Free Invoice Generator

DSO calculator

Enter what clients owe you now and what you invoiced over a recent period. The sheet works out your days sales outstanding, how far past your terms clients pay on average, and how much of your money they are holding.

What is owed and what you invoiced

Every unpaid invoice, overdue or not yet due.

The total of every invoice you sent, paid or not.

Your terms

The days your invoices give, as in Net 30. 0 for payment on receipt.

Your overdraft or loan rate, or what the money would earn in savings.

GuideChasing a late invoiceHow this works, and where the figures go wrong →

How the DSO calculator works

Days sales outstanding (DSO) is what clients owe you now, divided by what you invoiced over a period, times the days in that period. It answers one question: on average, how many days of your invoicing are sitting unpaid?

Say clients owe you £18,000 today, and you invoiced £30,000 over the last 90 days. That is about £333 a day, so the £18,000 is 54 days of invoicing. On 30-day terms, clients are paying 24 days late on average, and 24 days of invoicing, £8,000, is your money in their bank. If you run an overdraft at 8% a year to cover the gap, that costs you £640 a year.

The figure that matters most is the gap between DSO and your terms. A DSO of 40 is fine on 45-day terms and slow on 14-day ones. Accountants call the gap average days delinquent, and measure it against the best DSO you could have if everyone paid on time; your terms are the usual stand-in for that.

Getting the inputs right

  • Count the same way on both sides. If what is owed includes VAT, GST or sales tax, so should what you invoiced.
  • Leave out cash sales. Money taken on the day was never owed, so it makes DSO look better than it is.
  • Use a period long enough to be steady. Over 30 days, one large invoice sent last week can double DSO. 90 days or a year smooths that out.

What the DSO calculator does not do

  • It does not say who is late. DSO is an average across every client. One client 90 days late and the rest on time can give the same figure as everyone a little late.
  • It does not work out statutory interest. What you can charge a late payer under the UK's late payment law or the EU directive is the job of the late payment interest calculator.
  • It does not keep anything. Nothing is written to your browser's storage and nothing you type is sent to us.

Related

To bring DSO down, the guide to chasing a late invoice sets out what to send and when, and the payment reminder email generator writes each email. Shorter terms help too: the payment terms generator writes them, and the early payment discount calculator shows what a discount for paying early costs you.

Last updated 5 October 2026