SAFE Dilution Calculator
Stack your SAFEs and convertible notes, price the round, and see the cap table that actually results — including the option-pool increase almost nobody models. A post-money SAFE sells a percentage, a note converts at a price, and the two are measured against different denominators; multiplying headline percentages together gets the answer wrong every time.
If you are still choosing between the instruments, start with SAFE vs convertible note. If the priced round itself is the unfamiliar part, read what equity financing is first. The calculator loads with a worked example — Bramblewick Labs, Inc., a fictional two-founder Delaware C-corp invented for teaching — so you can see the shape of the answer before you type your own numbers over it.
1. Capitalization before any SAFE
Reserved but ungranted. It sits inside Company Capitalization, so the SAFEs are measured against it.
2. SAFEs (3 of 6)
3. Convertible notes (1 of 3)
The note converts at the lower of its cap price and this discounted price.
4. The priced round
Round price per share
$1.1738
Pre-money valuation ÷ pre-money fully diluted shares
Company Capitalization
12,926,886
The denominator every post-money SAFE percentage is measured against
Post-round fully diluted
17,038,408
Including a 703,841-share pool increase
Founders after the round
52.82%
The napkin said 57.60%
Pre-round: Company Capitalization = 12,926,886 shares
This is the only percentage a post-money SAFE actually promises: purchase amount ÷ post-money valuation cap, measured on founders plus the existing unissued pool plus every converting SAFE and note. The round’s new shares (3,407,682) and the pool increase (703,841) are deliberately outside this denominator.
| Holder | Instrument | Amount | Cap | Conversion price | Shares | % of capitalization |
|---|---|---|---|---|---|---|
| Founders (common) | Common stock | — | — | — | 9,000,000 | 69.62% |
| SAFE 1 — angels (month 0) | Post-money SAFE | $400,000 | $5,000,000 | $0.3868 | 1,034,151 | 8.00% |
| SAFE 2 — seed fund (month 7) | Post-money SAFE | $1,000,000 | $10,000,000 | $0.7736 | 1,292,689 | 10.00% |
| SAFE 3 — strategic (month 11) | Post-money SAFE | $350,000 | $14,000,000 | $1.0830 | 323,172 | 2.50% |
| Convertible note — family office | Convertible note | $260,000 | $10,000,000 | $0.9391 | 276,874 | 2.14% |
| Option pool (existing, unissued) | Reserved, ungranted | — | — | — | 1,000,000 | 7.74% |
| Company Capitalization | — | — | — | — | 12,926,886 | 100.00% |
Post-round cap table at $1.1738 per share
Pre-money fully diluted 13,630,727 shares, post-round fully diluted 17,038,408. The SAFE holders end at 15.55% against the 20.50% of Company Capitalization their post-money caps promised, because the round itself (20.00%) and the pool increase (4.13%) dilute them.
| Holder | Instrument | Conversion price | Shares | % post-round |
|---|---|---|---|---|
| Founders (common) | Common stock | — | 9,000,000 | 52.82% |
| SAFE 1 — angels (month 0) | Post-money SAFEconverted on its cap | $0.3868 | 1,034,151 | 6.07% |
| SAFE 2 — seed fund (month 7) | Post-money SAFEconverted on its cap | $0.7736 | 1,292,689 | 7.59% |
| SAFE 3 — strategic (month 11) | Post-money SAFEconverted on its cap | $1.0830 | 323,172 | 1.90% |
| Convertible note — family office | Convertible noteconverted on its discount | $0.9391 | 276,874 | 1.62% |
| Option pool (unissued, after top-up) | Reserved, ungranted | — | 1,703,841 | 10.00% |
| Series A investor (new money) | Series A Preferred | $1.1738 | 3,407,682 | 20.00% |
| Total | — | — | 17,038,408 | 100.00% |
Convertible note — family office converted at $0.9391 on its discount.A note that converts on its discount is a note whose cap never bound. Check which prong governed before you tell a founder the cap protected them.
