Summary

SAFEs and convertible notes both convert into equity later, but neither reliably dilutes you less. A post-money SAFE's dilution is easy to underestimate, while a convertible note adds interest and a maturity deadline that a SAFE does not. Which costs you more depends on the terms and your timeline to a priced round.

SAFEs and convertible notes perform the same basic function: let you raise money before you set a share price and turn that money into equity when a priced round or an acquisition arrives. That said, there are fundamental differences between the two.

A convertible note charges interest and sets a maturity date, so a slow priced round can leave you owing the money back. A SAFE carries neither and has become the simpler default, but a post-money SAFE can take more of your company than its cap implies. The wrong instrument can cost you real ownership or a repayment you can't afford, so each one is worth understanding in full before you sign.

For SAFE stacking across multiple rounds and 83(b) elections, see our full guide on SAFEs, 83(b) Elections, and Cap Table Hygiene.

How does a SAFE dilute your ownership?

A post-money SAFE fixes each investor's ownership as a percentage of the company, measured after all the SAFE money is counted. The math is deliberately simple. If the priced round values the company at or above the SAFE’s cap, the stake a SAFE converts into equals the investment divided by the valuation cap. On Y Combinator's post-money SAFE, $500,000 raised on a $6.7 million cap converts into about 7.5% of the company. If the round comes in below the cap, however, the SAFE converts at the lower round price instead, and the holder’s stake grows.

The word post-money is the catch. The cap sits on top of every dollar you raise across all your SAFEs. So each new SAFE you sign dilutes your own stake and the option pool, not the SAFE holders who came before. Their percentages are locked in and ride through to the priced round untouched.

Stack three or four SAFEs at different caps, and they begin to add up more than founders expect when they look at each one alone. The whole difference comes out of the existing equity. It’s also important to watch out for most-favored-nation (MFN) clauses and side letters as well, which can complicate the stack. An MFN lets an earlier SAFE holder adopt better terms you later give other investors, such as a lower cap or discount, which can raise that holder’s stake after the fact.

A convertible note also dilutes you through two levers. The discount lets it convert below the priced-round price, often by around 20%, and the valuation cap sets a ceiling on that price. A SAFE can carry a discount too, but most rely on the cap alone. Because most notes use a pre-money cap and accrue interest, the stake a note converts into depends on what else you raise and when the priced round closes.

What is the risk of raising on a convertible note?

A primary risk of a convertible note is its maturity date. Because the note is debt, the principal comes due on a fixed day. If you have not closed a priced round by then, the holder can demand repayment. A pre-revenue startup usually cannot pay, so you renegotiate an extension from a weak position, or you sit in technical default. 

A note's interest accrues until it converts or is repaid, usually at a few percent a year. That interest typically converts into extra shares, which adds to your dilution. Under the original issue discount rules, it can also be taxable to the investor as it accrues, before any cash is paid. A SAFE carries none of this, because it is not a loan and has no deadline to miss.

In return, a noteholder is a creditor and gets paid ahead of all stockholders if the company winds down. A standard SAFE holder has no creditor claim and is only paid after creditors, though ahead of common stockholders.

Qualified small business stock (QSBS), the federal exclusion that can shelter the greater of $15 million or 10 times your basis in exit gain per stockholder for qualifying stock acquired after July 4, 2025, is another consideration for both instruments. Among other requirements, Section 1202 requires you to hold C-corporation stock, and its holding period clock runs from when you hold that stock, not from when you fund the instrument. For stock acquired after July 5, 2025, a partial exclusion begins at three years and the full exclusion at five.

A convertible note is debt, so the clock starts when the note converts. For a SAFE the rule has not been settled; whether the clock runs from funding or from conversion depends on whether the SAFE counts as stock, which the IRS has not resolved. Plan for the clock to start at conversion unless your tax counsel tells you otherwise.

Model the dilution before you pick an instrument

Before you bring in a lawyer, you can roughly size your potential dilution yourself. Divide each SAFE or note you plan to raise by its cap, and count a note’s expected interest along with its principal. Add the stakes together, and compare the total against what you own today. If that leaves you with too little of the company, chances are you have a problem that should be fixed before you sign.

That said, this rough math misses the option pool and assumes a priced round comes in at or above the cap. A lead investor almost always demands a new or bigger option pool at the priced round, and its dilution comes out of your stake, not theirs. A lawyer prices that in and sets your cap and discount so you keep as much as possible. Once you sign, those terms are locked. Serotonin Legal regularly works through this process with founders.

Reach out to set up a free consultation.

Serotonin Legal advises technology founders on corporate, regulatory, and transactional matters at the intersection of AI, blockchain, and fintech. This guide is for general informational purposes and does not constitute legal advice. No attorney-client relationship is formed by reading this material.