A SAFE (Simple Agreement for Future Equity) is a convertible instrument created by Y Combinator in 2013 and rewritten as the post-money version in 2018, which is now the default form. It is not debt: there is no interest, no maturity date and no repayment obligation. The investor wires money today and receives a claim that resolves at the next equity round (shares, priced by a valuation cap, a discount, or both), at a change of control (the greater of the money back or the as-converted value), or at dissolution (the money back, ahead of common).
The post-money mechanics are what founders most often misread. The investor’s ownership equals investment divided by the post-money valuation cap, measured on the company’s capitalization including all converting securities (every other SAFE and note), granted and promised options, and the existing unissued pool, but excluding the new money and any pool increase adopted for the round. That makes each SAFE’s ownership a fixed promise at signature: issuing more SAFEs later does not dilute earlier SAFE holders, it dilutes the founders and every non-SAFE holder on the cap table.
Founders choose SAFEs for speed and simplicity: no valuation negotiation beyond the cap, no board seat, a few pages of standard text, closings that can happen investor by investor. The discipline this convenience removes has to be reintroduced by the founder: keep a running fully diluted cap table where every outstanding SAFE is converted at its cap, and read every new cap as a percentage sold, not as a flattering valuation headline.
