Your accounts receivable balance is lying to you. Not by a little, either. In the second quarter of 2026, small businesses waited an average of 29.3 days to get paid, with invoices landing nearly nine days past due — and almost three in five businesses carried invoices that were overdue by 30 days or more, owed an average of $17,700. Every overdue invoice sitting in your receivables is part real asset and part wishful thinking, and your books should say which part is which.
That is exactly what a bad debt reserve does. It is your honest, on-the-books estimate of how much of your outstanding receivables will never turn into cash. This guide covers the two ways to account for bad debts, how to estimate the reserve three different ways, why the reserve you keep on your books is not the deduction you take on your taxes, and the collection metrics that tell you whether your reserve — and your collections process — is telling the truth.
Why Your Receivables Need a Reserve
When you make a credit sale, you record revenue and an account receivable in full, as if every dollar will arrive. Experience says otherwise: some customers go quiet, some dispute the invoice forever, and some go out of business. If you leave receivables at gross value until each account individually dies, two things go wrong:
- Your balance sheet overstates assets. Lenders, investors, and your own cash planning all read receivables as near-cash. An unreserved balance pretends the doubtful dollars are just as good as the rest.
- Your income statement mistimes the expense. The sale was recorded this period, but the loss lands months later when you finally give up. That violates the matching principle — expenses should hit the same period as the revenue they relate to.
The fix is the allowance for doubtful accounts, a contra-asset account that sits against receivables on the balance sheet. Gross receivables minus the allowance equals net realizable value: the cash you actually expect to collect. Setting it up is not pessimism; it is the number your banker already assumes when they haircut your borrowing base.
The Two Methods: Allowance vs. Direct Write-Off
There are two ways to record bad debt expense, and they are not interchangeable. Which one you use depends on whether you are keeping GAAP books or filing a tax return — and many businesses use one for each.
The Allowance Method (Required by GAAP)
Under the allowance method, you estimate bad debt expense at the end of each period and record it before any specific account goes bad:
- To set up or top up the reserve: debit Bad Debt Expense, credit Allowance for Doubtful Accounts.
- To write off a specific dead account: debit Allowance for Doubtful Accounts, credit Accounts Receivable (naming the customer). Note that this write-off does not touch the expense account — the expense was already recognized when you built the reserve.
- If a written-off customer surprises you and pays: reverse the write-off (debit A/R, credit Allowance), then record the cash receipt normally.
Because the expense is estimated up front, the allowance method matches the cost of bad debts to the period of the sale. That is why GAAP requires it for any business whose financial statements need to be GAAP-compliant — which includes just about anyone borrowing from a bank, raising outside capital, or getting financial statements reviewed or audited.
The Direct Write-Off Method (Simple, but Usually Wrong for the Books)
Under the direct write-off method, you do nothing until a specific invoice is clearly uncollectible, then debit Bad Debt Expense and credit Accounts Receivable for that customer.
It is simple, and for a business with almost no credit sales the distortion is trivial. But it has two real problems: it violates the matching principle (the expense lands in a later period than the sale), and receivables sit overstated in the meantime. GAAP does not accept it except where the amounts are immaterial.
Where the direct write-off method is required is on your tax return, as covered below. This trips up many owners: the method the IRS demands is the method GAAP forbids, and vice versa.
Side-by-Side Comparison
| Allowance Method | Direct Write-Off Method | |
|---|---|---|
| When expense is recorded | Estimated each period, before accounts go bad | Only when a specific account is deemed uncollectible |
| Follows the matching principle | Yes | No |
| Accepted by GAAP | Yes — required | No (except immaterial amounts) |
| Accepted for tax deductions | No — reserves are not deductible | Yes — required |
| Best for | Accrual-basis books, lenders, audited statements | Tax return; tiny businesses with rare credit sales |
How to Estimate the Reserve: Three Approaches
The allowance is an estimate, so the question is how to make a defensible one. There are three standard approaches, and established businesses often blend them.
