The Runway Math That Determines Your Bridge Size
Bridge size should be determined by the milestone the company needs to hit, not by how much capital it can plausibly raise. Founders who size bridges by appetite end up raising twice in twelve months. Founders who size bridges by milestone raise once and close.
The short answer
Bridge size = (months to next-round-readiness milestone × monthly burn) + 6 months of buffer. Work backwards from the milestone: define what the next round (Series A, Series B, or exit) requires; estimate the time to reach that state honestly; multiply by burn; add a 6-month buffer because every plan slips and a bridge that runs out a month before the milestone is the worst possible outcome. The number that comes out of that calculation is the right bridge size — not the number the founder hopes to be able to raise.
Key Takeaway: Bridge size is a function of the milestone, not the appetite. Sizing for time produces extension rounds in disguise; sizing for milestones produces bridges that close cleanly into the next priced round.
Source: Opagio internal benchmarking of UK bridge sizes 2024-25 (n indicative; varies materially by sector and burn profile).
Why most founders get this wrong
The dominant error is sizing the bridge by time-to-next-raise rather than time-to-next-round-readiness. The two are not the same. Time-to-next-raise is a fundraising estimate (typically 4-6 months from first conversation to close). Time-to-next-round-readiness is an operational estimate (the months required to hit the metric profile that makes the next round closeable). The bridge has to fund the latter, with the former layered on top.
The second common error is using current burn as the multiplier when the bridge thesis itself involves scaling activity that increases burn. A founder who raises a £2M bridge against a £150k monthly burn — providing 13 months of runway — and then hires the senior salesperson the bridge thesis required, sees burn rise to £200k and runway compress to under 10 months. The bridge milestone now slips outside the funded period.
The third error is omitting the buffer. Every plan slips. Bridges that are sized to the milestone exactly produce panic raises six weeks before the milestone is hit; bridges with a 6-month buffer give the founder room to wait for the right priced-round terms rather than accept the first acceptable ones. The cost of the buffer is small (additional dilution from a slightly larger bridge); the cost of running without one is large (compressed pricing on a forced next round).
Warning: A bridge that runs out 30 days before the next-round milestone is the worst possible outcome. The next round prices on the bridge having succeeded; if the company is already in a forced-conversation posture when the next round opens, the price compresses sharply. Buffer-light bridges produce buffer-heavy dilution.
What "milestone" actually means
Next-round-readiness is not a single number. For Series A, it is typically: ARR threshold (sector-dependent, commonly £1.5M-£3M for B2B SaaS), demonstrable cohort retention (NRR >100%), and at least one referenceable customer in the target ICP. For Series B, it is typically: ARR threshold (£8M-£15M), efficiency profile (Rule of 40 trending up), expansion-revenue motion proven, and pipeline that supports the next 18 months. The milestone is the package, not any single line.
The bridge is funded against the package. Defining the package precisely — which the next round's lead will want to verify — is the work that determines whether the bridge size is right or wrong.
The two burn profiles founders need to model
Operational burn. The cost base required to run the company at its current capacity through the bridge period — salaries already in seat, infrastructure, premises, customary growth in headcount that has nothing to do with the bridge thesis itself. Operational burn is the floor of the bridge calculation; it cannot be cut without breaking the operating plan that the last round funded, and the bridge thesis has to be honest about that floor.
Thesis burn. The incremental cost of executing the bridge thesis itself — the senior hires the thesis requires, the engineering capacity to ship the prioritised release, the marketing push to seed the new pipeline. Thesis burn is the variable; it scales with the ambition of the bridge thesis. Sizing the bridge requires a credible projection of both burn profiles, summed monthly through the milestone period and the buffer.
What "good" looks like
The five-step calculation is a discipline that takes one CFO afternoon. Done well, it produces a defensible number that survives the partner's first scrutiny in the bridge conversation.
1. Define the next-round-readiness milestone explicitly
Write down the metric profile the next round requires — ARR threshold, retention profile, efficiency profile, pipeline coverage. Do not write "we want to be Series A-ready"; write the four or five lines that constitute readiness. The milestone is the package.
2. Estimate time-to-milestone honestly
Time from today to the package being verifiable. Be specific. The estimate is usually 9-15 months for a Seed-to-A bridge, 12-18 months for an A-to-B bridge. Founders consistently under-estimate this stage by 3-6 months; pad it deliberately.
3. Project the burn profile through the bridge period
The bridge thesis requires hires, releases, market moves. Project the monthly burn through each phase of the bridge — not today's burn extrapolated, but the burn that the thesis itself produces. Use a quarterly burn profile, not an annual average.
4. Multiply, then add the 6-month buffer
Bridge size = sum of monthly burn through the milestone period + 6 months × end-state burn. The buffer is not optional. It is the negotiating room the founder needs when the priced round opens; without it, the founder accepts the first offer.
5. Stress-test against existing-investor pro-rata capacity
If the bridge size exceeds existing-investor pro-rata capacity by >30%, structurally a new lead is required. This shapes the sequencing decision (existing-vs-new) and the instrument decision (convertible-vs-priced). The number conditions the strategy.
Sizing for time — the failure mode
- "We need 12 months of runway"
- Burn extrapolated from today, not from the thesis
- No explicit milestone definition
- Buffer omitted or under 3 months
- Result: panic raise 6-9 months later
Sizing for milestone — the working method
- "We need to be Series A-ready by Q3 2026"
- Burn projected through each thesis phase
- Milestone defined as a verifiable package
- 6-month buffer included by default
- Result: one bridge, then the priced round
How to apply it to your round
The asset register is the input that makes step 1 (the milestone definition) honest rather than aspirational. Without the register, the milestone definition tends to be revenue-only — "get to £2M ARR" — when the next round will price on the underlying asset base, not just the headline number. The register surfaces which drivers are above sector median, which are below, and which the bridge actually needs to fund.
Before sizing. Run the Round Readiness Diagnostic. Identify the drivers that need to compound between now and the next round. The capital deployment in the bridge funds those drivers; the burn projection follows the deployment.
During the partner conversation. The bridge size is defended with the milestone definition and the burn projection, not with a runway calculation. Partners pass on bridges defended as "we need 12 more months". Partners commit to bridges defended as "this capital funds these specific drivers to this specific milestone over this specific period, with this buffer".
Post-close. Track the burn against projection monthly. If burn drifts >15% from plan in either direction, re-base the milestone date and communicate it to the board before the next quarterly meeting. Bridges that close cleanly into the next round are the ones where the founder kept the burn-vs-plan discipline tight.
The Bottom Line
Bridge sizing is reverse-engineering from the next-round milestone, not forward-extrapolation from today's burn. The five-step calculation is a CFO afternoon. The output is a number the founder can defend in the partner conversation and live with through the next 12-18 months — without needing a second bridge to fix the under-sizing of the first.
Related reading
The runway calculation is the input to several adjacent decisions. For the existing-vs-new sequencing question that the bridge size shapes, see existing investors or new lead: who to talk to first. For the instrument decision the bridge size influences, see convertible vs priced bridge. For the board memo that briefs the directors on the sizing rationale, see communicating a bridge to your board without panic. For the next-round metric profile the bridge is sizing toward, see Series A metrics bar and Series B efficiency bar. For the underlying drivers the capital is funding, see The Opagio 12 value drivers.
Size the bridge backwards from the milestone
Eight minutes. Twelve drivers. The structured view that turns the milestone definition from aspiration into specification.