When using a SAFE (Simple Agreement for Future Equity), the most critical distinction is between a pre-money and a post-money valuation cap. The difference fundamentally changes who bears the dilution from other SAFEs issued during the same round. A pre-money SAFE sets the valuation cap before any new investment is added. This means each new SAFE investor dilutes the founders and all previous SAFE investors. It creates uncertainty, as no investor knows their final ownership percentage until the entire round closes.

The post-money SAFE, introduced by accelerator Y Combinator in 2018, solves this problem. It sets the valuation cap after the new investment is included. An investor's ownership is calculated based on their investment relative to this post-money valuation. This provides immediate clarity. If an investor puts $1M into a round with a $10M post-money cap, they know they will own 10% of the company right after the conversion, regardless of how much money is raised from other SAFE investors. This transparency is why post-money SAFEs have become the standard.

Both SAFEs and convertible notes use two key mechanisms to determine how the investment converts into equity: the valuation cap and the discount rate. The investor almost always gets the benefit of whichever term is more favorable to them.

Let’s see how this works. An investor puts in $100,000 on a SAFE with a $5 million valuation cap and a 20% discount.

One year later, the startup raises a Series A round at a $10 million pre-money valuation, selling shares for $2.00 each.

We need to calculate the SAFE holder's conversion price under both scenarios:

1. Valuation Cap: The cap price is calculated by dividing the valuation cap by the pre-money valuation of the priced round. Price = $5M / $10M * $2.00 = $1.00 per share.
2. Discount Rate: The discount price is simply the Series A price minus the discount. Price = $2.00 * (1 - 0.20) = $1.60 per share.

The investor gets the lower of the two prices, which is $1.00 per share. Their $100,000 investment converts into 100,000 shares, whereas a new Series A investor would only get 50,000 shares for the same amount.

Unlike SAFEs, convertible notes are debt instruments. This introduces two important features: an interest rate and a maturity date.

Interest Rate: The principal investment accrues interest over time, typically between 2% and 8% annually. When the note converts, the accrued interest is added to the principal, giving the investor slightly more equity. For example, a $100,000 note with 5% annual interest held for two years would convert as if it were a $110,000 investment.

Maturity Date: This is a deadline, usually 18-24 months after the note is issued. If the startup hasn't raised a priced funding round by this date, the note comes due. At this point, the note holders typically have the option to demand repayment of the principal plus interest, or to convert the note into equity at a pre-agreed valuation, often the valuation cap.

Choosing the right instrument depends on the startup's stage and fundraising dynamics. SAFEs are generally faster and simpler, making them ideal for the earliest stages of funding (pre-seed or seed) when a formal valuation is difficult to set. Convertible notes, with their debt-like features, often appeal to more traditional investors or are used in slightly later "bridge" rounds leading up to a Series A, as they offer more defined downside protection through the maturity date and interest.

Ultimately, both instruments serve the same purpose: to get capital into a company quickly without having to set a firm valuation. Understanding their structural differences is key to negotiating terms that align the interests of both founders and investors.