1. Percentage of Credit Sales (Income Statement Approach)
Multiply this period's credit sales by your historical bad-debt rate. If you sold $400,000 on credit this quarter and historically lose 1.5% of credit sales, your bad debt expense is $6,000 — recorded regardless of the current allowance balance.
This method is fast and ties the expense directly to the period's sales, which makes it great for interim months. Its weakness is that it never looks at the actual receivables aging, so the allowance balance can drift away from reality over time. Treat it as a quick monthly estimate, then true it up with the aging method at least quarterly.
2. Aging of Receivables (Balance Sheet Approach)
Group outstanding receivables by how overdue they are, and apply a higher loss rate to older buckets, based on your own collection history. A typical schedule looks like this:
| Aging Bucket | Balance | Estimated Uncollectible | Reserve Needed |
|---|---|---|---|
| Current (not yet due) | $180,000 | 1% | $1,800 |
| 1–30 days past due | $60,000 | 4% | $2,400 |
| 31–60 days past due | $25,000 | 12% | $3,000 |
| 61–90 days past due | $12,000 | 30% | $3,600 |
| Over 90 days past due | $8,000 | 60% | $4,800 |
| Total | $285,000 | $15,600 |
The aging method targets the ending allowance balance ($15,600 here), not the expense. So you compare it against the allowance account's existing balance: if the allowance already holds a $4,000 credit, you record only $11,600 of additional expense. If it somehow holds a debit balance (last period's write-offs exceeded the reserve — a sign you under-reserved), you add enough to both erase the debit and reach the target.
This is the most defensible method because it starts from the actual invoices at risk. Auditors and lenders expect to see an aging schedule behind the number.
3. Individual Review (Specific Identification)
For large individual balances, supplement the formulas with judgment: review the biggest and oldest accounts one by one. A $40,000 invoice from a customer that just filed for bankruptcy is not a 12% risk — it is effectively gone, and the reserve should say so. Conversely, a slow-but-reliable anchor client in the 61–90 bucket may deserve a lower rate than the schedule applies.
A practical policy: apply the aging percentages mechanically, then specifically review every balance over a threshold (say, $10,000 or 5% of receivables) and every account over 90 days. Document the reasoning — a one-line note per account is enough to turn a guess into a supportable estimate.
Book vs. Tax: Your Reserve Is Not Your Deduction
Here is the split that confuses almost everyone the first time:
- On your books (accrual basis, GAAP): use the allowance method, as described above.
- On your tax return: the IRS does not let you deduct an estimated reserve. Under Section 166, a business bad debt is deductible only when it becomes wholly or partially worthless within the tax year — effectively the direct write-off method, supported by evidence that you took reasonable steps to collect or that collection is hopeless.
In practice, this creates a routine book-tax difference: your financial statements show bad debt expense from building the reserve, while your tax return deducts only specific accounts that died during the year. Track both. At year-end, reconcile them — the difference between the reserve addition and the actual write-offs is a standard Schedule M-1 style adjustment, and having the schedule ready saves your preparer (and your bill) real time.
Two related traps worth knowing:
- Cash-basis businesses generally cannot deduct unpaid invoices at all. You never recorded the income, so there is no loss to deduct when the customer stiffs you. The write-off is simply removing an invoice you never counted. (Loans of actual money you made to others are a different story.)
- Nonbusiness bad debts get worse treatment. A loan to a friend or a personal guarantee gone wrong is a short-term capital loss, deductible only when totally worthless — not an ordinary business deduction. Keep business and personal lending strictly separated in your records.
Collection KPIs That Keep Your Reserve Honest
A reserve estimated once and never revisited is decoration. These five metrics, reviewed monthly, tell you whether collections are slipping and whether the reserve still fits reality.
Days Sales Outstanding (DSO)
DSO measures the average number of days from invoice to cash:
DSO = (Average Accounts Receivable / Total Credit Sales) × Number of Days in Period
If average receivables are $95,000 on $1,000,000 of annual credit sales, DSO is about 35 days. As a rough benchmark, median DSO across small businesses runs in the high 30s — so a DSO drifting into the 50s or 60s means your collection process, your credit terms, or your customer mix has a problem. Track DSO as a trend, not a single number: three months of steady increase is the early warning.