What you told yourself vs what happened
- The napkin: 20.00% sold on SAFEs, then 20.00% sold in the round57.60%
- The same napkin, using the SAFEs’ true 20.50% of Company Capitalization57.24%
- What the founders actually own, after the note and the pool increase52.82%
Gap: 4.78 points. The napkin misses three things in this order: the SAFEs are 20.50% of Company Capitalization rather than the round number you remember, the convertible note converts too, and the option-pool increase is created in the pre-money, so founders and every converting holder pay for it while the new investor does not.
What if every SAFE had carried one cap?
Re-solves the whole stack with every capped SAFE moved to a single valuation cap, holding the notes and the round constant. It is the fastest way to see what an early, low cap on a small cheque cost.
| Holder | Shares as modelled | Shares at one cap | % at one cap |
|---|---|---|---|
| Founders (common) | 9,000,000 | 9,000,000 | 55.05% |
| SAFE 1 — angels (month 0) | 1,034,151 | 497,728 | 3.05% |
| SAFE 2 — seed fund (month 7) | 1,292,689 | 1,244,321 | 7.61% |
| SAFE 3 — strategic (month 11) | 323,172 | 435,512 | 2.66% |
| Convertible note — family office | 276,874 | 265,646 | 1.63% |
| Option pool (unissued, after top-up) | 1,703,841 | 1,634,744 | 10.00% |
| Series A investor (new money) | 3,407,682 | 3,269,488 | 20.00% |
Founders land at 55.05% instead of 52.82%, a difference of 2.23 points, and the round prices at $1.2234 instead of $1.1738.
What is 20% off each cap worth?
One SAFE at a time, cap cut by a fifth, everything else held constant. The point is the ranking, not the decimals: the earliest, smallest cheque usually has the largest cap sensitivity, because its cap is the lowest and its percentage is the one that compounds across everything issued afterwards.
| SAFE | Cap | Cap minus 20% | Its shares | Founders’ % | Δ vs base |
|---|---|---|---|---|---|
| SAFE 1 — angels (month 0) | $5,000,000 | $4,000,000 | 1,034,151 → 1,327,078 | 51.34% | -1.48 pp |
| SAFE 2 — seed fund (month 7) | $10,000,000 | $8,000,000 | 1,292,689 → 1,669,955 | 50.97% | -1.85 pp |
| SAFE 3 — strategic (month 11) | $14,000,000 | $11,200,000 | 323,172 → 407,263 | 52.36% | -0.46 pp |
Solved by damped fixed-point iteration: 73 iterations, final residual 0.00e+0 shares, tolerance 1e-9 shares, hard stop at 200 iterations. Shares are rounded for display only; percentage columns use the largest-remainder convention so each one sums to exactly 100.00%. This is an ownership model, not legal, tax or investment advice, and it ignores liquidation preferences entirely.
How does this calculator model a stack of SAFEs?
In plain English, in the order the math runs. First, each post-money SAFE is turned into a percentage: purchase amount divided by post-money valuation cap. That percentage is what the instrument sells, and it does not move. Second, that percentage is applied to Company Capitalization — the founders’ common, the existing unissued option pool, every converting SAFE, and every converting note, but not the new round’s shares and not the option-pool increase adopted with the round. Third, each convertible note converts its principal plus accrued simple interest at the lower of its cap price and its discount to the round price. Fourth, the round prices at pre-money valuation divided by pre-money fully diluted shares, where fully diluted includes everything that just converted plus the pool increase.
The definitions come straight from the instrument. The Y Combinator post-money SAFE defines the Safe Price as the post-money valuation cap divided by Company Capitalization, and defines Company Capitalization to include all converting securities — other SAFEs and convertible promissory notes — plus the unissued option pool, excluding any increase to that pool made in connection with the financing. YC’s own SAFE user guide states the consequence: the safes are not diluted by each other, but they are diluted by the new money raised in the equity financing, and the post-money cap is not “post” the new or increased option pool adopted as part of the round.
Why is the math circular, and how is it solved?
