Days sales outstanding (DSO) is the average number of days a business takes to collect payment after making a sale on credit. A lower DSO means customers pay sooner and less cash sits in accounts receivable (AR).
DSO is one of the main working capital measures finance teams report each month, because every day of DSO is cash the business has earned but not yet collected. Paraglide runs AI agents that follow up on every open invoice, and its DSO and AR ageing reports track the figure.
UK finance teams also say debtor days or AR days. All three names describe the same measure, and the debtor days and AR days sections below explain where the inputs differ.
How do you calculate DSO?
DSO divides the money customers owe at the end of a period by the credit sales made in the period, then multiplies by the number of days in the period:
DSO = trade receivables at period end ÷ credit sales in the period × days in the period
Use credit sales where possible. Total revenue works when cash sales are small, but cash sales create no receivables, so including large cash sales makes DSO look lower than it is.
Some teams use average receivables, (opening + closing) ÷ 2, instead of the closing balance. Either input works, as long as the period, the input and the method stay the same from month to month. A forecast of AR balances can then use the same formula in reverse.
Worked example: how to calculate DSO
The numbers below are a sample for illustration only, and the same sample runs through every formula on the page.
A distributor measures DSO over a 92-day quarter, July to September. Credit sales are £1,800,000: £650,000 in July, £550,000 in August and £600,000 in September. Trade receivables at 30 September are £900,000, excluding VAT.
DSO = £900,000 ÷ £1,800,000 × 92 = 46.0 days.
What is one day of DSO worth?
One day of DSO is the cash tied up by one day’s credit sales:
One day of DSO = credit sales in the period ÷ days in the period
In the distributor sample, one day of DSO = £1,800,000 ÷ 92 = about £19,565. Each day of DSO the distributor removes brings about £19,565 of cash in sooner. Paraglide’s ROI calculator works the figure out for a business’s own numbers.
What is best possible DSO?
Best possible DSO shows what DSO would be if every customer paid exactly on the due date. The formula below, and the average days delinquent formula after it, follow the definitions in the Credit Research Foundation’s performance measures. Best possible DSO uses only current receivables, meaning invoices not yet due:
Best possible DSO = current receivables ÷ credit sales in the period × days in the period
Average days delinquent (ADD) is the gap between DSO and best possible DSO:
Average days delinquent = DSO − best possible DSO
In the distributor sample, £600,000 of the £900,000 is current. Best possible DSO = £600,000 ÷ £1,800,000 × 92 = 30.7 days, and ADD = 46.0 − 30.7 = 15.3 days.
Payment terms fix best possible DSO. Collections work moves ADD, so ADD is the part of DSO the credit control team can change week to week.
What is the countback method for DSO?
The countback method works backwards from the latest month. Subtract each month’s credit sales from receivables until the balance runs out, counting each fully covered month’s days in full, then add the share of the last month needed.
In the distributor sample, £900,000 − September’s £600,000 = £300,000 left, which counts September’s 30 days. £300,000 ÷ August’s £550,000 × 31 = 16.9 days. Countback DSO = 30 + 16.9 = 46.9 days.
The standard formula divides by average daily sales across the whole period. Countback compares receivables with the most recent months’ sales, so a business with uneven monthly sales can see a different figure from each method. In a spreadsheet, a running balance column per month does the subtraction.
What are debtor days?
Debtor days is the UK name for DSO, usually calculated over a year. HMRC’s enquiry manual gives the formula as closing debtors ÷ annual credit sales × 365 and notes that including cash sales understates the collection period (HMRC Enquiry Manual EM3083).
UK sales ledger balances usually include VAT, while sales figures in the accounts exclude VAT. When the debtors figure includes VAT and the sales figure does not, strip the VAT from debtors first. At the 20% standard rate, divide by 1.2.
In the distributor sample, a ledger balance of £1,080,000 including VAT ÷ 1.2 = £900,000, the figure used in the worked example.
