Debtor days: what it means, how to calculate it, and what to do when it is too high

Debtor days is one of the most searched financial terms among UK small business owners — and for good reason. It measures how long your customers are taking to pay you. When that number creeps up, it puts direct pressure on your cashflow, even if your sales are growing. This article explains exactly what debtor days means, the formula for calculating it, what a good figure looks like for UK SMEs, and the practical steps to bring it down.

What are debtor days?

Debtor days — also called the debtor collection period or accounts receivable days — is a measure of how many days, on average, it takes your customers to pay their invoices. It is expressed as a number of days and tells you how quickly your business is converting its sales into cash.

A business with debtor days of 30 is collecting payment within a month of invoicing. A business with debtor days of 75 is waiting two and a half months — which creates a significant working capital gap that has to be funded somehow, usually from overdraft or reserves.

For most UK product and service businesses, debtor days is one of the clearest early warning signals of a cashflow problem developing.

The debtor days formula

The standard formula for calculating debtor days is:

Trade debtors is the total value of outstanding invoices owed to you at a given point in time — the figure shown under current assets on your balance sheet.

Annual revenue is your total sales for the year. For greater accuracy, you can use credit sales only (excluding cash sales), but for most SMEs using total revenue gives a workable and comparable figure.

Debtor days worked example

What is a good debtor days figure?

There is no single universal answer, because the right figure depends on your payment terms, your sector, and your customer base. However, here are the benchmarks used across UK SMEs:

Businesses that operate with 60-day or 90-day payment terms (common in construction, manufacturing, and public sector supply) will naturally have higher debtor days. The key is to compare your figure against your actual agreed terms — not just industry averages.

Why debtor days matters for UK SMEs

The impact of high debtor days is compounded when your own supplier payment terms are shorter than your customer payment terms. If you pay your suppliers in 30 days but your customers pay you in 55 days, you have a 25-day working capital gap that has to be funded from reserves or borrowing on every pound of revenue.

How to calculate debtor days — step by step

1. Pull your most recent balance sheet and find trade debtors (or accounts receivable) under current assets.
2. Take your total revenue for the last 12 months from your P&L.
3. Divide trade debtors by annual revenue.
4. Multiply the result by 365.
5. The result is your current debtor days figure.

For a live calculation, use the LumixAI 30-Day Cashflow Snapshot, which shows your debtor position alongside your full cash movement — letting you see exactly when collections pressure is going to hit your bank balance.

The debtor days formula — common variations

You may see debtor days calculated slightly differently depending on the source. Here are the most common variations and when to use each:

• (Trade debtors ÷ Annual revenue) × 365 — the most common version. Use this for a general overview.
• (Trade debtors ÷ Annual credit sales) × 365 — more precise if a significant portion of your revenue is cash-in-advance. Excludes upfront payments from the denominator.
• (Trade debtors ÷ (Annual revenue ÷ 12)) — expresses debtor days in months rather than days. Less common but sometimes used in management accounts.

For most UK SMEs, the first version using total annual revenue is the most practical and produces a figure that is directly comparable month to month.

How to reduce debtor days

Bringing debtor days down is one of the highest-return activities a business owner can focus on — it improves cashflow without requiring any new sales. These are the most effective approaches:

1. Invoice immediately

Every day between completing a job and raising the invoice is a day added to your debtor days. Businesses that invoice on the day of delivery or completion consistently collect faster than those that batch invoices weekly or monthly. If you use Xero, QuickBooks, or Sage, set up automatic invoice generation on job completion.

2. Shorten your standard payment terms

If you currently invoice on 30-day terms, consider moving to 14-day terms for new customers. For established customers, you can negotiate a transition over time. Many businesses accept shorter terms without complaint — they only stay on 30 or 60 days because no one asked for anything different.

3. Follow up immediately at the due date

Most businesses follow up on overdue invoices 7–14 days after the due date. The most effective businesses follow up on the due date itself — a brief, professional reminder sent the morning an invoice falls due. This signals that you track your debtors actively, which changes payment behaviour over time.

4. Offer early payment incentives

A 1–2% discount for payment within 7 days is often worth more than the cost of the discount. If your cost of borrowing is 8% and the discount costs 1.5% annualised, you are ahead. More importantly, it changes the dynamic — customers who take early payment discounts become your fastest payers.

5. Use direct debit or card-on-file

For recurring customers, GoCardless or Stripe card-on-file eliminates debtor days entirely for that revenue. The customer still sees an invoice, but payment is taken automatically on the due date. This is the single most effective structural change for service businesses with repeat clients.

6. Review your debtor position weekly

Debtor days is a lagging indicator — it only tells you the problem has already grown. A weekly aged debtors review (invoices grouped by 0–30, 31–60, 61–90, 90+ days overdue) gives you the early warning signal. Any invoice moving into the 31–60 day bucket should trigger immediate action.

Debtor days vs creditor days

Creditor days is the mirror metric — how long you take to pay your suppliers. The relationship between the two tells you whether your working capital cycle is self-funding or whether it requires external financing:

• Debtor days < creditor days — you collect from customers faster than you pay suppliers. Your working capital cycle is self-funding.
• Debtor days > creditor days — you pay suppliers before customers pay you. You need working capital to bridge the gap. The bigger the difference, the more capital is needed.

For a business turning over £1m, every 10 days of debtor days improvement is worth approximately £27,000 of cash released from working capital.

Frequently asked questions about debtor days