Days Sales Outstanding (DSO): Formula, Example and Why It Drains Startup Cash

What is days sales outstanding (DSO)?

Days sales outstanding (DSO) is the average number of days between making a sale on credit and receiving the cash. A low DSO means customers pay quickly. A high DSO means your revenue is sitting in unpaid invoices instead of your bank account.

Would you rather book $100,000 of revenue that arrives in 20 days or in 90? The income statement shows the same revenue in both cases, but your runway does not. Founders who watch only revenue and burn often miss that cash can run short even while the business grows.

DSO is a close cousin of the cash conversion cycle, but simpler: it looks only at the collection side. That makes it the first receivables metric most early-stage teams can calculate from an invoice list in a spreadsheet, without a finance department.

The DSO formula

The standard formula is DSO = (accounts receivable ÷ credit sales in the period) × number of days in the period. Accounts receivable is the total you have invoiced but not yet collected. Use credit sales only, because card payments and prepaid plans are collected immediately and would pull the number down.

Pick one period and keep it fixed. Monthly (30 days) and quarterly (90 days) are the common choices. Mixing them from one report to the next produces swings that look like changes in customer behavior but are only changes in method.

A worked example: when DSO slips from 30 to 75 days

Take a fictional startup that invoices $60,000 a month on credit, holds $420,000 in cash and has a net burn of $30,000, which is 14 months of runway. At a DSO of 30 days, the company carries about $60,000 of receivables, because $60,000 ÷ 30 days × 30 days = $60,000.

Now a large customer starts paying late and DSO drifts to 75 days. Receivables grow to $60,000 ÷ 30 × 75 = $150,000, which is $90,000 more cash trapped in invoices. If the rest of the model stays the same, cash drops to $330,000 and runway falls to 11 months ($330,000 ÷ $30,000). Nothing about revenue changed. The numbers are an illustration, not a benchmark.

Why DSO matters for runway

Runway is a cash measure, so anything that delays cash shortens it. Unpaid invoices are an interest-free loan to your customers, and a startup rarely has spare cash to lend. This is the same reason profit and cash flow can diverge, which our guide to cash flow vs profit explains in more detail.

DSO also hides inside growth. When sales double and payment terms stay at 60 days, receivables roughly double too, so a growing company can need more working cash exactly when it feels successful. Track DSO next to burn, not after a surprise in the bank balance.

A practical habit is to review the aging of receivables once a month: how much is current, how much is 30 days late and how much is past 60. The oldest invoices are the ones most likely to become write-offs, so they deserve a phone call before they deserve a place in your forecast.

DSO across B2B, B2C and B2G models

B2C businesses usually collect at checkout, so DSO is close to zero and the metric matters little. B2B companies selling on invoice commonly work on net 30 or net 60 terms, and large customers often pay later than the terms allow. B2G, selling to public bodies, tends to carry the longest and least predictable payment cycles because of approvals and budget calendars.

That is why a B2G pilot can look profitable on paper while cash stays tight. Our piece on how B2G sales cycles eat runway shows the same pattern from the sales side. In sectors such as Health-Tech, Green-Tech and Edu-Tech, institutional buyers can add further delay, so size your cash buffer for the slowest realistic payer, not the average one.

How to lower your DSO

Start before the invoice exists. Shorter payment terms, a deposit or milestone payment at signing, and annual prepayment all move cash forward. Deferred revenue from annual plans is the extreme case: you collect today and recognize revenue over the year.

Then tighten the process. Send invoices the day work is delivered, send a reminder before the due date and call overdue accounts instead of emailing again. Offer a small early-payment discount only if you have done the math, because a discount of 2% for paying 20 days early is a large annualized cost.

Finally, watch concentration. If one customer accounts for a big share of receivables, their delay becomes your delay, and customer concentration risk is worth tracking alongside DSO.

Feeling the cash timing gap in Founder Runway

In Founder Runway, a run lasts 20 turns and every decision has four options. You pick a B2B, B2C or B2G business model and start at Pre-Seed, Seed, Pre-Series A or Series A, then watch cash, burn and runway respond to your choices. It is a learning game, not financial advice, but it shows how a slow-paying deal can put your plan under pressure.

A run can end in failure, a promising outcome, EBITDA positive or high-value exit potential. Replaying with a different business model is a quick way to feel why collection speed matters.