A common DSO formula
DSO = (average trade accounts receivable / net credit sales for the period) × days in the period
A simple average uses opening and closing receivables divided by two. More frequent observations can better represent a volatile period.
Some reporting systems use closing receivables or other methods. Document the convention and use it consistently. The Intuit DSO guide illustrates the average-receivables approach.
Worked example
For a 90-day quarter:
- Opening trade receivables: $90,000.
- Closing trade receivables: $110,000.
- Average trade receivables: $100,000.
- Net credit sales: $300,000.
DSO = ($100,000 / $300,000) × 90 = 30 days.
Use consistent currencies, portfolio scope, and treatment of taxes, allowances, and credits. Do not mix quarterly sales with a monthly day count. A zero sales denominator makes this formula undefined.
What DSO tells you
A change can reflect collection timing, billing delays, payment terms, sales growth, seasonality, customer mix, credits, or write-offs.
A lower value does not always prove that collections improved. Writing off receivables may reduce the numerator without bringing in cash.
Compare responsibly
Compare like periods and disclose methodology changes. There is no universally good DSO for every industry or contract structure.
Pair DSO with aging buckets, overdue balances, dispute resolution time, and verified cash receipts. For an exact analysis of payment timing, examine invoice-level payment records rather than relying on the aggregate ratio alone.