Convertible Note & SAFE Modeling in Excel: Complete Guide (2026)

June 24, 2026 · VeloraAI Team
Financial Modeling Formulas Excel

A Series A lead once told a founder her cap table was "off by 3.2 percent" and walked away from a $14 million term sheet. The error wasn't fraud — it was a SAFE conversion that had been modeled at the post-money valuation cap when the actual instrument was a pre-money cap with a 20 percent discount. The 3.2 percent gap was the difference between a $24.6 million pre-money valuation and a $25.4 million headline — and a founder who didn't understand her own securities. Convertible note and SAFE modeling in Excel is not a niche skill anymore; if you cover early-stage companies, you will model a half-dozen of these instruments in a single closing week, and getting the math wrong is how rounds die at the eleventh hour.

This guide walks through the full mechanics — the conversion price math for both convertible notes and SAFEs, the valuation cap and discount waterfall, MFN provisions, accrued interest treatment, and the priced-round dilution calculation that every founder, lawyer, and investor will recompute the moment you hand them the spreadsheet. Every formula works in Excel 365.

What Is a Convertible Note vs. a SAFE?

A convertible note is a debt instrument that converts to equity at a future priced round. It carries an interest rate (typically 4–8 percent), a maturity date (12–24 months), a valuation cap, and usually a discount (15–25 percent). A SAFE (Simple Agreement for Future Equity), invented by Y Combinator in 2013 and rewritten in 2018, is not debt — it has no interest, no maturity, and no repayment obligation. It converts on the same triggering events but the math is cleaner because there is no accrued interest to roll into the conversion.

Both instruments solve the same problem: pricing an early-stage company is hard, so investors delay the valuation conversation until a later priced round has set a market price. The cap and discount protect the early investor for taking risk before that price is known.

Feature Convertible Note Pre-Money SAFE (2013) Post-Money SAFE (2018)
Legal form Debt Equity-like contract Equity-like contract
Interest Yes (4–8%) No No
Maturity date Yes (12–24 mo) No No
Valuation cap Pre-money Pre-money Post-money
Discount Common (15–25%) Common (10–20%) Optional
Dilution to founder Diluted by next round Diluted by next round Locked at issuance
Most common in 2026 Bridge rounds Rare (legacy) Standard YC SAFE

ℹ️ Note: The 2018 post-money SAFE fixed the most-confusing part of the original instrument: the SAFE holder's ownership is now locked at signing, not recomputed at the priced round. This shifts dilution risk entirely to the founder, which is why founders must model every SAFE before signing.

Convertible note modeling in Excel showing conversion price and dilution waterfall

How Does a Convertible Note Convert to Equity?

A convertible note converts at the lower of two prices: (1) the priced-round share price multiplied by (1 − discount), or (2) the valuation cap divided by the company's fully-diluted share count immediately before the priced round. The note holder receives shares equal to the principal plus accrued interest divided by that conversion price. The lower-price rule is the entire economic point of the instrument — it guarantees the investor gets the better of cap protection or discount protection.

The mechanics matter because the two prices answer different questions. The discount price rewards the investor for the time-value of writing the check months before the priced round. The cap price rewards the investor for finding a company before its valuation ran up. A high-growth company hits the cap; a slow-growth company hits the discount.

The Three Conversion Triggers

Every convertible instrument lists three events that can trigger conversion:

  1. Qualified Financing — A priced equity round above a threshold size (typically $1M+). Triggers conversion to preferred stock at the lower of cap or discount.
  2. Liquidity Event — Acquisition, merger, or IPO before a priced round. The note holder typically receives the greater of (a) 1× to 2× principal back in cash, or (b) shares at the cap as if converted.
  3. Maturity — For convertible notes only. If no qualified financing happens before maturity, the note becomes due. In practice it gets extended or converted to common at a negotiated price; SAFEs have no maturity so this trigger does not apply.
graph TD
    A[Note Issued: $500K] --> B{Trigger Event?}
    B -->|Priced Round| C[Compute Cap Price]
    B -->|Priced Round| D[Compute Discount Price]
    C --> E[Use Lower of Cap or Discount]
    D --> E
    E --> F[Shares = Principal + Interest / Lower Price]
    B -->|Acquisition| G[Greater of 1-2x Cash or Cap Shares]
    B -->|Maturity| H[Renegotiate or Convert to Common]

How Do You Build a Convertible Note Model in Excel?

