Share Dilution Calculator
Enter current shares outstanding and new shares issued (secondary offering, SBC, or convertibles). See how much your ownership stake shrinks if you don't buy more.
Are these calculators investment advice?
No. They do arithmetic. AlphaLens does the research. Every decision is yours.
Do the calculators store my portfolio?
Calculator inputs at /tools/ are processed in the browser. They are not treated as an account profile.
How to read this number
Dilution percent tells you how much of the newly enlarged share count the new shares represent. Ownership after tells you what happens to a holder who does nothing — their stake shrinks even though they didn't sell anything. Whether that trade-off is worth it depends entirely on what the company does with the capital raised, which this calculator doesn't and can't evaluate for you.
Formula & assumptions
Worked example
A company with 100,000,000 shares outstanding issues 10,000,000 new shares in a secondary offering priced around $20.
A holder who owned 1.00% of the company before the offering owns roughly 0.909% after — a 9.1% cut to their stake — unless they buy enough new shares to keep pace.
Common mistakes
- Reading "9% dilution" as "the stock drops 9%." Dilution changes ownership percentage, not price directly — what happens to the price depends on what the company does with the capital raised.
- Treating the naive capital-raised figure as real cash in hand. Real offerings rarely price at the pre-announcement price, and underwriting fees and discounts reduce actual net proceeds below the naive estimate.
- Ignoring buybacks happening at the same time. A company issuing shares for compensation while also repurchasing shares on the open market has a smaller — or sometimes negative — net dilution than the gross issuance alone suggests.
- Treating all "new shares" the same. Dilution from a cash-raising secondary behaves differently than dilution from stock-based compensation or convertible notes — see the SBC vs Buyback Dilution calculator for that split.
Common questions
What is stock dilution?
Stock dilution happens when a company issues new shares, which reduces each existing shareholder's percentage ownership of the company unless they buy proportionally more shares.
How do you calculate dilution percentage?
Dilution percentage equals new shares divided by the total of current shares plus new shares. A holder who does not buy more sees their ownership percentage fall by roughly that amount.
Is dilution always bad for shareholders?
Not necessarily. Dilution reduces ownership percentage, but whether it is good or bad for a holder also depends on how the company uses the capital raised from issuing those shares.
Go deeper
Want the full picture on a dilution event — not just the math? Run a research pass in AlphaLens.
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