Convertible promissory notes: How startup convertible notes actually work

A convertible promissory note is a loan that's built never to be repaid in cash. Instead, it turns into equity in the company that borrowed the money. Early-stage startups reach for this tool when they need capital quickly and nobody yet agrees on what the business is worth. Rather than argue over a price today, the founder borrows now and lets a later funding round set the terms. On paper it's a real debt, with a principal balance, an interest rate, and a maturity date, but everyone signing expects those numbers to convert into shares instead of a check.

What a convertible note actually is

The mechanics start out looking like any unsecured loan. An investor hands a startup, say, $100,000, and the company signs a note promising to repay it with interest. The twist sits in a conversion clause. When the startup later raises a priced equity round, the outstanding balance automatically converts into shares of that new round instead of coming due. The investor stops being a lender and becomes a shareholder. This lets a company incorporated in Delaware or anywhere else defer the hardest question in early fundraising, which is valuation, until it has traction and real numbers to justify a price. For the founder, that delay is the whole point. Setting a valuation too early, when the company is little more than an idea, risks either scaring off investors with a number that looks greedy or giving away too much of the company with a number that looks cheap.

The valuation cap and the discount rate

Two terms decide how good a deal the early investor gets when conversion happens. The valuation cap sets a ceiling on the company value used to price the investor's shares, no matter how high the priced round values the company. If the cap is $8 million and the round prices the company at $20 million, the note holder converts as if the company were worth $8 million, which buys far more shares per dollar. The discount rate does something similar by giving the note holder a percentage off the new round's share price, often 15 to 25 percent. Many notes carry both, and the investor gets whichever produces the better price. These two levers are the reward for taking early risk.

What happens at the maturity date

A convertible note still has a maturity date, usually 18 to 24 months out. If a priced round happens before then, conversion takes care of everything and the date never matters. The awkward case is when no qualifying round occurs and the note comes due with cash the startup does not have. Well-drafted notes plan for this. Some convert automatically at maturity using the cap as the price. Some give the investor a choice to extend, convert, or demand repayment. A few flip into a demand obligation the holder can call at any time. Founders who ignore this clause can find a supposedly friendly note turning into a solvency problem.

Interest that converts instead of paying out

Convertible notes accrue interest, commonly 2 to 8 percent, but that interest rarely gets paid in cash. It adds to the principal and converts into equity alongside it. So an investor who put in $100,000 at 6 percent for two years converts roughly $112,000 into shares, not $100,000. This keeps cash inside the startup, where it's needed, while still compensating the investor for time and risk. Because the rate is real, founders should still confirm it sits under their state's usury ceiling before signing. Our usury limit checker is a quick way to sanity-check the number.

How a convertible note differs from a plain loan note

A standard promissory note exists to be repaid. The lender wants principal and interest back on a schedule, and default triggers collection. A convertible note inverts that expectation. Repayment in cash is the fallback nobody wants, and success looks like the debt disappearing into stock. The paperwork also differs. A convertible note layers in conversion mechanics, a cap, a discount, and definitions of what counts as a qualifying financing. Treating one like the other is a common and expensive mistake, especially when a founder copies a plain note template and forgets the clauses that make conversion actually work.

Convertible note vs a SAFE

The SAFE, or Simple Agreement for Future Equity, was created to strip out the parts of a convertible note that founders disliked. A SAFE is not debt. It has no interest, no maturity date, and no repayment obligation. It simply promises equity when a future round happens. That makes it lighter, but it also removes the leverage a note gives investors, since there's no date forcing a resolution. Some investors prefer the note precisely because it keeps a deadline and a repayment claim on the table. Neither is automatically better. The choice turns on how much pressure each side wants baked into the deal.

Who tends to use convertible notes

Convertible notes show up most often in the earliest stage of a company's life, before it has revenue or a track record that a traditional lender or a formal valuation could rely on. Friends, family, angel investors, and accelerators use them to move money in quickly on light paperwork. The appeal for the founder is speed and lower legal cost, since a note can close in days without the full machinery of a priced equity round. The appeal for the investor is getting in early on favorable conversion terms. The tradeoff is uncertainty. Nobody knows exactly what percentage of the company the investor will end up owning until the priced round finally sets a number. A founder who sells too many notes with low caps can wake up to far more dilution than expected once they all convert at once, which is why tracking the total outstanding across every note matters as much as the terms of any single one.

Why these count as securities

One point founders underestimate is that a convertible note is a security. Selling one is selling an investment contract, which brings federal and state securities law into play. The Securities and Exchange Commission (SEC) treats convertible instruments as securities that generally must be registered or sold under an exemption, and it publishes plain-language investor material describing how convertible securities turn into stock. Startups usually rely on private-placement exemptions, but the paperwork and investor qualifications still matter. Skipping that analysis because the note feels informal is how a routine friends-and-family raise becomes a legal headache. Treat the note as the regulated instrument it is.

Sources

• SEC (Investor.gov): Convertible Securities