Build a convertible note model in Excel in five steps: (1) lay out the instrument inputs (principal, interest rate, issue date, cap, discount, MFN); (2) compute accrued interest using DAYS360 or a day-count fraction; (3) calculate the cap conversion price and the discount conversion price separately; (4) take the lower price and divide principal-plus-interest by it to get shares; (5) integrate the new shares into a pre/post-money cap table to show dilution to founders and existing holders. The model should handle multiple notes with different caps and discounts at the same priced round.

The rest of this section walks through each step with real formulas.

Step 1: Workbook Architecture

A clean convertible model has four sheets, in this order:

  1. Inputs — instrument terms (principal, rate, dates, cap, discount, MFN flags) for each note or SAFE, one row per instrument
  2. Priced Round — pre-money valuation, new investor check size, option pool refresh, fully-diluted share count before conversion
  3. Conversion Math — per-instrument cap price, discount price, lower-of, and shares issued
  4. Cap Table — pre-financing, post-conversion, post-new-money, with ownership percentages

Use the model-color convention so reviewers and lawyers can audit at a glance:

  • Blue font — hard-coded inputs (term sheet values, signed instrument terms)
  • Black font — formulas referencing the same sheet
  • Green font — formulas referencing a different sheet
  • Red font — external file links (avoid these — keep all data in one file for diligence)

⚠️ Warning: Founders' counsel will recompute every formula in your model before closing. A hard-coded number where there should be a formula — or a formula that hides a hard-coded assumption — will get flagged in red-line markup and delay closing. Use named ranges (Insert > Name > Define) for all term inputs so the lawyer can audit the logic in plain English.

Step 2: Accrued Interest Calculation

Convertible notes accrue simple (not compound) interest on the principal from the issue date to the conversion date. The standard day-count convention is actual/365. If your principal is in cell B5, the issue date is in C5, the interest rate is in D5, and the conversion date is in cell $B$2 on the Priced Round sheet, the accrued interest formula is:

=B5*D5*(PricedRound!$B$2-C5)/365

For a $500,000 note issued June 1, 2025 at 6 percent interest converting on September 15, 2026, this returns:

=500000 * 0.06 * (DATE(2026,9,15) - DATE(2025,6,1)) / 365
= $39,452.05

The principal-plus-interest balance at conversion is =B5+InterestFormula, or in one cell:

=B5*(1+D5*(PricedRound!$B$2-C5)/365)

SAFEs have no interest, so this step is omitted. The SAFE conversion uses just the original investment amount.

💡 Pro Tip: Use ISO 8601 date format (YYYY-MM-DD) in input cells and apply the date format to display. Notes signed in different jurisdictions sometimes get input as 06/05/2025 — which is June 5 in the US and May 6 in the UK. A four-month accrued-interest error is the kind of thing that gets caught in legal diligence and unsigned.

