Cash flow explained

“Profit is an opinion, cash is a fact.” It’s an old accountants’ saying, and it’s the single most important idea in small business finance. Plenty of profitable businesses fail because they run out of cash. Here’s how cash flow works and how to keep yours healthy.

Profit vs cash

• Profit is a calculation over a period: revenue minus costs. It says nothing about when money actually moves.
• Cash flow is the timing of money: cash in (from sales and invoices) minus cash out (bills, salaries, tax) over a given period.

If a customer buys £10,000 of work but pays you in 60 days, you’ve made the profit today — but the cash arrives in two months. Meanwhile you still have suppliers, rent and tax to pay.

Why businesses fail on cash

The classic story: fast-growing business, profit on paper, all its cash sitting in unpaid invoices and stock, and then a tax bill or a big supplier invoice lands. Common causes:

• Late-paying customers
• Seasonal dips in sales
• Big one-off bills (VAT, tax, insurance, equipment)
• Buying stock or paying deposits well before the money comes back
• Ignoring the forecast until it’s too late

Build a 13-week cash flow forecast

The standard tool is a rolling 13-week forecast:

1. List the cash you expect in each week (invoices due, regular income)
2. List the cash you expect out (rent, wages, VAT, suppliers, loan repayments)
3. Add your starting balance and work forward

When the forecast dips below zero, you know weeks in advance — which is exactly when you still have options.

Ways to keep cash healthy

• Invoice immediately and chase on time — money sitting in invoices is the biggest cash drain
• Ask for deposits or milestones on larger work, so you’re not financing the whole job
• Negotiate supplier terms — 30 or 60 days beats paying upfront
• Stagger big payments — spread insurance, licences and tax across the year
• Hold a buffer — a cash reserve of 1–3 months’ costs is the business version of an emergency fund
• Consider a credit line before you need it, not after

Manage short gaps

If you need a short-term bridge, compare the cost carefully: a small business loan or overdraft for a known gap is usually fine if you can see the money coming back. A loan calculator helps you compare the true cost. The goal is to plan gaps so they never become crises.

Keep the discipline

The habit that saves most businesses: check your forecast weekly, review your invoices on time, and always know your next three big cash-out dates. Cash flow is a skill, not luck — and it’s learnable.

Read next

Our how to calculate profit guide shows why the numbers on your P&L differ from your bank balance, and how invoices work covers the payment side of cash in.