Debtor Days Explained: The Formula and How to Cut It

Key terms in this article

What is DSO?

DSO (days sales outstanding) is the American name for debtor days. It measures the same thing in the same way. UK finance teams tend to say debtor days; software built in the United States usually says DSO.

What is a KPI?

A KPI (key performance indicator) is a single number tracked over time to show whether something is improving. Debtor days is one of the few cash-flow KPIs that a small business can calculate from figures it already has.

What is credit control?

Credit control is the work of getting paid on time. It covers payment terms, chasing overdue invoices, account holds and deciding when to stop supplying a customer.

What Are Debtor Days?

Debtor days estimates how long your customers take to pay you. It is worked out from your balance sheet, not from each individual invoice, so treat it as a good indicator rather than an exact average. You divide what you are owed by your annual credit sales, then multiply by 365. HMRC uses the same formula and warns that a normal figure varies a lot between trades (HMRC, 2026).

Debtor days = (trade receivables ÷ annual credit sales) × 365

So if you are owed £120,000 and your annual credit sales are £900,000:

(120,000 ÷ 900,000) × 365 = 48.7 debtor days

On 30-day terms, that is a gap of about 19 days. It does not mean every invoice was paid 19 days late. It means your ledger as a whole is running about that far behind your terms.

Key Takeaways

• Debtor days = (trade receivables ÷ annual credit sales) × 365
• It is the same measure as DSO, which is the American term
• Compare it to your own payment terms first, then to your sector
• The gap between your terms and your debtor days is the number that matters
• It is a snapshot estimate, so a few very old balances can distort it badly
• Track it alongside your aged debt report, never on its own
• Consistent chasing moves it more reliably than tightening terms does

What Counts as Good?

The honest answer is that there is no universal figure, and anyone quoting one is usually selling something.

The comparison that means something is against your own payment terms and your own past figures. As a rough guide, on 30-day terms we would call 32 healthy and 65 worth investigating, though that is a rule of thumb rather than a standard. Check the aged debt detail before you call it a collection problem, because sales timing, seasonality or one large invoice can all move the number.

Sector norms vary widely for structural reasons. Construction runs long because of project payment cycles and retentions. Businesses selling to large corporates run longer than those selling to consumers, because large corporates set the terms. None of that is within your control, so benchmarking against it tells you little about how well you are actually chasing.

Two rules of thumb are worth more than a benchmark:

• The gap. Debtor days minus your standard terms. That gap is the bit you can influence.
• The direction. Whether this quarter’s figure is lower than last quarter’s.

Where the Number Misleads You

Debtor days is an average, and averages hide things. Three traps are worth knowing about.

One large invoice distorts everything. If a single big invoice lands just before your measurement date, debtor days jumps even though nothing has got worse. If it lands just after, the figure flatters you. Use a rolling measure or several months of figures rather than a single snapshot.

Old bad debt inflates it permanently. A £15,000 balance from three years ago that nobody expects to collect will sit in your receivables forever, quietly adding days. Clean up genuinely uncollectable balances before you read too much into the trend.

Seasonal businesses need the right sales figure. The standard formula uses annual credit sales, which smooths seasonality. If you use a single month’s sales instead, a quiet month produces an alarming figure for no real reason.

From our experience: the most common cause of a bad debtor days figure among small resellers is not difficult customers. It is that nobody owns the chasing. When it is one person’s actual job, with a worklist and a routine, the figure improves within two billing cycles without anyone having a difficult conversation. When it is everybody’s job in principle and nobody’s in practice, no amount of tightening the terms helps.

How to Reduce Debtor Days

In rough order of how much difference they make relative to the effort involved.

• Chase on a schedule, not on a feeling. A fixed sequence of reminders at fixed intervals beats occasional intense effort. Consistency is what customers respond to, because it tells them your dates are real.
• Invoice faster. Your cash gap starts when you deliver the service, not when you get round to billing for it. Billing three days earlier will not take three days off the figure, because the formula also depends on your sales, but it does get the clock started sooner.
• Get the invoice right first time. A disputed invoice is a stopped clock. Wrong purchase order number, wrong contact, wrong line detail, all of it buys the customer a legitimate reason to wait.
• Collect by Direct Debit where you can. The strongest lever available, because you collect on the agreed date rather than waiting. Collections can still fail or be returned, so keep an eye on them.
• Deal with the oldest balances properly. Either collect them, agree a plan, or write them off. Leaving them is the worst of the three.
• Use pre-pay where it fits. Some customers suit a pre-pay arrangement with automatic top-up. Usage paid for up front never becomes a receivable, so it never reaches this figure at all.

Notice that tightening your payment terms is not on that list. Shortening terms from 30 days to 14 days does very little if you were not enforcing 30 in the first place. It usually just widens the gap between your terms and reality.

Debtor Days and Aged Debt Together

The two measures answer different questions, and neither works well alone.

Debtor days gives you one number you can trend over time and put in front of a board or a lender. It is good for direction and terrible for detail.

The aged debt report gives you the detail. It tells you which customers, which invoices and how late, which is what somebody actually needs in order to do anything.

Use debtor days to know whether things are getting better. Use the aged debt report to make them better.

Your Legal Position on Late Payment

If your debtor days is high because business customers simply pay late, you have statutory rights. Unless your contract sets its own rate, the Late Payment of Commercial Debts (Interest) Act 1998 gives you interest at 8% above the Bank of England base rate (GOV.UK, 2026), plus a fixed recovery sum of £40, £70 or £100 depending on invoice size (GOV.UK, 2026). The rate comes from the 30 June or 31 December before the debt went overdue, not from today’s rate, so it depends on when the invoice fell due. Our late payment interest calculator works that out for a given invoice.

Whether to charge it is a different question, and mostly the answer is no. That is covered on the product site in why statutory interest rarely works.

The rules may tighten. The Commercial Payments Bill would cap standard commercial terms at 60 days, with narrow exemptions. It went to Parliament in May 2026, is not law yet, and will not apply to past payments (GOV.UK, 2026). Whether it pulls sector averages down remains to be seen. We covered the announcement in our King’s Speech update.

Frequently Asked Questions

What is a good debtor days figure?

There is no single right answer, because it depends on your terms, your sales pattern and your sector. Compare your figure to your own standard terms first, then use the aged debt report to explain the gap. As a rule of thumb we would treat something in the 30s on 30-day terms as healthy, and anything past 50 as a sign the chasing is not keeping up. Treat that as a starting point, not a standard: HMRC notes that collection periods differ a great deal between trades.

Are low debtor days always good?

Not always. An unusually low figure can mean you are collecting well, which is good. It can also mean you are only selling to customers who pay upfront, or that you are being so aggressive on terms that you are losing business you would rather keep. It is worth checking that a falling figure reflects better collection rather than a shrinking or narrowing customer base.

What is the difference between debtor days and DSO?

None. Days sales outstanding is the American term for the same measure, calculated the same way. You will see DSO in software built for the United States market and in accountancy material written there. UK finance teams generally say debtor days, or occasionally the debtor collection period.

Making It Someone’s Job

Debtor days improves when the chasing has an owner, a worklist and a routine. That is easier when the worklist builds itself from live invoice data rather than from a spreadsheet somebody exports on a Monday.

That is what the credit control side of our CRM does. It knows what is overdue, by how much, and who has already been contacted about it.

If you want to talk through your own figure and where the days are actually going, the contact form is the way in.