What Is a Post-Money SAFE? A Founder's Guide to YC's Standard Documents
The post-money SAFE is the default early-stage instrument in the US. Most founders use it without fully understanding how it works. Here is a plain-language explanation.
Y Combinator introduced the SAFE (Simple Agreement for Future Equity) in 2013 as a simpler alternative to convertible notes. In 2018, they updated it to the post-money SAFE - the version that is now the standard instrument for pre-seed and seed fundraising in the United States.
The post-money SAFE is used in thousands of early-stage deals every year. Yet most founders who sign them do not fully understand the difference between pre-money and post-money, what the valuation cap actually caps, or how multiple SAFEs interact on the cap table. This guide explains all of it in plain language.
Pre-money vs post-money SAFE: the critical difference
The original SAFE was a pre-money instrument. The 2018 update made it post-money. The difference is in how you calculate the ownership percentage the SAFE holder receives at conversion.
On a pre-money SAFE, the cap represents the pre-money valuation at which the SAFE converts. If your cap is $8M and you raise a priced round at $12M pre-money, the SAFE converts as if at $8M. The ownership percentage is calculated on the pre-money shares only - which means the percentage you promised to the SAFE holder gets diluted by the new option pool before it is calculated.
On a post-money SAFE, the cap represents the post-money valuation that the SAFE converts into. The ownership percentage is locked in at the time the SAFE is signed: investment amount divided by post-money cap. A $500K SAFE on an $8M post-money cap = 6.25% ownership. That percentage does not change regardless of how many other SAFEs you issue, or what option pool you create, between signing and conversion.
The post-money SAFE locks in ownership percentage at signing
This is the most important thing founders miss. If you issue ten SAFEs on a $5M post-money cap, and each is for $500K, that is ten times 10% = 100% of the company promised before you have raised a dollar. The SAFEs do not stack in the way founders assume. Model your total SAFE dilution before you start issuing them.
How the valuation cap works
The valuation cap on a post-money SAFE sets the maximum post-money valuation at which the SAFE converts. If you raise a priced round at $15M post-money and your SAFE has a $5M cap, the SAFE holder converts at $5M - meaning they receive three times as many shares as an investor paying full price at the priced round.
The cap is not a valuation. It does not mean you are worth $5M when you sign the SAFE. It is an agreement that the SAFE will convert no worse than if the company were valued at $5M post-money. If you raise your priced round at below the cap (say, $3M post-money), the SAFE converts at the actual priced round valuation, not the cap.
Setting the right cap requires balancing two things: giving early investors a reward for their risk (the cap should be meaningfully below your expected Series A or priced round valuation), and not over-diluting yourself before you have built anything (the cap should not be so low that you give away 30% of the company before writing a line of code).
The discount: an alternative or additional reward
Many SAFEs include both a cap and a discount rate. The discount entitles the SAFE holder to convert at a percentage below the priced round price - typically 15-20%. When a SAFE has both a cap and a discount, the investor converts at whichever gives them the lower price per share.
At a low priced round valuation (below the cap), the discount often applies. At a high priced round valuation (well above the cap), the cap applies. In practice, at the valuations where most early-stage SAFEs convert, the cap is usually the more valuable protection for the investor.
MFN (Most Favoured Nation) clauses appear on SAFEs without a cap. An MFN SAFE allows the holder to adopt the terms of any subsequent SAFE you issue - including the cap. If you raise an MFN SAFE today and then issue a capped SAFE next month, the MFN holder can adopt the cap. MFN SAFEs are typically used in the earliest possible stage, before you have enough data to set a cap with confidence.
How Airbnb used SAFE-like instruments in their early rounds
YC batch
W09
Initial YC investment
$20K
Seed round structure
Convertible notes
Series A valuation
$2.4M pre (2010)
Airbnb went through Y Combinator in the winter 2009 batch, when SAFEs did not yet exist. Their early funding used convertible notes - the predecessor instrument that SAFEs were designed to improve on. The notes were simple enough to close quickly but carried interest and maturity dates that created ongoing administrative overhead.
When YC introduced the SAFE in 2013, it was a direct response to the friction that companies like early Airbnb experienced: lawyers arguing over note terms, maturity extensions, and interest calculations that added cost without adding value to either side. The SAFE eliminated interest, maturity dates, and most of the terms that needed negotiation.
The post-money update in 2018 added one more simplification: predictable dilution. Founders know exactly how much they are giving up when they sign. Investors know exactly what they own. The instrument does what early-stage financing should do - transfer capital quickly and unambiguously - without getting in the way of building the company.
Common SAFE mistakes founders make
Issuing too many SAFEs at the same cap is the most common mistake. Because each post-money SAFE locks in a percentage of ownership, issuing multiple SAFEs on the same cap compounds the dilution. Ten SAFEs at 5% each on a $5M cap is 50% of the company. Model your full SAFE stack before issuing each one.
Not disclosing the SAFE stack to priced round investors is a close second. When a Series A investor discovers undisclosed SAFEs during due diligence - and they always do - it damages trust and can delay or kill the deal. Be transparent about your full SAFE and note stack from the first conversation.
Setting the cap too low to close quickly is a trap. A $2M post-money cap might help you close angels fast, but it means giving up significant ownership before you have built anything. Set the cap at a level that reflects the real value you expect to create by the time you raise the priced round - not the lowest number an investor will accept today.