The YC SAFE Explained: What Changed With Post-Money
Y Combinator's standard deal is $125,000 for a fixed 7 percent, plus $375,000 on an uncapped SAFE with an MFN clause. That is the YC SAFE: two post-money instruments signed together, and the 2018 post-money SAFE switch changed how every SAFE you sign afterward dilutes you. Model it first with Round Funded's cap table calculator.
Y Combinator created the SAFE (Simple Agreement for Future Equity) in 2013, written by then general counsel Carolynn Levy. The original was a pre-money SAFE: dilution from later SAFEs was shared across every earlier SAFE holder, so nobody's percentage was locked until the priced round actually happened. In 2018, YC rewrote the instrument as the post-money SAFE, now the default for nearly every US pre-seed and seed round, including YC's own $500K check.
The difference sounds technical. It is not. A post-money SAFE fixes the investor's ownership percentage the moment you sign it: $500,000 on a $5,000,000 post-money cap is 10 percent of the company, full stop, no matter how many more SAFEs you issue afterward. A pre-money SAFE floated with every new instrument added. Post-money is cleaner for the investor and, if you stack too many afterward, quietly brutal for you. A SAFE is also not debt, which is the other question founders ask early: see our convertible note vs SAFE comparison if you are still deciding between the two instruments for your round.
How a YC Post-Money SAFE Actually Converts
A SAFE issues no shares at signing. It sits until a priced round triggers conversion, and it converts using whichever term gives the investor the better price: the valuation cap or the discount, if both are present.
Here is the sequence, start to finish:
- You sign the SAFE and receive the cash. No board seat, no valuation debate, no shares yet.
- You spend the money building the company.
- You raise a priced round, usually a Series A, sometimes a priced seed. Every outstanding SAFE converts simultaneously.
- Each SAFE converts against the new round's actual price, at its own cap or discount, whichever number is lower for the investor.
YC's $500K is not one SAFE at one cap. It is two SAFEs signed together: a $125K SAFE that converts to a fixed 7 percent, with no cap and no discount, just a flat percentage; and a $375K uncapped SAFE carrying an MFN (Most Favored Nation) clause, which inherits the best terms of any SAFE you issue between the batch and your priced round. Our full breakdown of the YC deal math walks both pieces line by line.
That two-instrument structure matters for the rest of this article: everything below about caps, discounts, and stacking applies to the SAFEs you sign after YC funds you, since YC's own fixed 7 percent piece has no cap to negotiate in the first place.
Worked Example: $500K on a $5M Post-Money Cap
A $500,000 SAFE at a $5,000,000 post-money valuation cap converts to exactly 10 percent of the company, no matter how high your priced round eventually values you. The math is the check divided by the cap: $500K divided by $5M equals 10 percent.
That fixed percentage is the entire point of a post-money SAFE, and it cuts both ways. Run the same $500,000 check at different caps and the founder's real cost becomes obvious fast:
| Check size | Cap | Converts to |
|---|---|---|
| $500,000 | $3,000,000 | 16.7% |
| $500,000 | $5,000,000 | 10.0% |
| $500,000 | $8,000,000 | 6.25% |
| $500,000 | $12,000,000 | 4.17% |
Two things to notice. First, the cap is not a valuation of your company; it is a ceiling on the price the investor pays at conversion, and a low cap is expensive even when it feels like an easy number to agree to over email. Second, because a post-money SAFE locks in the investor's percentage at signing, every SAFE you add after this one dilutes only you and your co-founders, never the investor who already converted. Test your own numbers in the pre-money vs post-money calculator before you agree to a cap in a term sheet email.
Stacking SAFEs After YC: The Dilution Trap
Taking YC's $500K does not end your SAFE stack; it usually starts it. Most batch companies raise additional angel or pre-seed SAFEs during or right after the program, and each one dilutes founders on top of what YC already holds.
Here is what that looks like, using YC's own published worked example where the $375K MFN SAFE converts at a $15M post-money cap:
| Instrument | Amount | Converts to |
|---|---|---|
| YC fixed SAFE | $125,000 | 7.0% |
| YC MFN SAFE at $15M cap | $375,000 | 2.5% |
| YC subtotal | $500,000 | 9.5% |
| Angel SAFE added post-batch, $6M cap | $300,000 | 5.0% |
| Total before Series A | $800,000 | ~14.5% |
That 14.5 percent sits before the Series A lead's typical 20 percent and before the option pool refresh, which usually adds another 10 to 15 percent on top. Add those in and a founder who felt fine about "just YC plus a small angel round" can arrive at Series A close to 50 percent diluted, without ever agreeing to a single term that looked unreasonable in isolation.
The discipline that keeps this survivable, per the same logic in our general SAFE guide: budget total post-YC SAFE dilution to 15 to 20 percent before your priced round, raise the additional SAFEs in one batch instead of a drip, and keep a live model of what every signed SAFE converts to at your realistic Series A cap.
