DSO (days sales outstanding) is the average number of days it takes a business to collect cash after a sale. Lower is better.
Why it matters in B2B collections
DSO is the headline metric of collections health because it converts the whole AR process into one number a CFO can track. Because it is calculated from the ledger rather than from the bank, it is also easy to distort: payments that arrived but were never matched to an invoice still count as outstanding, so a cash application backlog quietly inflates DSO even when collections are performing well.
How DSO is calculated
DSO = (Accounts Receivable / Total Credit Sales) x Number of DaysUse credit sales, not total sales. Including cash sales in the denominator understates DSO, because those sales never entered receivables in the first place.
Worked example
- Accounts receivable at period end: 900,000
- Total credit sales in the period: 2,400,000
- Days in the period: 90
- (900,000 / 2,400,000) x 90
DSO = 33.75 days
Read the trend, not the reading
A single DSO figure says almost nothing on its own, because it moves with seasonality, with the mix of customers invoiced that period, and with a handful of large invoices landing either side of the period end. The direction over four or more periods is the signal. A common rule of thumb is that healthy DSO sits within roughly 10 to 15 days of your stated terms, so under about 45 days on NET-30, but treat that as a sanity check rather than a target, because it varies widely by industry and customer size.
What quietly inflates DSO
Three things distort DSO without any collections failure behind them. Unapplied cash is the most common: the money is in the bank, but because it was never matched to an invoice the ledger still shows the invoice open. Disputed invoices are the second, because they age like late invoices while being a resolution problem. Credit notes not yet issued are the third. Before acting on a rising DSO, it is worth separating these out, because the fix for each is completely different.
Standard DSO versus countback DSO
The standard formula above averages across a period, which smooths out timing but reacts slowly and can mislead when sales are growing fast, because a larger recent sales base pulls the ratio down even if collections have not improved. Countback DSO (also called the exhaustion method) works backwards from the current balance through prior periods' sales until the receivable is exhausted. It tracks recent behaviour more closely and is the better choice for a business with volatile or rapidly growing revenue.
What each metric actually tells you
| Metric | Answers | Blind spot |
|---|---|---|
| DSO | How long cash takes to arrive | Distorted by unapplied cash and by sales growth |
| CEI | How much of what was collectable you collected | Needs a defined period; less familiar to boards |
| Aging profile | Where the risk is concentrated | No single number to trend or report |
DSO: common questions
What is a good DSO?
How do you calculate DSO?
Why is my DSO rising when collections are working?
What is the difference between DSO and CEI?
Go deeper: Calculate your DSO and see how much of it is fixable.
How this connects to other terms
- Unapplied cash
Unapplied cash inflates DSO without any collections failure behind it.
- Collections Effectiveness Index (CEI)
CEI answers the same question as DSO while being harder to distort with sales growth.
- Cash application
Slow cash application is the most common hidden cause of a rising DSO.