Because every input depends on an output. The SAFEs’ share counts depend on Company Capitalization; Company Capitalization includes the note’s shares; the note’s shares depend on the round price when it converts on its discount; the round price depends on the pre-money fully diluted count, which includes the SAFEs, the note and the pool increase; and the pool increase is sized from the post-round total, which depends on all of it. This is not a matter of judgement — the system has one answer.
The calculator finds it by damped fixed-point iteration: guess Company Capitalization, derive the pool increase, the round price and every conversion, add them up, average the result with the previous guess, and repeat until the answer stops moving. The bar is a residual of 1e-9 shares, with a hard stop at 200 iterations; the worked example settles in the seventies with a residual of zero or one unit in the last place of a double. The iteration count and the residual are printed under the tables, so you can check the solve rather than take it on faith. The same system also has a closed form — substituting the pool equation into the capitalization equation collapses it to a single linear equation in the pre-money share count — and the two agree to within about two billionths of a share.
What assumptions does the model make?
All of them, stated, because a dilution number is only as good as its assumptions:
- Everything is fully diluted. No warrants, no restricted stock units, no promised options, no most-favoured-nation or pro-rata side letters.
- The existing option pool is unissued and sits inside Company Capitalization, per the SAFE’s own definition. With no promised options outstanding, the pool increase adopted with the round stays outside it.
- Every included SAFE and note converts at the priced round. None terminates, none is repaid in cash, and note interest is simple rather than compounding, accrued over the months you enter.
- SAFEs convert into shadow preferred at their own conversion price, and one preferred equals one common on an as-converted basis.
- The round price is the standard pool-shuffle price: pre-money valuation divided by pre-money fully diluted shares including the converting securities and the pool increase, so the new investor is not diluted by the pool.
- Shares are rounded to whole numbers for display only; the solve itself is unrounded, and percentage columns use the largest-remainder convention so they total exactly 100.00%. A real closing rounds each certificate and leaves a residue of a share or two.
- It is an ownership model only. No anti-dilution, no liquidation preference, no participation, no waterfall.
One assumption is worth calling out separately. The original 2013 pre-money SAFE is no longer published on YC’s documents page, so the pre-money option in this calculator is modelled from YC’s own description of the original form in its user guide and from the clause as restated in issuer filings on SEC EDGAR full-text search. Treat the pre-money results as directionally right rather than as a quotation of a contract you signed. If you hold a legacy pre-money SAFE, the operative text is the one in your own document.
What does this calculator not do?
It is not legal or tax advice, and nothing on this page is an opinion about your instruments. Conversion turns on the words in the document you actually signed — a discount that stacks with a cap, a most-favoured-nation clause, a pro-rata side letter, a broad-based weighted-average adjustment, or a note that matures before the round all change the answer, and none of them are modelled here. It does not manage a cap table either: there is no ledger, no certificate numbering, no 409A, no board consents, and no audit trail of amendments. Use it to understand the shape of an outcome and to check a term sheet before the call, then let your cap-table software and your lawyer produce the closing numbers.
It also will not tell you what a good cap is. And to be plain about who wrote it: I run Peony, a data room company, not a cap-table platform. Peony is a data room used by 6,800+ customers; the signed instruments this calculator models usually live in a Data Room plan at $52 per admin per month, where per-viewer dynamic watermarks and page-by-page analytics matter once a lead’s counsel is reading them. A first tracked deck link is free at $0. That is the whole pitch, and it is beside the point of this page: the arithmetic above is yours whether or not you ever open a room.
SAFE dilution calculator FAQ
How does this SAFE dilution calculator work?
It solves the conversion the way the documents actually read, rather than multiplying headline percentages together. A post-money SAFE buys a fixed percentage of Company Capitalization equal to its purchase amount divided by its post-money valuation cap, so a $400,000 SAFE on a $5,000,000 cap is 8.00% and stays 8.00% no matter how many SAFEs sign after it. Company Capitalization is founders plus the existing unissued option pool plus every converting SAFE and note, and it deliberately excludes the new round's shares and the pool increase adopted with the round. Convertible notes convert their principal plus accrued simple interest at the lower of their cap price and a discount to the round price. Because each of those numbers depends on the others, the calculator solves the whole system by damped fixed-point iteration to a tolerance of 1e-9 shares, capped at 200 iterations, and reports the iteration count and residual so you can audit the answer instead of trusting it.