Cap vs Discount vs MFN: The Terms That Actually Protect You
A cap protects the investor from your valuation running away before they convert. A discount rewards them for coming in before the price is set. An MFN clause protects an early, uncapped investor from a later investor getting a better deal. All three can appear on a YC-style stack, and knowing which one is doing the work changes what you should actually negotiate.
| Term | What it controls | Where it shows up |
|---|---|---|
| Valuation cap | Ceiling price at conversion | The SAFEs founders add after YC; YC's own $125K piece is a fixed percentage, not a cap |
| MFN clause | An uncapped SAFE inherits the best terms issued later | YC's $375K piece, and any bridge SAFE you leave uncapped on purpose |
| Discount | Percent off the priced round's price, typically 10 to 20 percent | Common on angel SAFEs, rare on YC's own paper |
| Pro rata side letter | The investor's right to invest more in your next round to hold their percentage | Granted to YC via the YC Agreement, and to larger checks generally |
Red flags to check before you sign a modified SAFE: any deviation from a standard template deserves a second read. Watch for a cap quietly lower than what was discussed verbally, a discount stacked on top of an already-low cap (a form of double dipping), MFN language broadened beyond future SAFEs to cover your priced round too, and pro rata rights extended past the standard "next round only" scope. None of these are automatic dealbreakers, but each one is a term a sophisticated investor put there on purpose, and each one has a specific cost.
Where Round Funded Fits: Model Your SAFE Before You Sign
Round Funded's free calculators run the exact conversion math investors already ran on you, before you sign anything.
| The problem | The Round Funded tool |
|---|---|
| Not sure what a cap actually converts to | Cap table calculator: stack every SAFE and see founder ownership at Series A |
| Comparing a cap against a discount | Pre-money vs post-money calculator |
| Sizing the raise so you stop stacking out of desperation | Funding goal calculator |
| SAFE terminology that reads like legalese | Fundraising glossary |
Model the whole stack, YC's $500K plus every SAFE you plan to add afterward, before you agree to a cap over email.
Model your SAFE stack on Round Funded →
How to Use a YC Post-Money SAFE: Step by Step
- Find investors who actually write post-YC SAFE checks on the Round Funded directory. Filter by stage and sector so you are pitching angels who fund this exact instrument, not generalist checks that expect a priced round.
- Take YC's $500K unmodified. The deal is standard for a reason; do not spend legal fees negotiating terms that are not negotiable.
- Set one cap for every SAFE you add afterward, justified by comparable rounds at your traction level, not by your ambition.
- Cap your total post-YC SAFE dilution at 15 to 20 percent using the cap table calculator before you accept the second check.
- Raise the post-YC batch in one wave, not a drip. Every reopened round tends to reset caps under pressure, and each reset costs you more of the company.
- Log every SAFE the day it is signed: amount, cap, discount if any, MFN status, and pro rata terms. Update your conversion model after every signature, not at Series A.
- Stop when you hit your dilution ceiling, even if more checks are on the table. The next dollar should come from a priced round, not another SAFE.
Frequently Asked Questions
What is a YC SAFE?
It is the SAFE Y Combinator issues to every accepted company: a $125,000 SAFE that converts to a fixed 7 percent, plus a $375,000 uncapped SAFE with an MFN clause. Both are the standard 2018 post-money SAFE, non-negotiable and identical across every batch. See the full deal math.
How is a post-money SAFE different from the original 2013 SAFE?
The original 2013 SAFE was pre-money: dilution from every new SAFE was shared across all existing SAFE holders, so nobody's stake was fixed until the priced round. The 2018 post-money version fixes each investor's percentage at signing instead, which protects investors but means every SAFE you add afterward dilutes only the founders.
How much does a $500K YC SAFE actually cost in equity?
At YC's own worked example, a $15M post-money cap on your next round, the $375K MFN piece converts to about 2.5 percent, putting YC's total near 9.5 percent alongside the fixed 7 percent piece. Weaker rounds cost more: a company raising at an $8M cap gives up closer to 11.7 percent.
What happens if I stack more SAFEs after YC's?
Each additional SAFE dilutes founders only, since post-money SAFEs lock in earlier investors' percentages. A $300,000 angel SAFE at a $6M cap on top of YC's roughly 9.5 percent pushes total non-founder SAFE dilution to about 14.5 percent before the Series A lead and option pool are even added. Model it with the cap table calculator.
What is an MFN clause on an uncapped SAFE?
MFN stands for Most Favored Nation: an uncapped SAFE with an MFN clause automatically adopts the best terms, lowest cap or biggest discount, of any SAFE you sign afterward. YC's $375K piece works this way, so it converts at exactly the price your next investors pay, never worse and never better.
Does the pro rata side letter affect my dilution math?
Not directly at signing. A pro rata right only lets the investor buy more in your next round to hold their existing percentage; it does not change what the SAFE itself converts to. It does affect how much room is left for new investors in that round, so budget for it when sizing a priced round on Round Funded.
When does a YC SAFE actually convert into shares?
At your next priced round, typically Series A, when every outstanding SAFE converts simultaneously into preferred shares. Until then it sits as an unconverted agreement with no shares, no board seat, and no valuation attached. A standard SAFE also converts on an acquisition, paying back the greater of the cash invested or its converted value.
Final Word
YC's post-money SAFE fixed a real problem: nobody knew what they actually owned until the priced round hit. It also created a new one, because every SAFE you sign after it comes straight out of your side of the table. Know your cap, know your stack, and run the numbers before the next check lands.
Model your full SAFE stack on Round Funded →
The YC piece is fixed and predictable. Everything you sign after it is where you win or lose. Model it before you sign.