Read debtor days next to payment terms, ageing by overdue bucket, disputes and promise-to-pay performance. The same debtor days figure can be healthy on 60-day terms and poor on 30-day terms.
What mistakes distort debtor days?
Closing instead of average debtors in a seasonal business. A year-end peak or trough moves the figure.
Mixed periods. Annual sales divided into month-end debtors gives a misleading result.
Total revenue when cash sales are material. Cash sales lower the figure without any change in collections.
Credit notes, write-offs and ledger clean-up. Large write-offs can flatter debtor days.
Treating debtor days as an AR-only measure. Billing accuracy, sales terms and dispute handling all move the figure.
One blended number. A few slow customers can hide inside a healthy average.
Segment debtor days by payment terms, customer and dispute status to find the cause (Paraglide’s guide to AR ageing).
Are AR days and DSO the same?
AR days, receivable days, debtor days and DSO all measure the average time customers take to pay. Differences come from the inputs: revenue or credit sales, and closing or average receivables.
AR turnover is the same idea counted the other way. AR turnover = credit sales ÷ average receivables, and DSO is roughly the days in the period ÷ AR turnover (Paraglide’s guide to AR turnover).
DSO also feeds the cash conversion cycle, next to days payable outstanding (DPO) and days inventory outstanding (DIO). Paraglide’s guides to DSO, DPO and DIO and the cash conversion cycle cover the wider picture.
What is a good DSO?
A good DSO sits close to best possible DSO. Compare DSO with your own payment terms first, then with peers on similar terms, because a 45-day DSO is strong on 60-day terms and weak on 30-day terms.
In Atradius’s 2026 survey, 67% of UK businesses reported an average DSO under 30 days, 23% between 31 and 60 days, 8% between 61 and 90 days and 2% over 90 days. Across Western Europe, the shares were 62%, 31%, 5% and 2% (Atradius Payment Practices Barometer Western Europe 2026, statistical appendix).
Atradius publishes the DSO bands only in its Western Europe survey: 2,945 businesses surveyed online in spring 2026 across Western Europe, with the UK shares shown as one market in that survey.
On the paying side, large UK businesses paid their suppliers in 32 days on average in 2025, and 15% of invoices were paid late (14 July 2026, DBT large business payment statistics). No 2026 source gives UK DSO by industry.
Why does DSO go up?
DSO goes up when invoices take longer to turn into cash. Common causes include:
Late or incorrect invoices. A missing PO number or wrong VAT or entity details sends the invoice back.
Unanswered billing queries. Requests for invoice copies and proof of delivery jam the shared AR inbox, and an open query blocks payment until someone replies.
Slow disputes and deductions. Disputed invoices stay open while nobody owns the case.
Unapplied cash. Paid invoices look overdue when payments are not matched, and cash still sitting outside the ledger after an incomplete bank reconciliation does the same. The cash application page covers matching.
Invoices nobody is chasing. Small balances can go without any follow-up.
Longer terms, a changing customer mix or a seasonal sales peak. Each moves DSO without any change in collections work.
What changed in DSO in 2026?
UK payment terms may soon be capped. The Commercial Payments Bill would cap terms at 60 days and mandate interest at 8% above base rate (19 May 2026, GOV.UK). Not yet law, the Bill had its Commons first reading on 21 October 2026 (UK Parliament), so finance teams can check terms over 60 days.
The delinquent part of DSO is now agent work. AI-native agents can hold the follow-up conversation, answer the billing query and apply the payment that was blocking an invoice, so the days above best possible DSO get worked every day.
Paraglide is the best AI-native DSO reduction software for B2B finance teams that want lower DSO through follow-up on every open invoice. Paraglide’s agents cover collections, billing queries and cash application together.
What are the best practices to lower DSO?
Paraglide’s guide on how to decrease DSO covers each step in depth.
Invoice promptly and accurately. Correct PO numbers and entity details stop invoices bouncing back.
