party server just to see what happens if they raise another round.

The calculators use vanilla JavaScript and Chart.js for visualizations. The Pro tier adds PDF export, scenario saving, and side-by-side comparison for $19/month — but the core calculations are and will remain free.

What's Next

I'm actively developing FounderMath based on what founders tell me they need. Upcoming features include:

• Multi-round dilution simulation with up to 10 rounds
• Convertible note calculator with interest and maturity
• Team sharing for cofounders to collaborate on scenarios
• Scenario comparison side-by-side

Have feedback or a feature request? I'd love to hear from you at hello@foundermath.com or through the contact form.