UK Startup Runway Calculator: How Long Will Your Cash Last?

The question every UK founder must be able to answer

How many months until you run out of cash?

If you cannot answer this question within five seconds, you have a problem. Not a financial modelling problem — a survival problem. For UK SaaS founders, runway is the most fundamental metric, and getting it wrong has consequences that no pivot, no product improvement, and no team restructure can undo. UK-specific cash drains often catch first-time founders off guard: quarterly VAT payments to HMRC, monthly PAYE and Employer NIC, annual Corporation Tax, and the lag between filing and receiving the R&D tax credit (typically 4–8 weeks once submitted, sometimes longer).

Running out of cash is permanent.

How to calculate runway

The basic formula

Runway (months) = Cash balance / Net monthly burn rate

Net monthly burn is your total monthly costs minus your total monthly revenue. If you spend £80,000 per month and collect £30,000, your net burn is £50,000. With £600,000 in the bank, you have 12 months of runway.

Simple enough. But the basic formula has three significant weaknesses that catch founders out.

Why the basic formula lies to you

Problem 1: Burn rate is not constant

Your costs change every month. You hired two engineers last month (£12,000 additional monthly cost). You are signing a new office lease next quarter (£3,000/month). Your annual insurance payment of £8,000 hits in March. A flat burn rate misses all of this.

Problem 2: Revenue is growing (hopefully)

If your MRR is £30,000 and growing 10% per month, your burn rate is shrinking. In three months, MRR will be approximately £40,000, reducing your net burn from £50,000 to £40,000. The basic formula does not capture this.

Problem 3: Cash is not the same as revenue

You might recognise £30,000 of revenue this month, but if half your customers are on net-30 terms, only £15,000 actually arrived in the bank this month. The rest is sitting in accounts receivable. Runway should be calculated from actual cash movements, not accounting profit.

The better formula

Instead of dividing by a single burn rate, build a month-by-month cash flow projection:

For each month going forward:

• Opening cash = prior month's closing cash
• Cash in = expected collections (from AR ageing + new invoicing)
• Cash out = committed costs (payroll, rent, subscriptions) + planned costs (new hires, marketing)
• Closing cash = Opening + Cash in - Cash out

Your runway is the month where closing cash hits zero.

This approach captures varying burn rates, revenue growth, one-off payments, and the timing differences between revenue and cash.

Understanding burn rate

Gross burn vs net burn

Gross burn = Total monthly expenses (regardless of revenue). If you spend £80,000 per month, that is your gross burn.

Net burn = Gross burn minus revenue. If you spend £80,000 and collect £30,000, your net burn is £50,000.

Both are useful. Gross burn tells you what you need to cover if revenue drops to zero. Net burn tells you the actual rate of cash consumption at current revenue levels.

What is driving your burn?

For most SaaS startups, the cost breakdown looks roughly like this:

People costs dominate. This means the single largest lever for extending runway is hiring decisions. Every hire you make or delay changes your runway by months, not days.

The hiring calculation

Before approving any new hire, run this calculation:

A mid-level developer at £65,000 base salary costs approximately:

• Gross salary: £65,000
• Employer NIC (13.8% above £9,100): approximately £7,700
• Pension (5%): £3,250
• Equipment, software, benefits: approximately £3,000
• Total annual cost: approximately £79,000
• Monthly cost: approximately £6,600

If you have 10 months of runway and this hire reduces it to 9 months, is the hire worth it? What will they deliver in 9 months that justifies the reduced safety margin?

This is not an argument against hiring. It is an argument for making every hire count and timing them deliberately.

Runway targets

By stage

These are post-fundraise targets. If you have just raised and have less than 18 months of runway, you either raised too little or your burn is too high.

Before fundraising

Start the fundraising process when you have at least 9-12 months of runway remaining. A typical raise takes 3-6 months for Seed and Series A:

• Month 1: Preparation (model, deck, data room)
• Months 2-3: First meetings, follow-ups, partner meetings
• Months 4-5: Due diligence, term sheet negotiation
• Month 6: Legal documentation, closing, funds hit the account

If you start at 6 months of runway, you are fundraising under duress. Investors can sense urgency, and it weakens your negotiating position. Valuations drop. Terms get worse. Options narrow.

How to extend runway

When runway gets shorter than you are comfortable with, you have two categories of lever: increase cash in and decrease cash out.

Increase cash in

Shift to annual billing: If 80% of your customers pay monthly, moving 30% to annual upfront billing (with a 10-15% discount incentive) generates a one-time cash windfall. A customer paying £2,000/month switching to annual saves them £2,400-£3,600 but gives you £20,400-£21,600 today instead of £2,000 per month.

Tighten payment terms: Move from net-30 to net-14 or payment on invoice for new customers. For existing customers, offer a small discount for early payment.

Collect overdue invoices: Check your accounts receivable ageing. If you have £50,000 in invoices over 30 days past due, that is cash you have already earned that is not in your bank. Chase it.

R&D tax credits: If you have not claimed them, start immediately. UK SaaS companies with qualifying R&D expenditure can claim 20% of qualifying costs (under the merged scheme from April 2024). For a company spending £300,000 on qualifying R&D, that is a £60,000 cash benefit.

Decrease cash out

Defer non-critical hires: Every hire you delay by one quarter extends runway. Be ruthless about which roles are genuinely critical for the next 6 months versus nice-to-have.

Renegotiate contracts: Annual software subscriptions, office leases, and vendor contracts can often be renegotiated. Many suppliers would rather reduce the price than lose a customer.

Cut discretionary spend: Conference sponsorships, team off-sites, premium software tiers, office perks. These are the first things to go when cash is tight.

Consider contractor vs full-time: A contractor at £500/day is expensive per month, but you can stop the cost instantly. A full-time hire at the same effective rate requires notice periods, redundancy considerations, and severance.

The danger zones

Below 6 months: Emergency

You are in crisis territory. Every decision should be evaluated through the lens of cash preservation. Fundraising from this position is extremely difficult — investors will either pass or extract punitive terms.

Actions: Immediate cost reduction. Explore bridge financing from existing investors. Consider revenue-based financing or venture debt if eligible.

6-9 months: Urgency

You should be actively fundraising or implementing significant runway extension measures. There is still time, but not much margin for error.

Actions: Accelerate fundraising timeline. Implement the highest-impact runway extension levers immediately.

9-12 months: Planning

This is the right time to start fundraising. You have enough runway to run a proper process without desperation.

Actions: Begin fundraising preparation. Financial model, data room, investor list.

12-18 months: Comfortable

You have time. Use it to build the business, hit milestones, and strengthen your position for the next raise.

18+ months: Strong position

You can be selective about when and whether to raise. Consider whether you need external capital at all, or whether the business can reach profitability on current resources.

The weekly discipline

Check your runway every week. Not monthly. Not quarterly. Weekly.

Update your 13-week cash flow forecast every Monday morning. Compare last week's forecast to actual. Adjust the forward view. Know your exact cash balance, your expected receipts this week, and your committed payments.

This takes 30 minutes per week. It is the single most important 30 minutes a founder can spend on financial management. Because the companies that run out of cash almost never do so suddenly. They do so gradually, one ignored week at a time, until the options are gone.