Do post-money SAFEs dilute each other?
No, and that is the entire design change from the original 2013 form. Y Combinator's own post-money SAFE user guide puts it plainly: the safes are not diluted by each other, but the safes will be diluted by the new money raised in the equity financing. Every post-money SAFE's percentage of Company Capitalization is fixed the day it is signed, so when a second and third SAFE arrive, the shares needed to deliver the first SAFE's percentage grow, and those extra shares come out of the founders. The original pre-money SAFE worked the other way: each SAFE converted on a capitalization that excluded the other converting SAFEs and included the round's option pool increase, so the SAFEs diluted each other and nobody could compute anyone's percentage on the day they signed. Set a SAFE's cap basis to pre-money in the calculator to watch both effects at once.
Is a convertible note's valuation cap pre-money or post-money?
Read the instrument, because the same headline number produces two different answers. A cap struck on a pre-money basis against the fully diluted capitalization excluding the converting securities gives a fixed cap price the day the note is signed: a $10,000,000 cap against a 10,000,000-share cap table is $1.00 per share, full stop. The same $10,000,000 read as a post-money cap behaves like a SAFE percentage instead, and the noteholder's stake grows with everything else that converts. In the worked example on this page that single word moves the noteholder from 276,874 shares to 338,101 shares, a swing of 61,227 shares, and moves the founders by roughly a third of a point. The tell that a model is wrong is an implied conversion price below both the stated cap price and the discount price.
Why does the option pool increase come out of the pre-money?
Because that is what the lead is asking for when it says the pool should be a set percentage of the post-round company. Sizing the pool to 10% of post-round fully diluted shares and creating the increase in the pre-money means the new shares are issued before the round closes, so they dilute the founders, the SAFE holders and the noteholders, and they do not dilute the incoming investor. It is also the largest single item founders forget: in the worked example the increase is 703,841 newly reserved shares, 4.13% of the post-round company, and it costs the founders about 2.88 points on its own, more than the converting note does. The calculator lets you move the increase outside the pre-money so you can price the difference rather than argue about it.
Where do the signed SAFEs and notes actually live during a round?
In whatever room your lead's counsel will be reading them from during confirmatory diligence, which in practice means a data room rather than an email thread. I run Peony, a data room company used by 6,800+ customers, and this is the honest split: Data Room at $52 per admin per month is where most signed instrument sets end up once a lead's counsel is reading them, because it adds dynamic per-viewer watermarks, Advanced NDA with a signed PDF and granular per-file permissions; Business at $30 per admin per month covers the angel phase with revoke access, download prevention and a Simple NDA gate; page-by-page analytics and link expiry are on every tier, including free, so a first tracked deck link costs nothing. Be clear about the limits, though: Peony does not manage a cap table, does not maintain a ledger of your SAFEs, and does not offer e-signature of SAFEs. This calculator is a modelling tool, your cap-table software is the system of record, and your lawyer is the one who reconciles them at closing.
Where do these rules come from?
- Y Combinator, post-money SAFE forms and definitions of “Safe Price”, “Company Capitalization” and “Converting Securities” — ycombinator.com/documents (checked 9 September 2026).
- Y Combinator, Safe Financing Documents User Guide, on SAFEs not diluting each other and on the pool increase sitting outside the post-money cap — SAFE User Guide (PDF) (checked 9 September 2026).
- The legacy pre-money SAFE capitalization clause as restated in issuer filings — GenesisAI Corp, EX-99 filed 12 January 2023 (CIK 0001797609, accession 0001669191-23-000048; checked 9 September 2026), one of the filings EDGAR full-text search returns for that clause.