About the Startup Cap Table & Dilution Simulator
Founders sign SAFEs one at a time, each one reasonable on its own, and meet the cumulative result at the Series A closing table. This simulator shows it before the signatures: build the cap table from founders and an option pool, add post-money SAFEs with a valuation cap, a discount or both, add a priced round with its pre-money valuation and pool top-up, and watch every holder’s fully diluted percentage move event by event.
The conversion follows the mechanics in Y Combinator’s post-money safe user guide, including the circular case where a SAFE converts at the round price because it is lower than the cap price. The maths runs on exact fractions, not floating point, then rounds the way a real closing does: price per share to four decimals, share counts down to whole shares. Given the pool top-ups YC printed, it reproduces every share count in YC’s worked examples; left to compute the top-up exactly (YC rounds theirs to the nearest thousand shares), the price moves by a hundredth of a cent. The exit waterfall then shows what each holder receives at a sale price, with each preferred holder choosing between its 1× preference and converting to common.
It is a simulation, not legal, tax or investment advice. Everything is calculated in your browser, and the share link carries the whole table inside the URL, so nothing is stored on a server.
Everything else stays as built. Scenario B changes only the terms of the last priced round — for example $2M at $10M pre against $3M at $12M pre.
A link that opens straight to the tables with the builder hidden — for a co-founder, an employee or an investor who should see the numbers, not edit them. Like every link here it carries the table in the URL; it is a clean view, not a password.
How to Use the Cap Table Simulator
Start with the company as it stands: each founder’s shares, the options already granted (include promised grants, as the SAFE does) and the unissued pool. Add financing events in the order they happened or will happen. A SAFE needs the amount and a post-money cap, a discount, or both; tick pro rata if the investor has a side letter to buy their percentage of the next round. A priced round needs the pre-money valuation, the new money and the pool the investors want available afterwards. The ownership table then shows every holder after every event, and the waterfall shows what each receives at the exit value you enter.
What a Post-Money SAFE Really Sells
A post-money cap makes the arithmetic honest: amount ÷ cap = ownership, measured on the “Company Capitalization” just before the priced round, which includes every share, every option, the unissued pool and all converting SAFEs. $200,000 on a $4 million cap is 5%; $800,000 on $8 million is 10%. Because each SAFE’s percentage is fixed, every additional SAFE comes entirely out of the founders and the pool. That is the feature founders miss: five SAFEs raised over eighteen months at rising caps can quietly add up to 20% before the Series A term sheet even arrives, and the simulator’s running columns make the total visible.
When the Round Prices Below the Cap
A SAFE converts at whichever price gives the investor more shares: the cap price (cap ÷ Company Capitalization), the round price less any discount, or, for a capped SAFE with no discount, the round price itself if that is lower. When the round comes in low, a SAFE can convert at the round price, which in turn changes the share count the round price is computed from. The simulator solves that circular definition exactly, as a fixed point in rational numbers, and labels each conversion with the price it used — in YC’s $11 million Series Seed example, the $8 million-cap investor ends up with 10.30% rather than 10%.
The Option Pool Shuffle
Term sheets typically say the pre-money valuation “includes an unissued option pool equal to 10% of the post-closing fully diluted capitalization.” The new pool shares are created before the investors buy, so they lower the price per share and the whole cost lands on existing holders. On a $15 million pre-money, that clause alone can be worth a couple of percentage points of the company. Flip the pool to “post-money” on the round to see the difference in the founders’ column; it is a legitimate negotiating point.
Reading the Exit Waterfall
- Non-participating preferred takes the larger of its liquidation preference or its as-converted share, and the simulator finds the combination of choices where no holder would do better by switching.
- Unconverted SAFEs at a sale receive the greater of their purchase amount or their as-converted share, measured on the “Liquidity Capitalization”, which leaves out the unissued pool.
- Options are treated as exercised common and strike prices are ignored, which slightly overstates option holders and understates everyone else. The unissued pool receives nothing.
Related tools: the Business Valuation Calculator for what the company might be worth, and the Runway Calculator for how long the round needs to last. Browse every Freelance & Business tool for more.
Simulation only — not legal, tax or investment advice. Models post-money SAFEs only (not pre-money SAFEs or convertible notes), ignores dividends, vesting, warrants and option strike prices, and rounds price per share to $0.0001 and share counts down to whole shares. Real closings follow their own documents; have counsel prepare the pro forma. SAFE mechanics follow Y Combinator’s published post-money safe user guide.
Frequently Asked Questions
How much of my company does a post-money SAFE take?
With a post-money valuation cap, the SAFE's ownership is simply the amount divided by the cap, measured just before the priced round: $500,000 on a $5 million post-money cap is 10%. That percentage includes all other SAFEs and the existing option pool in the denominator, so each additional SAFE dilutes the founders and the pool, not the earlier SAFE holders. The new money and any pool increase in the priced round then dilute everyone, SAFE holders included.
What is the difference between pre-money and post-money SAFEs?
A pre-money SAFE's cap is measured before the SAFE money, so its final ownership depends on how many other SAFEs and notes convert alongside it and is hard to predict. Y Combinator switched to the post-money SAFE in 2018 so that each investor's ownership is fixed at amount divided by cap. This simulator models post-money SAFEs, which are now the common standard for seed financings.
Why does the option pool top-up dilute founders more than investors?
Term sheets usually say the pre-money valuation includes an unissued pool of, say, 10% of the post-closing company. The new shares for that pool are counted before the new investors buy in, so the price per share falls and only the existing holders, founders and converting SAFEs, bear the dilution. Switch the pool to come from the post-money to see how much that one clause is worth.
What does 1x non-participating preferred mean at an exit?
Each preferred investor chooses the better of two outcomes: take back what they invested (1x their money) ahead of common stock, or convert to common and share the proceeds pro rata. In a low sale they take their money back; above a certain price converting pays more. Participating preferred, by contrast, takes its money back and also shares in the rest, which can shift millions from founders at a modest exit.
Is this simulator a substitute for a lawyer or cap table software?
No. It is a simulation for understanding dilution and payouts, not legal, tax or investment advice. Real financings add rounding conventions, promised options, notes, warrants, vesting, option strike prices, dividends and negotiated terms that change the numbers. Have your counsel or cap table provider produce the pro forma before you sign anything.
