TL;DR. The Post-Money SAFE is the Y Combinator form that ate the early-stage fundraising world. It’s the simplest way to take a check at pre-seed and seed. It’s also more dilutive to founders than the original pre-money SAFE — a feature, not a bug, because it makes the math predictable.
This post is part of the Montague Entrepreneur Forms Library — a free, plain-English collection of the legal documents every startup needs between its first day and its Series A. Pillar: Raise the Money.
What this document actually does
A SAFE — Simple Agreement for Future Equity — is a contract under which an investor gives the company cash now in exchange for the right to receive shares at the next priced round. The SAFE is not debt. It has no interest rate and no maturity date. It’s not equity either — the investor doesn’t get shares, voting rights, or dividends today. It’s a contract that converts into preferred stock at the next qualified financing, on terms defined at the time of the SAFE.
The "post-money" in Post-Money SAFE refers to how the valuation cap is measured. Under the original 2013 pre-money SAFE, the cap was applied to the company’s pre-money valuation, which meant that if multiple SAFEs stacked on top of each other, founders couldn’t reliably predict their dilution until the next round closed. The 2018 post-money SAFE fixes this by calculating the cap on a post-money basis: the investor’s percentage ownership is locked in at the moment they sign, subject only to dilution from subsequent new money. The tradeoff is that, dollar-for-dollar, the post-money SAFE is more dilutive to founders than the pre-money SAFE was — but the dilution is predictable, which matters more.
When you’ll encounter it
Pre-seed and seed. The SAFE is the default instrument for friends-and-family rounds, angel checks, accelerator checks (YC itself invests via SAFE), and most pre-priced seed rounds. You’ll use SAFEs until the round is big enough — or the investor sophisticated enough — to justify the cost and complexity of a priced Series Seed round. Most founders raise their first $500K–$3M on SAFEs and then do a priced Series Seed or Series A when they cross that threshold.
The narrative — why YC published it and why the market accepts it unmodified
The SAFE is a public standard published under a Creative Commons license. Y Combinator released it in 2013 to replace the then-prevailing convertible note, which had become a complex and expensive instrument for very small checks. The SAFE stripped out the debt features — no interest, no maturity, no creditor priority — and turned the instrument into a pure pre-equity contract. In 2018, YC revised the form to move to a post-money cap, and the post-money version is now the industry default. Big-law practice treats the YC post-money SAFE as unmodified standard. Investors expect the form as-is. Changes are confined to four negotiated fields: the valuation cap, the discount rate, the MFN clause, and the pro-rata side letter. Everything else is boilerplate. Founders who try to negotiate the SAFE itself waste time and signal inexperience.
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The four negotiated fields
Lever 1 — Valuation cap
The valuation cap is the maximum post-money valuation at which the SAFE will convert. If the next priced round prices above the cap, the SAFE investor converts at the cap (and gets more shares per dollar than the new money). If the round prices below the cap, the SAFE investor converts at the round price. The cap is the single most negotiated term — and the single term that most directly determines the investor’s return.
Lever 2 — Discount rate
An alternative or complement to the cap: the SAFE converts at the next round price minus a negotiated discount (commonly 10–20%). Discount-only SAFEs are rare; cap-only SAFEs are common; cap-plus-discount SAFEs give the investor the better of the two at conversion.
Lever 3 — MFN (Most Favored Nation)
An MFN clause allows the SAFE investor to elect to adopt the terms of any subsequent SAFE the company issues to a different investor, if those terms are more favorable. Early investors in a company that keeps raising want an MFN to protect against being "capped" out of favorable terms later. Companies resist because it complicates the cap table.
Lever 4 — Pro-rata side letter
The base YC SAFE does not include a pro-rata right — the right to participate in the next priced round at the same terms to maintain percentage ownership. Sophisticated investors request a pro-rata right separately in a side letter. This is market practice and almost always granted to a meaningful investor.
The conversion mechanics
At the next priced round (the "Equity Financing" in the YC form), the SAFE converts into a new sub-series of the preferred stock being issued ("Safe Preferred Stock") at a per-share price equal to either (a) the round price, or (b) the price implied by the valuation cap, whichever results in more shares for the investor. The "cap price" is calculated as the cap divided by the company’s post-money capitalization immediately before the financing. The mechanics are meticulous; the YC form is the canonical source and in any real deal you should use the version published at ycombinator.com/documents rather than a re-typed copy.
Traps for the unwary
- Raising too much on SAFEs. If you raise $4M on SAFEs at a $10M cap, and then try to price a Series A at $20M, your effective founder dilution from the SAFEs alone can be surprisingly large. Model the cap table before you sign.
- Stacking caps. Multiple SAFEs at different caps are legal but painful at conversion. Track them carefully.
- Modifying the YC form. Don’t. Investors expect the unmodified form. Negotiate the four fields and leave the rest alone.
- Forgetting that a SAFE is not debt. There is no interest. There is no maturity. The investor has no right to get their money back if the company doesn’t raise a priced round. This is a feature, but it’s worth being honest with investors about.
- MFN without a floor. An MFN that applies to all future SAFEs, forever, can be triggered in unpredictable ways. Consider a floor or a sunset.
How this fits into the founder journey
The SAFE is the fundraising instrument for the stage before you do a priced round. You’ll raise on SAFEs through pre-seed and often most of seed. When you cross the threshold into a priced Series Seed or Series A, the SAFE converts into preferred stock as part of the closing, and from then on every future round is priced. The more SAFE money you take at low caps, the more dilution you’ll feel at the priced round — so be thoughtful about how much SAFE money you raise and at what caps. Related posts in this pillar: the Series Seed SPA and the Investor Rights Agreement.
Get this reviewed by Montague Law
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This post is for general information only and is not legal advice. No attorney-client relationship is formed by reading it. If you’re about to sign something that matters, talk to a lawyer — preferably one who’s seen at least a hundred of these.