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Model equity conversion for SAFE notes, convertible notes, UK ASA / SeedFAST, and French BSA Air — before you sign anything.

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🌍 US SAFE (YC post-money) 📄 Convertible Note 🇬🇧 ASA / SeedFAST 🇫🇷 BSA Air

SAFE Note Calculator — post-money cap (YC standard)

Used primarily in the US. Ownership % = Investment ÷ Post-money cap — locked at signing.
e.g. $1M on a $10M cap = exactly 10%, regardless of share count.
Exception: if the next round valuation is below the cap, the investor converts at the round price and may receive more than the headline ownership %.

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SAFE Note Inputs
Post-money valuation cap model
Next priced round (for discount comparison)
Note on terminology: A "20% discount" means the investor pays 20% less than the round price (80% of round price). YC uses "discount rate" to mean the rate itself (e.g. 80%) — this calculator uses the simpler term: just enter 20 to mean 20% off.
Post-money cap formula: Ownership % = Investment ÷ Cap. If the round valuation is below the cap, the SAFE converts at the lower round price — which means the investor gets more shares than the headline % implies.
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Fill in the inputs to model conversion

Convertible Note Calculator — principal + interest → shares

Convertible notes are debt instruments. They accrue interest (simple or compound), which converts alongside principal. Common globally but especially in the US and Europe.

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Convertible Note Inputs
Principal + accrued interest → shares
Priced round details
Unlike a SAFE, the full principal + accrued interest converts. Investors receive more shares than the headline investment implies. Choose simple or compound interest to match your note terms.
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Fill in the inputs to model note conversion

ASA / SeedFAST Calculator — UK standard

In the UK, founders don't use SAFEs. The standard instruments are the ASA (Advance Subscription Agreement) and SeedFAST.
Both convert at a pre-money valuation — not post-money like a YC SAFE. The discount and cap apply to the pre-money valuation at the qualifying round.

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ASA / SeedFAST Inputs
Converts at pre-money valuation cap
Qualifying round details
ASA / SeedFAST key differences vs SAFE:
• Converts at a pre-money cap (not post-money)
• Commonly used with SEIS/EIS — ensure your structure is SEIS/EIS compatible
• No interest, no maturity date (similar to SAFE in that respect)
• Cap and discount apply to the pre-money valuation at the qualifying round
• Investor gets whichever gives the lower price (more shares)
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Fill in the inputs to model ASA / SeedFAST conversion

BSA Air Calculator — French standard

In France, the standard early-stage instrument is the BSA Air (Bon de Souscription d'Actions — Accord d'Investissement Rapide).
Like the UK ASA, it converts at a pre-money valuation cap. A discount can be applied to the qualifying round price. No interest, no maturity date.

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BSA Air Inputs
Converts at pre-money valuation cap
Qualifying round details
BSA Air key features:
• Converts at a pre-money cap (same as ASA, different to YC SAFE)
• A French SAS or SA legal structure is typically required
• No interest accrual, no maturity date
• BSA Air can qualify for preferential French tax treatment (under certain conditions)
• The discount is applied to the share price at the qualifying round
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Fill in the inputs to model BSA Air conversion
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FAQ

Instruments explained
simply

What is the difference between a post-money SAFE and a pre-money SAFE?
A post-money SAFE (YC standard since 2018) locks the investor's ownership at signing: Ownership % = Investment ÷ Cap. A $1M investment on a $10M cap guarantees exactly 10%, regardless of share count. A pre-money SAFE (older format) calculates ownership at conversion using existing shares — the investor typically ends up with slightly less than expected (e.g. 9.09% instead of 10%). Post-money is more transparent and is now the global standard for SAFEs.
What does "20% discount" mean — is it different to "discount rate"?
Yes — and this trips everyone up. A "20% discount" means the investor pays 20% less than the round price (they pay 80% of the round price per share). YC uses the term "discount rate" to mean 80% — because that is the rate at which conversion happens. This calculator uses the more intuitive plain-English version: just enter 20 if the investor gets 20% off.
What happens if the next round valuation is below the SAFE cap?
If the qualifying round's pre-money valuation is below the cap, the SAFE converts at the round price (not the cap price). This means the investor gets a larger ownership % than the headline cap implied — because they're converting at a lower price. This is the correct behaviour for post-money SAFEs and is important for founders raising multiple SAFEs to model carefully.
What is an ASA or SeedFAST and why is it used in the UK instead of a SAFE?
UK companies cannot easily issue SAFEs because the SAFE structure is optimised for US Delaware C-Corps. UK founders use Advance Subscription Agreements (ASA) or SeedFASTs instead — both are legally similar to SAFEs but adapted for UK company law. A key difference is that ASAs/SeedFASTs convert at a pre-money valuation cap, not post-money. They are also commonly structured to be SEIS/EIS compatible, which gives investors significant UK tax reliefs.
What is a BSA Air and how does it work in France?
A BSA Air (Bon de Souscription d'Actions — Accord d'Investissement Rapide) is the French equivalent of the SAFE/ASA. It is issued by French SAS or SA companies and gives investors the right to subscribe to shares at a future qualifying round. Like the ASA, it converts at a pre-money valuation cap. A discount can be negotiated. No interest accrues and there is no maturity date. Under certain conditions it can qualify for preferential French tax treatment.
Should I use simple or compound interest for a convertible note?
Simple interest is far more common for convertible notes and is the default assumption unless your term sheet explicitly states otherwise. Simple interest = Principal × Rate × Years. Compound interest accrues interest on top of previously accrued interest — it results in more shares being issued to the investor and is generally more investor-friendly. Always check your note terms and clarify with your lawyer.
What is an MFN clause?
MFN stands for Most Favoured Nation. An MFN clause means that if you later issue another SAFE (or ASA/BSA Air) on better terms, the original investor automatically receives those better terms retroactively. It is considered founder-unfriendly because it creates uncertainty about the economics of future fundraises. Negotiate it out if you can — particularly if you plan to raise from multiple angels before a priced round.