Step 3: Calculate the Cap Price and Discount Price

The cap conversion price is the valuation cap divided by the fully-diluted share count immediately before the new round closes. "Fully diluted" includes common, preferred, options (granted and reserved), warrants, and — critically — every other convertible note or SAFE in the round. If the cap is in cell E5 and the pre-money fully-diluted share count is in PricedRound!$B$5, the cap price is:

=E5/PricedRound!$B$5

The discount conversion price is the priced-round per-share price (pre-money valuation divided by pre-money fully-diluted shares) multiplied by 1 minus the discount. If the discount is in cell F5 and the priced round per-share price is in PricedRound!$B$6:

=PricedRound!$B$6*(1-F5)

The conversion price for the note is the lower of these two:

=MIN(E5/PricedRound!$B$5, PricedRound!$B$6*(1-F5))

Example: A $500K note with a $10M cap and a 20% discount, converting at a $20M pre-money round with 10M fully-diluted shares: Cap price = $10M / 10M shares = $1.00. Discount price = ($20M / 10M) × 0.80 = $1.60. The note converts at $1.00 (the cap was the better protection). The note holder receives $539,452 / $1.00 = 539,452 shares.

Step 4: Calculate Shares Issued

Shares issued to the note holder equal the principal-plus-interest balance divided by the conversion price:

=(B5*(1+D5*(PricedRound!$B$2-C5)/365))/MIN(E5/PricedRound!$B$5, PricedRound!$B$6*(1-F5))

If you have multiple notes, repeat this row for each one — keep one row per instrument. The total new shares from conversion is =SUM(G5:G20) where column G holds the per-note shares issued. Add this to the new investor's shares (their check divided by the priced-round per-share price) to get total new shares issued in the round.

Step 5: Build the Post-Conversion Cap Table

The full cap table should show three columns: (1) pre-financing shares and percentages, (2) post-SAFE/note-conversion but pre-new-money, and (3) post-new-money fully diluted. The formula for ownership percentage in column 3 for each holder is:

=Shares/SUM($Shares$Column$)

The founder dilution is the difference between column 1 percentage and column 3 percentage. This is what the founder will scrutinize, and it must be exactly what their attorney calculates independently. For the complete cap table build — multiple rounds, option pool top-ups, liquidation preferences, and exit waterfall formulas — see our cap table modeling guide in Excel.

How Does a Post-Money SAFE Differ in the Math?

A post-money SAFE locks the investor's ownership percentage at signing, not at the priced round. The math runs in reverse: the SAFE holder's ownership equals the SAFE amount divided by the post-money cap. A $500K SAFE with a $10M post-money cap converts to exactly 5.0 percent ownership of the company post-money, no matter what happens in the priced round.

This shifts all dilution risk to the founder. In a pre-money SAFE world, if a second SAFE is signed at a higher cap later, the original SAFE holder's percentage decreases because the cap-implied price per share rises. In a post-money SAFE world, each new SAFE dilutes the founder directly — the percentages of existing SAFE holders are unchanged.

The Excel formula for post-money SAFE ownership is brutally simple:

=SAFEAmount/PostMoneyCap

The complexity moves to the founder's side of the equation. Post-money SAFE conversion shares are calculated as:

=PostConversionFullyDilutedShares * (SAFEAmount/PostMoneyCap)

But "post-conversion fully diluted shares" depends on every other SAFE in the stack, which is why post-money SAFE cap tables become recursive when you have multiple notes at different post-money caps. The clean way to model it in Excel is to compute each SAFE holder's percentage independently, then back-solve for the founder's percentage as 1 − SUM(SAFE percentages) − new money percentage − new option pool.

💡 Pro Tip: When stacking three or more post-money SAFEs, do not try to compute conversion in one cell. Build a small table that lists each SAFE's locked percentage, then compute the founder's residual ownership last. Recursive formulas in convertible math are how you introduce circular references that nobody can audit at 2 a.m. before closing.

How Do You Model an MFN (Most Favored Nation) Provision?

