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DSO Calculator: Days Sales Outstanding, Count-Back DSO and CEI

Days sales outstanding (DSO) tells you how many days of credit sales are sitting in accounts receivable. A lower number means cash arrives sooner. A single DSO figure hides a lot, though: it blends invoices that are on terms with invoices that are overdue, and it moves with sales timing as well as collections.

This calculator gives you simple DSO, count-back DSO, best possible DSO, average days delinquent and the collection effectiveness index side by side, plus the dollar value of one day and the cash released if you cut DSO. The example values are illustrative; replace them with yours.

Core inputs

Ending accounts receivable (AR) ($)Credit sales in the period ($)Exclude cash sales. Use revenue if you have no cash sales.Days in the period (days)90 for a quarter, 365 for a year.Current (not yet due) AR ($)For best possible DSO and average days delinquent.

Collection effectiveness index

Beginning AR ($)Ending AR ($)Ending current AR ($)

Count-back DSO: monthly credit sales

July sales ($, 31 days)August sales ($, 31 days)September sales (most recent month) ($, 30 days)Count-back walks the ending AR above back through these months. Rename to your own three most recent months; the day counts here are 30, 31, 31.

Scenario

Reduce DSO by (days)

Your results

Simple DSO

40.5 days

AR ÷ credit sales × days.

Count-back DSO

37.8 days

Walks AR back through monthly sales. Less distorted by seasonality or growth than simple DSO.

Best possible DSO

31.5 days

The DSO you would have if nothing were past due.

Average days delinquent

9.0 days

Simple DSO minus best possible DSO.

Collection effectiveness index

90.3%

Share of collectible receivables actually collected in the period.

Value of one day of DSO

$133,333

Credit sales ÷ days in period.

Cash released by -5 days

$666,667

One-time cash freed if DSO falls by that many days at the same sales level.

Move this number with us →

Free template · Excel workbook

Collections Log and AR Aging Workbook

Paste your AR aging, rank collections by expected cash per hour, and track promises to pay and roll rates.

Illustrative example values are prefilled; replace them with your own. Everything is calculated in your browser, and Performis does not store what you enter. Our analytics record which calculator was used and a few summary figures, such as a rounded result.

The formulas

Simple DSO: DSO = AR ÷ credit sales × days in period
Count-back DSO: Take ending AR, subtract the most recent month of sales and add that month’s days; repeat until the next month’s sales exceed what remains, then add (remaining AR ÷ that month’s sales) × that month’s days.
Best possible DSO (BPDSO): BPDSO = current AR ÷ credit sales × days in period
Average days delinquent (ADD): ADD = DSO − BPDSO
Collection effectiveness index (CEI): CEI = (beginning AR + credit sales − ending AR) ÷ (beginning AR + credit sales − ending current AR) × 100
Value of one day of DSO: Credit sales ÷ days in period
Cash released by reducing DSO by N days: N × (credit sales ÷ days in period)

How to interpret your result

Compare DSO with your own terms

If you sell on net-30 and your DSO is well above 30, some customers are paying late. Your terms, not a published average, are the yardstick that matters for your business.

Read average days delinquent as the recoverable part

Average days delinquent is the portion of DSO caused by past-due balances. That is the slice collections work can act on; the rest is set by your terms and billing timing.

Cross-check simple and count-back DSO

If count-back DSO is much lower than simple DSO, recent sales growth or a late-period spike is inflating AR, not slow payment. If it is higher, older invoices are lingering.

Use CEI to judge collections effort

A CEI near 100% means nearly everything collectible was collected in the period. A falling CEI over successive periods signals collections slipping even if DSO looks stable.

Track the trend, not one reading

Compute the same measures the same way each month. Direction and cause (terms, disputes, concentration in a few accounts) are more informative than any one value.

Frequently asked questions

How do you calculate DSO?

Divide accounts receivable by credit sales for the period and multiply by the number of days in the period. For example, $5.4M of AR against $12.0M of credit sales over 90 days gives 40.5 days.

What is the difference between simple DSO and count-back DSO?

Simple DSO divides total AR by average daily sales across the whole period, so a spike or dip in sales inside the period distorts it. Count-back DSO walks the ending AR back through the most recent months of actual sales, so it reflects when the open invoices were really billed.

What are best possible DSO and average days delinquent?

Best possible DSO uses only current (not yet due) AR, so it shows the DSO you would have if every customer paid on terms. Average days delinquent is simple DSO minus best possible DSO: the days of sales stuck in past-due balances.

What does the collection effectiveness index measure?

CEI compares what you collected with what was collectible during the period, as a percentage. It isolates collections performance from how many days of sales are outstanding, so it moves independently of sales terms.

Is my data stored or sent anywhere?

No. The calculator runs entirely in your browser, collects no personal information, and does not save what you type.

Go deeper

Take it further with a template

Free template · Excel workbook

Collections Log and AR Aging Workbook

Paste your AR aging, rank collections by expected cash per hour, and track promises to pay and roll rates.

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