Anti-Dilution Protection in Bridge Rounds: What Founders Miss
The anti-dilution clause in the original Series A term sheet is the document founders skim past in the negotiation, then meet again — under very different conditions — when the bridge prices below the last round. Broad-based weighted average, narrow-based weighted average, and full ratchet behave very differently. The post-money cap table diverges sharply.
The short answer
Anti-dilution clauses adjust the conversion price of preferred stock when a subsequent round prices below the original. Three structural variants exist. Broad-based weighted average — the UK Seed and Series A default — adjusts mildly, taking the new round's size and the company's existing share base into account. Narrow-based weighted average — more common in cross-border syndicates — adjusts more aggressively because the calculation excludes some share categories. Full ratchet — rare in UK Seed but seen in some US-led syndications — adjusts the original conversion price to the new round's price entirely, producing the most dilutive outcome for founders by a wide margin. The bridge price negotiation is, in part, a negotiation about which clause activates.
Key Takeaway: The anti-dilution clause is the document the bridge negotiation actually turns on when the price moves below the last round. Read it before pricing, model the founder dilution under each candidate price, and negotiate the bridge price into the most cap-table-efficient zone.
Source: Opagio internal benchmarking of UK Seed and Series A term sheets 2024-25 (n indicative; varies materially by lead-investor jurisdiction).
Why most founders get this wrong
The dominant failure is not noticing the clause until it activates. Founders skim the anti-dilution section of the original term sheet because the language is dense and the scenario feels remote. Eighteen months later, when the bridge prices 20% below the last round and the conversion-price adjustment compresses the founder cap table by 6-12 percentage points, the founder discovers what the clause actually does.
The second failure is mis-modelling the clause when the bridge price is being negotiated. A 15% step-down under broad-based weighted average is materially less dilutive than a 5% step-down under full ratchet — a counter-intuitive result that founders without the model in front of them frequently get wrong. Negotiating the bridge price without the model is negotiating in the dark.
The third failure is treating the clause as fixed once it activates. Anti-dilution clauses can be waived or restructured at the bridge round. A founder who knows what the clause does and proposes an explicit restructuring — typically in exchange for a slightly better bridge price — sometimes recovers a meaningful share of the founder cap-table position that the clause would otherwise have compressed.
Warning: If the original term sheet contains full ratchet, the bridge negotiation is a different conversation from the one the founder thinks they are having. Every percentage point below the last round's price triggers a one-for-one conversion-price adjustment on the affected preferred stock. The founder dilution can be 2-4× the dilution under broad-based weighted average at the same nominal price.
How each variant actually calculates
Broad-based weighted average. The conversion price is adjusted by the formula CP2 = CP1 × (A + B) / (A + C), where CP1 is the original conversion price, A is the company's existing share base (broad: includes options, warrants and convertibles on a fully-diluted basis), B is the share-equivalent of the new investment at the original price, and C is the share-equivalent at the new price. The adjustment is mild because the broad share base dilutes the impact.
Narrow-based weighted average. Same formula, but A is restricted to the existing preferred-stock base (excluding options, warrants and convertibles). The narrower denominator produces a sharper adjustment.
Full ratchet. The conversion price is reset to the new round's price entirely. CP2 = price of new round. There is no weighting, no offset for the size of the affected investment. The most dilutive of the three by a wide margin.
Why jurisdiction matters for the clause
UK Seed and Series A term sheets — drafted on the British Venture Capital Association model documents — default to broad-based weighted average. US-led syndications, drafted on NVCA model documents, also default to broad-based weighted average but more frequently negotiate to narrow-based or full ratchet at the lead investor's request. Cross-border syndicates that include US, European, and UK lead investors sometimes adopt the more aggressive variant by default — the document gravitates to the lead's home convention. Founders signing into cross-border syndicates should read the anti-dilution mechanism as a primary negotiating point, not a boilerplate paragraph.
What "good" looks like
A worked example illustrates how sharply the variants diverge. Consider a company with a £10M post-money Series A (£8M raised at £2M pre-money equivalent on the affected preferred), now raising a £2M bridge at a 20% step-down to £8M post-money equivalent.
1. Read the original term sheet before the bridge price is offered
Pull the Series A or Seed term sheet. Identify the anti-dilution mechanism explicitly. If the mechanism is broad-based weighted average (the most common in UK), document the formula inputs. If narrow-based or full ratchet, flag the situation to the chair and to legal counsel before any price conversation.
2. Model the founder dilution at three candidate bridge prices
Build the cap-table model at flat, -15%, and -30% to the last round, under the actual anti-dilution mechanism. The output is a table showing founder ownership at close under each scenario. The differences are usually larger than founders expect — and inform the bridge price negotiation directly.
3. Negotiate price into the most cap-table-efficient zone
The model shows where each percentage-point of price-down translates into dilution. Use it. A 5% concession from the founder on price often produces a 0.3% concession in founder ownership — but a 15% concession on price under full ratchet can produce 4-6% in founder ownership. The model decides which trade-offs are worth making.
4. Consider explicit clause restructuring at the bridge round
Anti-dilution clauses can be waived or restructured at the bridge round, typically in exchange for slightly better bridge terms for the protected investors. A clean restructuring at bridge — converting full ratchet to broad-based weighted average, for example — recovers founder cap-table position that the original clause would otherwise have compressed indefinitely.
Same bridge under broad-based weighted average
- 20% step-down on £10M to £8M post-money
- Conversion-price adjustment: ~3-5%
- Founder dilution effect: ~1-2 percentage points
- Negotiable; cap-table absorbs the bridge cleanly
Same bridge under full ratchet
- 20% step-down on £10M to £8M post-money
- Conversion-price adjustment: 20% (full reset)
- Founder dilution effect: ~4-8 percentage points
- Restructuring at bridge usually warranted
How to apply it to your round
The anti-dilution clause sits inside the original term sheet, often unread since signing. The first action of any bridge process is to pull that document, read the clause, and model the founder dilution under three candidate prices. Without that model, the bridge price negotiation is a negotiation about a number; with it, the negotiation is about cap-table outcomes — which is the conversation that actually matters.
Before the bridge price is discussed. Pull the original term sheet. Identify the anti-dilution variant. Build the founder-dilution model. The model is the negotiating tool.
During price negotiation. Use the model. The founder is not negotiating the headline number; the founder is negotiating the cap-table outcome the headline number produces under the existing anti-dilution mechanism. These are different conversations and they produce different results.
If full ratchet activates. Open an explicit restructuring conversation. The protected investors usually accept a restructuring in exchange for slightly improved bridge terms — they understand that the cap-table compression a full-ratchet activation produces damages the next round's pricing as well, which is in nobody's interest.
The Bottom Line
Anti-dilution is the document the bridge negotiation actually turns on when the price moves below the last round. Read it first, model it second, negotiate price third — in that order. The Series A clause founders skim past becomes the bridge clause founders are constrained by. Reading it eighteen months early is the cheapest hour of cap-table work a founder ever does.
Related reading
Anti-dilution mechanics inform several adjacent decisions. For the bridge pricing decision the clauses condition, see flat, up or down: bridge valuation mechanics. For the down-round structural alternative when the clause would activate punitively, see down rounds: when to accept, when to reprice. For the founder-dilution arithmetic each clause produces, see dilution math every founder should own. For the pro-rata picture that often accompanies a clause-activating bridge, see the pro-rata problem and how to solve it. For the underlying drivers a defensible bridge price relies on, see The Opagio 12 value drivers.
Read the clause before the bridge price is offered
Eight minutes. Twelve drivers. The structured view that builds the bridge-price defence before the anti-dilution clause activates.