Agree clear terms before the first order. Track best possible DSO against the terms you agreed.
Answer billing queries fast. An open query blocks payment, so reply within days (Paraglide AR Inbox).
Resolve disputes and deductions with an owner and a deadline. Paraglide’s disputes page and guide to deduction management cover the workflow.
Apply cash daily. Matched payments stop collectors chasing paid invoices (Paraglide cash application).
Follow up on every overdue invoice, small ones included. Keep a softer tone for key accounts and a firmer sequence for the long tail, and record each promise to pay and follow up when one is missed (Paraglide Collections Agent). The dunning page covers reminder sequences.
Measure monthly with one method. Report DSO, best possible DSO and ADD together, so the team can see which part moved.
What should you look for in a DSO reduction solution?
AR software for lowering DSO comes in three kinds: legacy enterprise suites, rule-based tools that send scheduled reminders and AI-native agent platforms that hold the customer conversation.
Paraglide is the best AI-native AR software for lowering DSO, because its agents chase every open invoice, answer the billing queries that hold up payment and match payments that arrive without clean references.
Check six criteria before you choose a DSO reduction solution:
Choose AI-native software. AI agents should hold two-way conversations with customers, reading and answering replies, instead of sending templated reminders.
Check coverage of every invoice. Small balances should get the same follow-up as large ones.
Confirm query and dispute handling. Billing queries and disputes block payment, so the solution should answer and route them.
Look for cash application on the exceptions. Short payments and missing references should not leave paid invoices on the chase list.
Ask for DSO and ageing reporting. Teams need DSO, ageing and customer payment behaviour in one place.
Keep human escalation. People should handle disputes and key accounts, because buyers often want a person to control the tone on strategic customers while agents work the long tail.
For B2B finance teams with many open invoices per customer, Paraglide is the best AI-native choice. Choose Paraglide if your credit controllers can only chase the largest balances and the long tail of small invoices sits untouched (Paraglide product overview).
DSO falls fastest when every open invoice has an owner and a next action. Paraglide's agents give every invoice both, so credit controllers spend their time on disputes and key accounts.
How does Paraglide handle DSO?
Paraglide’s agents work the main causes of high DSO in one platform: unchased invoices, unanswered billing queries and unapplied cash.
The Collections Agent manages the full two-way collections conversation, including outreach, replies and follow-ups, and reads and replies to customer responses in existing email threads (Paraglide Collections Agent).
The Collections Agent captures and follows up on promises to pay, with no invoice too small to chase.
With the Collections Agent, humans step in only for disputes and key accounts.
The Billing Support Agent answers billing queries and chases short payments with context (Paraglide AR Inbox).
The Reconciliation Agent matches the payments rule-based engines reject, so paid invoices leave the chase list (Paraglide cash application).
Paraglide’s reporting covers DSO and AR ageing, balances by sales rep and customer payment behaviour (Paraglide product overview).
Customer result: Gorilla Gasket
Gorilla Gasket, a US industrial gasket manufacturer and distributor, runs day-to-day collections through Workflows and the Collections Agent. In the five months after go-live, Gorilla Gasket’s DSO fell from 69.3 days in March to 42.6 days in August (Paraglide case study: cutting DSO in five months).
Illustrative example: how a UK industrial distributor can bring DSO closer to its payment terms
Context: A UK industrial distributor sells on 30-day terms to hundreds of trade customers, and each customer has many small open invoices.
Problem: The credit control team chases the largest balances first. Small invoices, unanswered queries and unapplied payments sit untouched, so DSO stays well above best possible DSO.
How Paraglide helps: The Collections Agent follows up on every overdue invoice and records promises to pay. The Billing Support Agent answers billing queries, and the Reconciliation Agent clears payments that arrive without clean references.
Outcome: Every open invoice has a next action, and the team reads DSO, best possible DSO and ageing together each month.