Founder equity dilution calculator
A founder who owns 80% and gives 20% of the company to new investors keeps 64%, a drop of 16 percentage points. Enter today's stake and the round size to see what you keep before the term-sheet meeting. Free in your browser.
- Free
- Runs in your browser
- No sign-up
- Your data stays on your device
Run — free
Worked example
- Ownership pct
- 80
- Round pct
- 20
- Retained fraction
- 0.8
- Post round ownership pct
- 64
- Dilution points
- 16
Every funding round that issues new shares dilutes existing owners. Founders, co-founders, and early employees need a clear answer to a simple question: if I own this much today and the next round gives this much of the company to new investors, what percentage do I keep? This founder equity dilution calculator applies the standard percentage-based model—post-round ownership equals current ownership times one minus the round percentage over one hundred—and returns the diluted stake, the retained fraction, and how many percentage points were lost. It rejects impossible rounds of one hundred percent or more so you never see a silent zero that looks like a bug.
How simple founder equity dilution works
In the simplest and most common planning model, a funding round is described by the percentage of the fully diluted company that new investors receive after the close. Existing holders keep their relative ranking among themselves, but every pre-round percentage is scaled by the retained fraction: one minus the investor percentage divided by one hundred. If a founder owns eighty percent before a twenty-percent primary round, post-round ownership is eighty times zero point eight, which is sixty-four percent.
The sixteen percentage points that disappeared are dilution points—the absolute drop in the ownership bar—while the retained fraction of zero point eight is the multiplier every other pre-round holder should apply to their own stake under the same round. This model does not need share counts, option pools, or pre-money versus post-money labels when the only input you have is “investors take X percent of the company.”
It is the mental math founders use on whiteboards and the first pass lawyers and CFOs still check before building a full capitalization table. Edge cases matter: a round of zero percent leaves ownership unchanged; a round approaching one hundred percent drives post-round ownership toward zero; a round of one hundred percent or more is rejected because it would leave no residual ownership for existing holders and is not a meaningful primary dilution scenario for this calculator.
Where a founder dilution calculator earns its keep
Startup founders use a founder equity dilution calculator before term-sheet meetings so they can answer “what do I own after this raise?” without waiting for counsel to rebuild the spreadsheet. Accelerators and venture studios embed the same arithmetic in onboarding checklists so every team sees how a twenty-percent seed or a fifteen-percent Series A reshapes the founder bar.
Educators teaching entrepreneurship and corporate finance need a deterministic oracle that marks a one-hundred-percent round as invalid rather than silently returning zero. Failed validation does not consume balance. Keep requests small, pin the rounded fields in tests, and you get reproducible post-round ownership for decks, LMS plugins, and edge workers without standing up a full capitalization-table engine of your own.
What you can do with it
Term-sheet sketching for founders
Enter current founder ownership and the investor percentage in a proposed round to see post-round stake and dilution points before the full cap table is updated.
Accelerator and venture-studio checklists
Standardize the “what do I own after this raise?” step so every portfolio team applies the same retained-fraction math to their pre-round percentages.
Entrepreneurship and finance classrooms
Give students a deterministic dilution oracle that rejects rounds of 100% or more and returns ordered numeric fields for homework and auto-grading.
FAQ
How is post-round founder ownership calculated?
Example: 80% ownership and a 20% round → 80 × 0.80 = 64% after the round.
What happens if the round percentage is 100 or more?
A funding round of 100% or more would leave no residual ownership for existing holders and is rejected rather than returning a silent zero.
Does this model option pools or share classes?
No. It is the simple percentage-based dilution model when you already know the investor percentage of the round. Full cap tables, pool top-ups, and multi-class structures need a dedicated capitalization tool.
What is the difference between dilution points and the retained fraction?
Dilution points are the absolute drop in your stake: going from 80% to 64% is 16 points. The retained fraction, 0.8 after a 20% round, is the multiplier every existing holder applies to their own percentage.
Do I need share counts or a pre-money valuation?
No. You only need your current percentage and the share of the company new investors receive. It is the quick whiteboard check founders run before anyone builds a full capitalization table.
Related tools
For developers — API access
Everything on this page is available programmatically. This section is for teams who want to wire it into their own systems; everyone else can just use the tool above.
API endpoint
Prefer to automate it? One authenticated POST creates the task; the result comes back by webhook or a signed link. The same capability also runs here on the web, by email and from Telegram — and soon from our app too.
Call it from your stack
curl -X POST https://api.kit.forhosting.com/biz/equity-dilution-calc \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"ownership_pct":80,"round_pct":20}'const res = await fetch("https://api.kit.forhosting.com/biz/equity-dilution-calc", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"ownership_pct": 80,
"round_pct": 20
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/biz/equity-dilution-calc",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"ownership_pct": 80,
"round_pct": 20
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/biz/equity-dilution-calc", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"ownership_pct":80,"round_pct":20}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"ownership_pct":80,"round_pct":20}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/biz/equity-dilution-calc", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)Example request
{
"ownership_pct": 80,
"round_pct": 20
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "biz.equity_dilution_calc",
"status": "queued",
"_links": {
"result": "/tasks/tsk_…/result"
}
}The API is asynchronous: the call returns a task_id immediately and the result arrives by webhook. Polling is capped at 1 req/s per task.
Pricing
Published price — no tokens, no invented credits. A failed task is never charged.
Errors
| HTTP | Code | Meaning |
|---|---|---|
401 | unauthorized | Missing or invalid API key. |
402 | insufficient_balance | Your balance doesn't cover the task price. |
404 | unknown_type | That task type doesn't exist. |
429 | rate_limited | Too many requests. Use the webhook instead of polling. |
Technical notes
The same pure analytic path powers the free browser tool and the API, so term-sheet sketches and automation pipelines always agree on the arithmetic before anyone opens a full cap-table spreadsheet.
Input contract, response fields, and error behaviour
Use it when you already know the investor percentage of the round and only need the founder’s post-round percentage. POST a JSON body with two required finite numbers. ownership_pct is the founder’s current ownership as a percentage from zero through one hundred inclusive—eighty means eighty percent today. round_pct is the percentage of the company issued to new investors in this round; it must be non-negative and strictly less than one hundred. Values of one hundred or more fail with invalid_input and a message that the round must be less than one hundred so existing holders retain some claim. Missing fields, empty strings, and non-finite values also fail with invalid_input naming the field; those validation failures are not billed. On success the body echoes ownership_pct and round_pct, returns retained_fraction as one minus round_pct over one hundred, post_round_ownership_pct as the diluted founder stake, dilution_points as the absolute drop in percentage points, and a short formula string you can display next to the numbers. Numeric strings are accepted the same way as JSON numbers. This endpoint does not model option-pool top-ups, multiple classes of stock, secondary sales, or convertibles converting at a discount; those require a full cap-table tool.
Where a founder dilution calculator earns its keep
Product and pricing teams building fundraising planners or SAFE calculators call a tiny parse API instead of re-implementing percentage dilution in every client.
Technical questions
What do retained_fraction and dilution_points mean?
retained_fraction is 1 − round_pct/100, the multiplier applied to every pre-round stake. dilution_points is ownership_pct minus post_round_ownership_pct, the absolute drop in percentage points.
What does the founder equity dilution calculator cost?
The browser tool is free. API requests use prepaid KIT balance at $0.002 per successful call; invalid input is not billed.