A Most Favored Nation clause grants a SAFE or note holder the right to retroactively adopt the terms of any later instrument with better terms. In Excel, model MFN by adding a flag column for each instrument and using a MAX/MIN logic across all notes flagged MFN to pick the best cap and discount. If column H holds the MFN flag (TRUE/FALSE), the effective cap for an MFN holder is:

=IF(H5, MIN(IF($H$5:$H$20, $E$5:$E$20)), E5)

Enter this as an array formula in older Excel (Ctrl+Shift+Enter); in Excel 365 it spills automatically. The effective discount is:

=IF(H5, MAX(IF($H$5:$H$20, $F$5:$F$20)), F5)

MFN is one of the most overlooked sources of dilution surprise. A founder signs three SAFEs over six months, the third has a lower cap because the market shifted, and the first two — if they have MFN — automatically adopt the lower cap. Suddenly the founder is more diluted than the spreadsheet showed at the time each SAFE was signed.

⚠️ Warning: Always model MFN propagation BEFORE signing each new SAFE. If the new instrument has more favorable terms for the investor, every prior MFN holder will benefit. Run the conversion math twice — once at "current terms" and once at "post-new-SAFE terms" — and show the founder both before they sign.

Convertible Note vs. SAFE: Which Should You Recommend?

For pre-seed and seed rounds in 2026, the post-money SAFE has effectively won. It is fast, founder-and-investor friendly (with one critical caveat), legally standardized, and avoids the messy interest accrual and maturity-extension paperwork of a convertible note. Convertible notes remain dominant in two specific cases: bridge financings between priced rounds (where the lender wants debt seniority) and rounds with non-institutional investors who want interest payments.

Use Case Recommended Instrument Why
First check, pre-revenue startup Post-money SAFE Standard YC docs, fast close
Bridge between Series A and B Convertible note Lender wants debt seniority and interest
Angel round with high-net-worth individuals Convertible note Investors want interest accrual
Accelerator program Post-money SAFE Standardized across cohort
International (non-US) startups Convertible note SAFE legal status varies by jurisdiction
Equity crowdfunding (Reg CF) Post-money SAFE Required by most platforms

Common Modeling Errors That Sink Term Sheets

The five errors below account for almost every "the cap table was wrong" closing delay we have seen on convertible rounds.

  1. Using the post-money cap as a pre-money cap — The 2018 post-money SAFE explicitly states the cap is post-money. Plugging a $10M post-money cap into a pre-money formula understates dilution by roughly the SAFE size as a percentage of the cap. On a $1M SAFE with a $10M post-money cap, the modeling error is roughly 10 percent of cap, or 1 percent of the company.
  2. Forgetting accrued interest — A 6 percent note outstanding for 18 months accrues 9 percent of principal. On a $500K note that is $45,000 of additional principal that converts to additional shares.
  3. Excluding option pool from fully-diluted count — Most term sheets define the priced-round per-share price using a fully-diluted count that includes the new option pool refresh. Convertibles convert at that same price (or a discounted version of it). Forgetting the new pool overstates the per-share price and underdilutes the converters.
  4. Stacking MFN cascades wrong — If three SAFEs have MFN and a fourth comes in with a better cap, all four adopt the better cap. If only two of the first three have MFN, only those two upgrade.
  5. Modeling at issuance rather than at conversion date — Cap price depends on the FD share count at conversion, not at issuance. A new option grant between issuance and conversion changes the cap price.

ℹ️ Note: Founders should run the conversion math themselves before signing every convertible instrument. Counsel can verify the legal terms, but the dilution arithmetic is the founder's responsibility. A founder who cannot model their own cap table will lose negotiating leverage in every priced round.

What Excel Functions Are Essential for Convertible Modeling?

Five Excel functions cover almost all the math: MIN for the lower-of cap-or-discount conversion, IF for trigger-event branching, DAYS360 or simple date subtraction for interest accrual, SUMIFS for rolling up multiple notes by series or cohort, and XLOOKUP for pulling instrument terms from a master inputs table. For multiple-note stacks, dynamic arrays (FILTER, SORT, UNIQUE) make the conversion table maintainable without copy-paste.

