Stock Dilution Calculator for Public-Company Offerings
Model exactly what a follow-on offering, ATM draw, or PIPE does to your ownership stake — before the underwriter sends a term sheet. Enter your cap table once and see percentage dilution, fully-diluted overhang, and a scenario table across offering sizes.
Share count, pre- and post-offering
The same figures as the calculator above, laid out as a cap table so you can see exactly which line moved and which didn't.
| Metric | Pre-offering | Post-offering | Fully-diluted |
|---|---|---|---|
| Shares outstanding | — | — | — |
| Your shares owned | — | — | — |
| Your ownership % | — | — | — |
| New shares issued (this offering) | — | — | — |
| Warrant/option overhang | — | — | — |
Scenario table: what different offering sizes do
Holding your current cap table constant, here's your post-offering ownership and relative dilution if the company issues new shares equal to 10%, 25%, 50%, or 100% of shares currently outstanding — regardless of what raise size or price you entered above.
| New issuance | New shares | Shares outstanding after | Your ownership after | Relative dilution |
|---|---|---|---|---|
| 10% of current O/S | — | — | — | — |
| 25% of current O/S | — | — | — | — |
| 50% of current O/S | — | — | — | — |
| 100% of current O/S | — | — | — | — |
Percentage dilution and economic dilution are different things
The calculator above measures percentage dilution: the drop in your ownership percentage caused by a larger share count in the denominator. That number is purely mechanical — it happens any time new shares are issued, full stop, regardless of price.
Economic dilution is a separate question: does the new money come in at a price above or below the company's per-share book value (or, more usefully for a growth company, above or below what you believe intrinsic value per share to be)? If new shares are sold at a price above book value, book value per share for existing holders actually rises even as their percentage ownership falls — the pie got a smaller slice for you, but the slice is now worth more per unit. If shares are sold below book value (a "dilutive" raise in the stricter finance-textbook sense), existing holders take a hit on both axes at once: smaller percentage, and a lower per-share value on what's left.
This is why board decks distinguish the two terms carefully, and why a founder should never treat "we got diluted 20%" as a complete sentence — the follow-up question is always "at what price, relative to what the shares were worth." Enter a book value per share above to see this calculator flag which side of that line your scenario falls on.
Why dilution math looks different for a public company
Dilution calculators built for venture-backed private companies assume a single, discrete priced round with a term sheet, a cap table update, and 409A implications. Public companies raising follow-on capital operate under a different toolkit, and the mechanics of how shares get issued matter as much as how many.
Once a company has an effective Form S-1 or a Form S-3 shelf registration on file, it can sell registered shares directly into the market without a fresh road show for every tranche. Three structures dominate:
- Firm-commitment follow-on offering. A single priced block sold through an underwriter, similar in mechanics to an IPO — this is the "raise amount + price per share" mode in the calculator above, and it's the cleanest case to model because the new share count is fixed at pricing.
- At-the-market (ATM) offering. Shares are drip-fed into the market over weeks or months at prevailing prices through a sales agent, off an S-3 shelf. Dilution accrues incrementally and the effective price is a volume-weighted average, not a single print — model an ATM by running this calculator at your expected average price and cumulative dollar draw to date, then re-running it as the program continues.
- PIPE (private investment in public equity). A negotiated block sold directly to institutional investors, often at a discount to market and frequently bundled with warrant coverage — which is exactly what the warrant/options overhang field above is built to capture, since PIPE investors' warrants sit off the primary share count until exercised but belong in any fully-diluted view a board or analyst should be looking at.
The overallotment toggle matters specifically for firm-commitment deals: underwriters typically negotiate a 15% overallotment option (the standard "greenshoe"), giving them the right to sell 15% more shares than the base deal if demand supports it. Whether that option is exercised is usually known within 30 days of pricing, and it changes both the final share count and the company's proceeds — model both the base case and the with-overallotment case before you commit to a number in an investor update.
Worked example
Take the calculator's default scenario: a company with 20,000,000 shares outstanding, where a shareholder holds 2,000,000 shares — a 10% stake. The company runs a firm-commitment follow-on and raises $10,000,000 at $2.00 per share, no overallotment exercised. That prices out to 5,000,000 new shares, bringing shares outstanding to 25,000,000.
The shareholder's 2,000,000 shares are unchanged in count, but now represent 2,000,000 ÷ 25,000,000 = 8.00% of the company, down from 10.00%. That's a 2.0 percentage-point drop in absolute terms, but a 20% relative reduction in the size of their stake — the number the calculator reports as "relative dilution," and the more useful figure when you're comparing offerings of different sizes against each other.
Now flip on the +15% overallotment: the underwriter sells 5,750,000 shares instead of 5,000,000, shares outstanding rises to 25,750,000, and the same shareholder's stake falls to 7.77% — another quarter-point of dilution purely from the greenshoe being exercised. Add a 1,500,000-share warrant overhang from an earlier PIPE and the fully-diluted stake drops further, to 7.34% of a 27,250,000-share fully-diluted count. None of these numbers required a new priced round or a cap table software subscription to compute — they're all mechanical consequences of the share count getting larger, which is exactly what this calculator is built to make visible before the pricing call, not after.
Frequently asked
What percentage dilution means, what it doesn't, and how to talk about it with your board.
Is a stock split the same thing as dilution?
No. A forward or reverse stock split changes the share count and price proportionally for every holder at once — nobody's percentage ownership moves. Dilution specifically means new shares are issued to someone who wasn't a proportional holder before (an underwriter's investors, a PIPE buyer, an option holder), which shrinks everyone else's percentage of the company. If you're modeling a split instead, use the Reverse Stock Split Calculator linked above.
Does getting diluted always mean I'm losing money?
No — that conflates percentage dilution with economic dilution. If new shares are priced above what the existing shares were worth per unit, your percentage of the company shrinks but the value of what you still hold can rise. See "Economic dilution vs. percentage dilution" above; the price-versus-book-value comparison is exactly this question.
How is an ATM offering different from a traditional follow-on for dilution purposes?
Mechanically the dilution math is identical — more shares outstanding, same denominator effect. The difference is timing and price certainty: a follow-on prices at a single moment, so you know the exact new share count at signing. An ATM sells shares incrementally over an extended window at prevailing market prices, so the effective dilution builds gradually and the average price is a moving target until the program is paused or exhausted.
Should I count warrants and options in my dilution math?
For any decision that matters — voting control thresholds, a sale process, board seat math — yes, use the fully-diluted view. Warrants and unexercised options don't dilute your percentage today, but they represent claims on shares that convert into real dilution the moment they're exercised, often at exactly the moment the stock price makes exercise attractive, which is the worst time to be surprised by it.
What does the 15% overallotment option actually do?
It's a standard clause (also called a greenshoe) giving underwriters the right, for roughly 30 days after pricing, to sell up to 15% more shares than the base deal size if investor demand supports it. If exercised, it increases both the company's proceeds and the total share count — model your dilution both with and without it before you quote a single number to your board.
Does filing an S-3 shelf registration cause dilution by itself?
No. A shelf registration only gives the company the ability to sell registered shares quickly when it chooses to — filing one doesn't issue a single share or move anyone's ownership percentage. Dilution only happens when shares are actually sold off that shelf, whether in one priced follow-on or incrementally through an ATM program.