How Much Cash Should a Small Business Keep in Reserve?

A cash buffer should cover the gap in your forecast

“Keep three months of expenses” is a tidy rule. It is not a calculation. A business paid at the till has a different cash problem from a contractor that pays staff and suppliers weeks before a customer settles an invoice.

The useful number is the lowest cash balance in a realistic stress forecast. If the forecast falls to minus £40,960 before recovering, the business needs at least £40,960 of opening cash to avoid a negative month-end balance. That is the bare mathematical floor, not a comfortable target.

We built three 12-month scenarios to show how the answer changes with the business model. Every figure is an editorial assumption in pounds sterling. These are not observed averages, survey results or recommendations for a particular business.

Download the 36 monthly scenario rows as CSV.

What the three businesses needed

Each profile starts with an established trading pattern and a positive normal month. We then apply a payment or sales disruption in month 2 and add one disclosed one-off cost.

“Minimum opening cash” is the amount that brings the worst month-end balance to exactly £0. It leaves no room for an expense arriving a day early, a customer paying a day late or an input being wrong. Real businesses should round up and allow for risks missing from the model.

The results also show why a fixed number of months can mislead. The retailer survives this particular stress with less than one normal month of outflows because it generates £13,000 of cash in month 1 and still takes £30,000 in month 2. The contractor needs £71,000 because a large receipt disappears temporarily while £83,000 leaves the bank in the stressed month.

One month was not enough in two profiles

We tested opening cash equal to half, one, two and three months of each profile's normal core outflows.

Two months clears all three tests, but that does not make two months a universal answer. It only means two months covers the shocks we chose. Double the delayed invoice, bring a tax payment into month 2 or remove the established receivables pipeline and the result changes immediately.

The British Business Bank notes that even a profitable company can face a cash shortage while it waits for customers to pay. It also suggests moving from one month of available cash towards two or three as one possible cash-management measure, not as a guarantee that every business is safe at that level. Its cash-flow guidance is refreshingly clear on the difference between profit and cash.

The assumptions behind each profile

The model uses monthly cash movements. Normal collections and core outflows remain flat unless the stress changes them.

The delayed cash is recovered rather than written off. The agency receives £72,000 in month 3 and the contractor £120,000 in month 4. Later recovery matters to the annual closing balance, but it does not pay month 2's payroll. Cash does not care that the invoice is technically revenue.

The retailer's stock payment stays at £27,000 in the weak sales month. This represents inventory already ordered. If the retailer could cancel or defer that purchase, its required buffer would fall. If the sales drop lasted longer, the buffer would rise.

How the buffer calculation works

For each month:

Net cash flow = collections − fixed outflows − variable outflows − one-off costs

We then add each month's net cash flow to the prior cumulative result. The opening cash needed to avoid a negative month-end balance is:

Minimum opening cash = the absolute value of the lowest negative cumulative cash flow

The agency generates £3,520 in a normal month. In month 2 it collects nothing and pays £26,000 of fixed costs, £6,480 of variable costs and the £12,000 shock. Its cumulative position falls from positive £3,520 to negative £40,960. Opening cash of £40,960 brings that low point to £0.

The same calculation produces £29,000 for the retailer and £71,000 for the contractor. Python Decimal calculations generated every monthly row. A separate JavaScript calculation reproduced all three minimums.

Government guidance says a cash-flow forecast should reflect when money reaches the bank, not when an invoice is issued. It also recommends including income, expenses and a running balance, and looking across the next 12 months. That is the method used here, although the assumptions are ours rather than figures taken from a real company. See the current Business.gov.uk forecasting guidance.

Build the reserve from your own cash cycle

Start with actual bank timing. Export at least a year of receipts and payments if the business has that history. Separate recurring outflows from VAT, corporation tax, annual insurance, equipment purchases, dividends and other costs that arrive irregularly. Bank charges belong in the recurring list too, so price your business bank account over a full year rather than trusting the monthly fee.

Then stress the few events that can do real damage:

1. Move one or two large customer payments to the date they arrived in a bad month, not the date the invoice says they should arrive.
2. Keep payroll, rent and committed supplier payments on their real due dates.
3. Test a fall in sales without assuming every cost falls at the same speed.
4. Add known annual payments and a plausible equipment, refund or repair bill.
5. Find the lowest projected balance and measure the opening cash needed to keep it above zero.
6. Add a separate margin for forecast error and events the model cannot name.

The British Business Bank's working-capital guide links the amount a business needs to the length of its cash-flow cycle. Costs incurred while producing, invoicing and waiting for payment all need funding. Longer customer terms and more stock generally increase the cash tied up before collection.

Use the cash runway calculator when revenue and expenses can be represented by a stable monthly burn. Use a month-by-month forecast when collections, stock purchases or tax payments move around. A single burn-rate number cannot see an £80,000 receipt slipping from one month to the next. For a longer view of how many registered UK businesses remain active, see our UK five-year business survival rates.

What this model excludes

The simulation excludes VAT, corporation tax, payroll taxes, debt repayments, interest, overdraft limits, financing fees, owner withdrawals, dividends, capital expenditure beyond the stated shock and cash held in restricted accounts. It assumes all monthly movements occur at month-end, so it can miss a shortage within the month.

It also starts with established businesses whose normal collection pipelines already exist. A new agency or contractor may pay costs for several months before the first customer receipt. Applying these results to a start-up would understate that gap.

The one-off costs are deliberately round stress assumptions. They are not estimates of a typical repair bill, refund, legal cost or tax demand. The profiles do not represent sector averages.

If the forecast shows a shortfall, deal with it before the bank balance reaches the line. Reduce or defer spending where that is commercially possible, tighten invoicing and collection, negotiate supplier timing, or arrange suitable finance. Our UK late-payment cost analysis shows why bridging a delayed receipt can create a separate financing cost. If borrowing is part of the plan, put the actual payment into the forecast; the business loan calculator can model the principal, rate, term and fee. New debt is not free reserve cash.

For a separate view of historical expense structure, our Australian small-business expense benchmarks compare ten ATO industries. Their 2023–24 tax-return ratios are Australian benchmarks; they do not supply the assumptions or a recommended reserve for this pound-denominated simulation.

Frequently asked questions

Is three months of expenses enough for a small business?

It may be, but the number of months is not enough information. Three months of average outflows can still fail if a large tax payment, stock order or payroll run arrives before a major customer receipt. Test the dates and amounts in a month-by-month forecast.

Should the reserve include tax money?

Do not count cash already owed for VAT, payroll taxes or corporation tax as freely available reserve cash. Put expected tax payments into the forecast on their due dates. Tax is excluded from our three illustrative profiles.

Is a cash buffer the same as cash runway?

Not quite. Runway usually estimates how long cash lasts at a continuing net burn. A buffer covers temporary timing gaps and shocks even when the business is profitable over the year. The two measures can use the same bank balance but answer different questions.

Where should a business keep its reserve?

Keep enough accessible to meet the payments the reserve is meant to protect. Interest, deposit protection, account access and notice periods depend on the bank and product. This simulation does not compare deposit accounts or give treasury advice.