What Is Antidilution?

The Sibyl Un-Glossary · What it actually means

Antidilution

A term that protects investors on price by effectively repricing their shares if the company later raises financing at a lower valuation.

The Sibyl Un-Glossary starts with the definition, then goes beyond it to where founders misinterpret the term.

More VC terms in the Un-Glossary →

Antidilution sounds like a promise that an investor will not be diluted. It is not. In venture financing, the term usually means something narrower: protection against the company later selling shares at a lower price. That distinction matters because the wrong antidilution provision can turn an already painful down round into far more dilution for founders and employees.

1. Antidilution does not stop dilution

An investor can have antidilution protection and still own a smaller percentage after your next round.

Standard antidilution does not preserve an investor’s ownership percentage every time new shares are issued. It protects the price at which preferred shares convert if the company later sells shares more cheaply. Keeping a fixed percentage is usually handled by a separate pro rata right, which requires the investor to put in more money.

Imagine an investor owns 20% after your Series A. A Series B at a higher valuation dilutes them to 15%. Antidilution does nothing to reverse that. Now change one factor: the Series B prices below the Series A. That is when antidilution comes into play.

The mechanism does not prevent dilution generally. It protects against dilution caused specifically by a lower share price.

2. Not all antidilution is created equal

The important question is not whether a term sheet contains antidilution. It is what kind.

Broad-based weighted-average antidilution is standard. Full-ratchet antidilution is far more aggressive.

Suppose an investor buys Series A shares at $10. Two years later the company issues new shares at $2. Under full ratchet, the investor’s conversion price could reset all the way to $2, even for a small issuance. A weighted-average formula factors in both the lower price and the size of the new round, so a small issuance creates a smaller adjustment than a large one.

Antidilution is not a yes-or-no term. The formula is the term.

3. The investor’s protection comes from somewhere

Antidilution does not erase the economic damage of a down round. It reallocates it.

When an adjustment gives an investor a better conversion ratio, that investor ends up with more common shares on an as-converted basis. That value does not appear from nowhere: other shareholders, usually founders and employees, own less as a result.

Say a founder expects a new down-round investor to take 20% and models dilution as if everyone else absorbs that 20% proportionately. But existing investors with antidilution protection get their conversion price adjusted too, diluting the founder twice: once from the new shares, again from the adjustment.

Modeling a down round using only the size of the new financing can badly understate what happens to founder ownership.

Why does antidilution matter to early stage founders?

At the moment you negotiate antidilution, a future down round feels remote. Everyone expects the next round to be better. But antidilution exists precisely for the scenario where that expectation is wrong.

Its significance is asymmetric. If valuations keep rising, the provision may never matter. If the company has to raise below the previous price, it can become one of the most economically important provisions in the deal.

Founders should know three things before agreeing to it: what triggers the protection, how the adjustment is calculated, and whose ownership absorbs the dilution. The goal is not eliminating antidilution. Standard protection is normal. The goal is knowing what version you are giving away.

How wrong is too wrong?

Thinking antidilution simply means “investors are protected from dilution” is wrong but easily fixed once the mechanics are explained.

Treating all antidilution provisions as equivalent is far more dangerous. A founder who cannot distinguish weighted-average from full ratchet can agree to dramatically different economics without realizing it. A founder who models a down round using only the new investor’s stake can badly overestimate what founders and employees keep.

You do not need to memorize the formula. You need to ask: what triggers it, which formula applies, and what the cap table looks like if it happens.