How SAFEs Actually Convert: Post-Money vs Pre-Money Math

The One Formula That Governs Every SAFE Conversion

Every SAFE conversion reduces to a single question: what price per share does the SAFE holder pay when the priced round closes? Under the standard post-money SAFE, that price is the post-money valuation cap divided by the company capitalization. The number of shares the investor receives is their investment divided by that price. Because the company capitalization in the post-money SAFE includes every other SAFE and convertible, the arithmetic collapses to something a founder can do on a napkin: ownership sold = investment ÷ post-money cap. Raise $500K on a $5M post-money cap and you have sold exactly 10% — before the priced round’s own dilution, which we will get to.

If the SAFE carries a discount instead of a cap, the conversion price is the priced round’s share price multiplied by the discount rate — a 20% discount means the investor pays 80% of what the new money pays. When a negotiated SAFE has both mechanisms, the investor converts at whichever produces more shares. There is no scenario where the SAFE holder does worse than the new investors in the round: the cap and discount exist precisely to reward the earlier, riskier check.

This page works entirely from the current standard instruments: the post-money SAFE forms published on Y Combinator’s documents page (valuation cap only, discount only, and uncapped MFN, plus an optional pro rata side letter). For the history and clause-by-clause walkthrough of the agreement itself, see our Y Combinator SAFE agreement guide.

Post-Money vs Pre-Money SAFEs: What Actually Changed in 2018

Y Combinator introduced the original SAFE in late 2013 as a replacement for convertible notes. That original version was a pre-money SAFE: its valuation cap was stated pre-money, and — critically — the conversion denominator excluded all the other SAFEs the company had signed while including the option pool increase the priced round would demand. The result was that no investor, and no founder, could compute final ownership until the round actually closed. Each additional SAFE changed everyone’s math retroactively.

The 2018 post-money SAFE flipped both of those choices. YC’s own framing is that SAFE rounds had evolved from short bridges into standalone seed rounds, so the instrument should price like its own round. “Post-money” here means the cap is measured after all the SAFE money but before the priced-round money that converts the SAFEs. Two consequences follow directly from the definitions:

Because the post-money denominator includes the other SAFEs, each investor’s percentage is locked at signing and SAFEs cannot dilute each other. And because it excludes the priced round’s pool increase, that increase dilutes the SAFE holders along with everyone else when the round closes. The transparency cuts both ways: investors know exactly what they bought, and founders bear exactly the ownership they sold — every point of it.

Conversion at a Priced Round: A Worked Example

The following is a hypothetical with round numbers chosen so you can check every line yourself — it is not market data. A company has 9,000,000 shares outstanding (founders plus an existing option pool). It raises $500K on a post-money SAFE with a $5M post-money valuation cap, then later closes a $2M Series A at a $12M pre-money valuation, with no new pool increase (we add that wrinkle below).

Step 1: The cap sets the SAFE’s ownership and price

Ownership sold = $500K ÷ $5M = 10%. For the investor to own 10% after conversion, the 9,000,000 existing shares must be 90% of the post-conversion total: 9,000,000 ÷ 0.90 = 10,000,000 shares, so the SAFE converts into 1,000,000 shares. Equivalently, the SAFE price per share is the cap divided by the company capitalization: $5,000,000 ÷ 10,000,000 = $0.50, and $500K ÷ $0.50 = the same 1,000,000 shares. Both routes agree — that internal consistency is the whole design of the post-money SAFE.

Step 2: Cap vs discount — the investor takes the better price

The Series A prices at $12M pre-money across 10,000,000 fully diluted shares (which include the SAFE’s conversion shares), so the round’s price per share is $12M ÷ 10,000,000 = $1.20. Had the SAFE investor simply invested at the round price, $500K would buy 416,667 shares. The $5M cap instead gives them 1,000,000 shares at $0.50 — 2.4x more stock for the same check, exactly the ratio of the $12M pre-money to the $5M cap. A discount-only SAFE at 20% would convert at $1.20 × 0.80 = $0.96 for 520,833 shares: better than paying full price, far worse than this cap. Discounts only beat caps when the round prices low relative to the cap — which is also the scenario where founders keep the most.

Step 3: The new money dilutes everyone — including the SAFE

The Series A investor’s $2M at $1.20 buys 1,666,667 new shares, taking the total to 11,666,667. Final ownership: founders 9,000,000 ÷ 11,666,667 = 77.1%; SAFE holder 1,000,000 ÷ 11,666,667 = 8.6%; Series A investor 14.3% (which is just $2M ÷ $14M post-money). Notice the check the algebra hands you: the SAFE holder’s 10% times (1 − 14.3%) equals 8.6%. The SAFE’s 10% was never a post-Series-A number — the SAFEs are their own round, and the priced round dilutes them like everyone else. Add a typical option pool increase (which, in the standard NVCA-style priced round documents, is carved out of the pre-money side) and the founders’ share drops further while the new investors’ percentage holds.

Want to run your own numbers instead of ours? Our free SAFE calculator models cap, discount, and round-size scenarios, and the dilution calculator layers rounds on top of each other.

Multiple-SAFE Stacking: Why the Math Compounds Against Founders

Because post-money SAFEs do not dilute each other, stacking them is brutally additive for founders. Each SAFE’s ownership is its investment divided by its own cap, and the founders absorb the sum. Here is a hypothetical stack — again, round numbers chosen for verifiability, not market benchmarks — followed by a Series A that sells 20% to new investors and adds a pool equal to 10% of the post-round company:

Verify any line: SAFE 1 is $250K ÷ $4M = 6.25%; SAFE 3 is $750K ÷ $10M = 7.5%. Together the three SAFEs sold 20% for $1.5M, and the founders were at 80% before the Series A term sheet arrived. The round then takes 30% of the whole company (20% new money + 10% pool), so every pre-round holder keeps 70% of their stake: the SAFEs land at 14%, the founders at 56%. A founding team that mentally anchored on “we only gave up 10% at the first cap” discovers it owns just over half the company at the Series A close.

The discipline this math demands is simple: keep a running total of investment ÷ cap across every SAFE outstanding, and re-run the stack against your realistic next-round size before signing the next one. If you also granted pro rata side letters, model those too — YC’s User Guide walks through how pro rata participation adds several more points of dilution on top of the round itself. For how these conversions land on your cap table structurally, see our cap table guide.

Cap, Discount, or MFN: Choosing the Conversion Mechanism

The current YC standard offers three US post-money SAFE forms, each with a different conversion trigger. The valuation cap version fixes ownership at signing and is the default for most rounds — it is the only version where both sides can state the deal in one sentence (“$500K for 10%”). The discount version defers pricing entirely to the next round, minus a negotiated percentage; it suits investors who trust the next round to price fairly and founders who expect a strong markup. The uncapped MFN (most favored nation) version has neither cap nor discount, but automatically adopts the terms of any better SAFE the company issues later — common for the very first small checks written before anyone can credibly argue a valuation.

Whichever mechanism you choose, remember what the SAFE is not: it is not a priced round, it carries no board seat, no formal governance, and no investor protections like the anti-dilution provisions that arrive with preferred stock (covered in our anti-dilution guide). If you are weighing whether to raise on SAFEs at all versus pricing the round now, our SAFE vs priced round comparison covers that decision, and the term sheet guide covers what the eventual priced round will negotiate.

Three Mistakes Founders Make With SAFE Math