Collection Effectiveness Index (CEI)
DSO can be distorted by sales swings, so pair it with CEI, which measures what collectors actually collected out of what was available to collect:
CEI = (Beginning Receivables + Credit Sales − Ending Receivables) / (Beginning Receivables + Credit Sales − Ending Current Receivables) × 100
A CEI near 100% means you collected essentially everything that came due; sustained readings in the low 90s or below mean collectible dollars are aging past due on your watch. Many credit managers treat CEI as the fairer scorecard for collection effort because current receivables — not yet due — are excluded from the target.
Aging-Bucket Mix
Boil the aging schedule into one headline number: the percentage of receivables that are current. Healthy B2B businesses typically keep 70–80%+ of receivables in the current bucket. When the current share shrinks while total receivables grow, cash is quietly converting into hope. Set an internal tripwire (for example, current share below 70%, or over-90-day balances above 10% of A/R) that triggers a collection push and a reserve review.
Bad Debt to Sales Ratio
Divide actual write-offs (or bad debt expense) by credit sales for the period. This is the reality check on your percentage-of-sales estimate: if you reserve at 1.5% but write off 2.5% year after year, the reserve is systematically light. Review this ratio annually at minimum and reset your estimating percentages to what the business actually experiences.
Average Days Delinquent (ADD)
ADD strips payment terms out of DSO to isolate collection performance:
ADD = DSO − Best Possible DSO (receivables if every customer paid exactly on terms)
Two businesses with identical DSO can have very different problems: one offers 45-day terms and collects on time, the other offers 15-day terms and collects terribly. ADD exposes the second one. If ADD keeps climbing while terms stay flat, the issue is enforcement, not policy.
A Monthly Routine That Takes Thirty Minutes
Reserves go stale because nobody owns them. Put one person in charge of this monthly cycle:
- Run the aging report as of month-end and compute the current-share percentage and over-90-day balance.
- Update DSO and CEI, and compare each against the prior three months, not just last month.
- Refresh the reserve using the aging method (or percentage-of-sales for speed, with a quarterly aging true-up), and specifically review large and very old balances.
- Work the buckets: reminders before due dates, calls at 15–30 days past due, payment plans or holds at 60, and a firm decision — agency, attorney, or write-off — for anything crossing 90.
- Log every write-off decision with the reason and supporting evidence (collection attempts, bankruptcy notice, unresponsive after X contacts). That log is what substantiates the tax deduction later.
Businesses that run this cadence rarely get surprised. The write-offs still happen, but they arrive pre-reserved, pre-documented, and roughly the size you predicted — which is the entire point.
Common Mistakes to Avoid
- Using the direct write-off method on accrual-basis books. It overstates receivables and mistimes expenses. Fine for the tax return; wrong for GAAP financials your lender reads.
- Deducting the reserve on the tax return. Only specific worthless debts are deductible. Claiming the allowance addition is one of the most common small-business tax errors in this area.
- Setting the reserve once and forgetting it. Loss rates drift with your customer mix and the economy. Re-estimate at least quarterly.
- Ignoring small balances in bulk. Ten $2,000 dead invoices hurt exactly as much as one $20,000 corpse. The aging schedule catches what individual review misses.
- Writing off without documentation. "We decided it was uncollectible" is not evidence. Keep the collection log, the returned mail, the bankruptcy filing — whatever proves worthlessness and timing.
- Chasing new sales while collections bleed. A customer who pays in 90 days at full price is often worse than a smaller customer who pays in 10. DSO by customer tells you who your profitable customers really are.
Simplify Your Financial Management
As you tighten up receivables and reserves, keeping the underlying records clean is what makes all of this routine instead of a year-end scramble. Separating gross receivables from the allowance, tracking write-offs against the reserve rather than straight to expense, and running aging and DSO reports every month takes minutes with well-organized books — and hours with messy ones. Beancount.io provides plain-text accounting that gives you complete transparency and control over your financial data — no black boxes, no vendor lock-in. Get started for free and see why developers and finance professionals are switching to plain-text accounting.
