13-Week Cash Flow Forecast for UK SaaS: The Founder's Walk-Through
How to build a 13-week rolling cash forecast for UK SaaS - weekly inflows + outflows, VAT and PAYE timing, R&D credit timing, the runway trigger every founder should track.
For the impatient - the 13-week cash forecast for UK SaaS in one screen:
- Why 13 weeks not 12 months A quarter is the right operational horizon for SaaS founders - close enough that the numbers stay real, far enough that you can act on what you see
- What goes in Weekly cash inflows (paid customer invoices, VAT recovered, R&D credit) plus outflows (payroll including employer NIC and pension, supplier payments, rent, software, VAT due, PAYE & NIC due)
- UK timing items that catch out US templates VAT quarterly payment cycle, monthly PAYE/NIC due 22nd, R&D claim 6-9 months after year end, SEIS investor cash typically 4-6 week lag
- The runway trigger Raise when 9-12 months of runway remain. The 13-week view tells you when the runway window starts to close.
- Build your own in Google Sheets in under an hour - the column structure and rows are at the end of this post
Most cash forecasts on a UK SaaS founder's screen are American. They model federal payroll tax and US sales tax, and they miss the three items that break a UK SaaS month: VAT quarterly bills, monthly PAYE/NIC, and the long lag on R&D credit refunds. A US template will hide a £30k+ cash dip sitting four weeks out.
A 13-week forecast built around UK timing fixes that. It pairs with the full 3-statement financial model and the monthly board pack as the operational layer between board meetings.
Why 13 weeks, not 12 months
A 3-5 year model tells investors where the business is going. It cannot tell you whether payroll clears on the 28th.
The 13-week forecast lives at the other end: weekly cash in and cash out, refreshed every Monday with what settled last week. A quarter is close enough that the numbers stay real, and far enough out to act on what you see - delay a hire, chase an invoice, accelerate an R&D claim.
The monthly management accounts tell you whether the business is performing. The 13-week forecast tells you whether it survives the next 90 days. Both matter.
What goes in - the weekly line items
One tab. Weeks across the top, line items down the side. Every line is a cash movement, never an accrual.
| Section | Line | Source |
|---|---|---|
| Opening cash | Bank balance at start of week | Bank feed / Xero |
| Inflows | Paid customer invoices (annual upfront, monthly direct debit, ad-hoc) | AR ageing + contracted MRR |
| Inflows | VAT recovered (input VAT refunds when in repayment position) | VAT return |
| Inflows | R&D tax credit refund | HMRC submission date + 6-9 weeks |
| Inflows | SEIS / EIS / equity round proceeds | Term sheet + completion date |
| Outflows | Payroll (net pay to employees) | Payroll software, 25th/28th |
| Outflows | PAYE & NIC (employer + employee combined) | HMRC, 22nd of following month |
| Outflows | Pension contributions | Pension provider, monthly |
| Outflows | VAT due (output VAT - input VAT) | Quarterly, 1 month + 7 days after quarter end |
| Outflows | Corporation tax instalment | 9 months + 1 day after year end (small co) |
| Outflows | Supplier payments (AWS, software, contractors) | AP ageing |
| Outflows | Rent + utilities | Lease + DDs |
| Outflows | Director loans / dividends | Board decision |
| Closing cash | Opening + inflows - outflows | Computed |
Each week's closing balance becomes next week's opening. That recursive line is what surfaces the lowest cash point in the next 13 weeks - and that point is where you decide.
UK timing items that US templates miss
Five UK-specific cash events almost always sit inside a 13-week window:
| Item | Timing | Typical size |
|---|---|---|
| VAT (Value Added Tax, 20% standard rate on B2B invoices) | Quarterly, 1 month + 7 days after quarter end (e.g. 7 May, 7 Aug, 7 Nov, 7 Feb) | £15-50k per quarter for growing SaaS |
| PAYE & NIC (income tax + national insurance on payroll) | Monthly, 22nd of following month | ~£10-12k/month at £400k annual salary bill |
| R&D tax credit (refunds 27-33% of qualifying R&D spend) | Filed after year-end, HMRC pays 6-12 weeks later (so 6-9 months post year-end) | Often £50-200k for Seed-stage SaaS |
| SEIS / EIS proceeds | 4-6 weeks from term sheet to bank, sometimes 8 | Round size |
| Director dividends + personal tax | January / July payment on account drives dividend timing | Owner-managed only |
Two traps the table hides. R&D credit is the most-mistimed line in UK SaaS forecasts - a March year-end claim filed in June lands in September or October, not April. And a signed SEIS term sheet is not cash until legals complete and money is wired.
How to update it weekly - the 30-minute Monday routine
The discipline is half the value. Three steps, every Monday:
- Reset opening cash - pull the actual closing bank balance from Xero, overwrite this week's opening, mark last week's column as actual.
- Roll forward one week - drop the week just closed, add a new week 13, re-check the AR ageing and any VAT/PAYE date now in the window.
- Send the trough - one line to the CEO: "lowest point in 13 weeks is week X, £Y."
After eight weeks of variance discipline, the receipt timings calibrate to actual debtor behaviour and the forecast lands within 5% of actual.
When the runway trigger fires
The rule: raise when 9-12 months of runway remain. The 13-week forecast tells you when that window opens, because it shows the actual cash trough rather than the average burn.
- 12 months runway - fundraise process should already be in motion (investor list, deck refresh, intro requests).
- 9 months - conversations live.
- 6 months - negotiating from weakness.
Two levers sit alongside the raise:
- Cost levers - which hires slip a quarter, which software is killed, which contractor flexes down. The 13-week view prices each saved pound in weeks of runway.
- Cash levers - which customers move to annual upfront, which owe over 60 days, which suppliers accept extended terms. A single £30k annual-upfront conversion adds 3-4 weeks of runway with zero P&L impact.
Build your own - the structure
Under an hour in Google Sheets. 14 columns (label + 13 weeks). Three sections:
| Section | Rows |
|---|---|
| Inflows | Opening cash (week 1 only); paid customer invoices; AR collections by expected week; VAT recovered (refund quarters only); R&D credit (payment week only); SEIS/EIS once landed; Total inflows |
| Outflows | Payroll net pay (25th/28th); PAYE & NIC (22nd of following month, separate row); pension; software & infra; rent & utilities; marketing; contractors; VAT due (4 weeks/year); Total outflows |
| Answers | Net cash movement; Closing cash (running total); runway weeks remaining; first week closing cash drops below raise-trigger (~6 months forward burn) |
This sits on top of the MRR schedule that feeds the cash forecast, so recurring inflows stay consistent with the rest of the reporting stack.
Common UK SaaS mistakes in cash forecasts
- Booking R&D credit too early. Cash lands 6-9 months after year-end at the earliest.
- PAYE/NIC merged with net pay. Two events, four weeks apart (25th/28th and 22nd of following month).
- VAT smeared monthly. It is a quarterly £15-50k spike, not a monthly accrual.
- SEIS term sheets treated as cash. £0 until legals complete and money is wired.
- No weekly variance discipline. A forecast left untouched is wallpaper.
When you need help
A 13-week cash forecast is one of the first things ScaleWithCFO builds for every new client. Pre-revenue or under £500k ARR, see the 5 things pre-revenue founders should focus on. Post-Seed, the fractional CFO engagement covers the whole reporting stack with this forecast updated weekly.