A robust conversion calculation for a stack of notes, using LET for readability:

=LET(
  principal, B5,
  rate, D5,
  issueDate, C5,
  convDate, PricedRound!$B$2,
  cap, E5,
  discount, F5,
  fdShares, PricedRound!$B$5,
  newPrice, PricedRound!$B$6,
  balance, principal*(1+rate*(convDate-issueDate)/365),
  capPrice, cap/fdShares,
  discPrice, newPrice*(1-discount),
  convPrice, MIN(capPrice, discPrice),
  balance/convPrice
)

This single LET formula handles the entire per-note conversion. Drop it down the column once for each instrument, and you have a fully auditable conversion table that any reviewer can read top to bottom.

Frequently Asked Questions

What happens to a convertible note if the company is acquired before a priced round?

The acquisition is a triggering event, and the note converts under the "change of control" clause. Most modern term sheets give the investor a choice between (a) repayment of 1× to 2× the principal plus accrued interest in cash, or (b) conversion at the cap as if a priced round had occurred at the cap valuation. The investor will pick whichever is greater. Model both paths and let the founder see which one wins under acquisition price scenarios above and below the cap.

Can a founder negotiate the cap and discount, or are they take-it-or-leave-it?

Both are fully negotiable. The cap is the more contested term because it sets the worst-case dilution for the founder. Discounts are more standardized (15–20 percent is market). Founders with leverage — strong traction, multiple term sheets — push for higher caps and lower discounts. Founders without leverage often accept the lead investor's first offer. The negotiation should always happen with a fully-built dilution model showing the founder's percentage at multiple priced-round valuations. A two-way sensitivity table in Excel across cap levels and implied priced-round valuations turns this into a grid the founder can reference in real time during negotiations.

How is a SAFE treated for tax purposes?

A pre-money SAFE has historically been treated as equity-like for tax purposes, but the IRS has never issued definitive guidance. A post-money SAFE is even less settled. Most founders and investors treat SAFEs as not-yet-equity until conversion, meaning the QSBS five-year holding period for Section 1202 starts at conversion, not at SAFE signing. This is a major hidden cost for early investors — always consult tax counsel before treating SAFE issuance dates as the QSBS start.

What is the difference between a discount and a valuation cap in practice?

A discount gives the investor a fixed percentage off the next round's per-share price. A cap sets a maximum implied valuation at conversion. The discount rewards the investor for writing the check earlier; the cap rewards the investor for finding the company before its valuation grew. The note converts at the lower of the two implied prices, so high-growth companies always trigger the cap and low-growth companies trigger the discount. Most modern SAFEs use a cap only; most convertible notes use both.

How do you handle multiple convertible notes with different caps?

Each note converts independently at its own lower-of (cap-price, discount-price). Build one row per note in the conversion table. The aggregate dilution is the sum of all individual conversions plus the new money. The order of conversion does not matter mathematically because all conversions happen simultaneously at the priced round. Watch for MFN cascades, which can collapse multiple cap structures into one effective cap across the stack.

Closing: From Spreadsheet to Term Sheet

Convertible note and SAFE modeling is one of those skills where the difference between a junior and a senior analyst is invisible until closing day. For most early-stage companies issuing these instruments, the conversion mechanics sit inside a broader SaaS or startup operating model — our SaaS startup financial model guide in Excel shows how to build the ARR waterfall, burn rate schedule, and post-close capitalization table that give these conversion calculations their full context. The senior analyst built a model the founder, lead investor, and counsel can all recompute independently and arrive at the same number. The junior analyst built one that needs to be rebuilt every time someone asks "what if we raised the cap by ten percent?" — and the round closes a week late.

If you want to skip the manual workbook setup and let AI generate the conversion formulas, accrued interest calculations, and post-money cap table directly inside Excel from a plain-English description of your term sheet, VeloraAI's Excel add-in handles convertible and SAFE conversion math natively — and produces the same dilution waterfall lawyers will recompute by hand. Build your next conversion model in minutes, not hours, and spend your time negotiating the cap instead of debugging the spreadsheet.