In accounting, an invoice is the source document that records a commercial transaction between a seller and a buyer, showing what was sold, how much is owed, and when payment is due. It is both a formal request for payment and the evidence behind nearly every entry that follows. For the seller, an invoice triggers revenue recognition and an accounts receivable entry. For the buyer, it creates a liability in accounts payable. Because these documents drive financial statements, tax reporting, and cash flow, small mistakes on them cause outsized problems later.
Invoices also serve as a legal record of debt. If a customer never pays, the invoice becomes your evidence when pursuing collections. And when the IRS audits a business, it traces reported income and deductions back to source documents. Invoices are usually the first thing examiners ask for.
What a Standard Invoice Contains
A useful invoice carries a specific set of data points. Missing any of them creates headaches ranging from delayed payments to disputed charges.
- Seller and buyer details: the vendor’s legal business name, address, and contact information, along with the customer’s name and address. For import transactions, federal customs regulations require the name and complete address of the foreign party responsible for invoicing the merchandise.1eCFR. 19 CFR 142.6 – Invoice Requirements
- A unique invoice number, sequential or system-generated, that prevents duplicate payments and simplifies tracking.
- The transaction date and, separately, the payment due date.
- Line-item descriptions with quantities and unit prices. Customs regulations specifically require an adequate description of the merchandise for imported goods.1eCFR. 19 CFR 142.6 – Invoice Requirements
- Payment terms such as “Net 30” (full payment due in 30 days) or “2/10 Net 30” (2% discount if paid within 10 days, otherwise full in 30).
- Applicable taxes, broken out as separate line items. Statewide sales tax rates range from 0% to 7.25% across U.S. states, and local taxes can push the effective rate higher.
- The seller’s tax identification number, which the buyer may need for year-end 1099-NEC reporting.
Businesses that pay independent contractors should collect a completed Form W-9 before issuing the first payment. If a vendor fails to provide a correct taxpayer identification number, the paying business must withhold 24% of each payment as backup withholding and remit it to the IRS.2Internal Revenue Service. Backup Withholding That rate remains 24% for 2026.3Internal Revenue Service. 2026 Publication 15
Electronic invoices carry the same legal standing as paper. The federal ESIGN Act (15 U.S.C. ยง 7001) provides that a contract or record cannot be denied legal effect solely because it is electronic.4Office of the Law Revision Counsel. 15 USC 7001 – General Rule of Validity The practical requirements are the same either way: the components above, delivered in a format the buyer can store and reproduce.
Common Types of Invoices
Not every transaction calls for the same invoice format. The type depends on timing, project structure, and whether goods have shipped.
Pro Forma Invoices
A pro forma invoice is a preliminary quote formatted to look like an invoice, sent before goods are delivered. It gives the buyer a cost estimate but is not an actual demand for payment. In international trade, buyers often need one to apply for an import license, open a letter of credit, or arrange currency transfers.5International Trade Administration. Pro Forma Invoice The document also informs customs about the contents and value of a future shipment, so changes should not be made without the buyer’s consent.
Recurring Invoices
Subscription businesses and ongoing service contracts generate recurring invoices at set intervals. A web hosting company billing monthly issues the same invoice each cycle with updated dates. Automating these reduces administrative work and helps both parties predict cash flow.
Interim and Progress Invoices
Large projects, particularly in construction and consulting, rarely get billed as a single lump sum at completion. The contractor and client agree on milestones upfront, and the contractor sends an interim invoice as each milestone is finished. A building project might trigger separate invoices for site preparation, foundation, framing, and finishing. Cash keeps flowing to the contractor throughout the project, and the buyer avoids paying for work not yet performed.
How Invoices Are Recorded in the Ledger
Once an invoice is sent or received, it must be recorded. The entries differ depending on which side of the transaction you sit on, and the timing of recognition depends on your accounting method.
Seller-Side Entries
When a seller issues an invoice for a credit sale, the entry debits Accounts Receivable (increasing assets) and credits Sales Revenue. If the sale involves inventory, a second entry debits Cost of Goods Sold and credits Inventory to reflect the reduction in stock. Accounts Receivable is a current asset representing money customers owe you. Watching its age helps identify slow payers before they turn into bad debts.
Buyer-Side Entries
When a buyer receives an invoice for a credit purchase, the entry debits either Merchandise Inventory (if buying goods for resale) or an expense account (for supplies or services), and credits Accounts Payable. Accounts Payable is a current liability showing what you owe vendors. Tracking due dates matters because many invoices carry early-payment discount windows, and missing the deadline by a day means paying full price.
Cash Basis vs. Accrual Basis
The timing of when invoice income hits your books depends on your accounting method. Under cash basis accounting, you report income only when payment actually arrives. An invoice sent in December but paid in January falls into the next tax year.
Under accrual basis accounting, you report income when you earn it, regardless of when payment arrives. The IRS applies the “all events test”: income is recognized when all events have occurred that fix your right to receive it and the amount can be determined with reasonable accuracy. Issuing an invoice typically satisfies that test. If you have audited financial statements or other applicable financial statements, the IRS requires you to recognize income no later than when it appears as revenue on those statements.6Internal Revenue Service. Publication 538, Accounting Periods and Methods
The difference matters at year-end. An accrual-basis business that sends $50,000 in invoices during December owes tax on that revenue even if none has been collected yet. Cash-basis businesses avoid that timing pressure but lose the ability to match revenue with the expenses that produced it.
Payment Terms and Corrections
Payment terms on an invoice do more than set a due date. They shape cash flow for both parties. The most common format is “Net 30” or “Net 60,” meaning the full balance is due 30 or 60 days after the invoice date. Discount terms such as “2/10 Net 30” give the buyer 2% off for paying within 10 days.
When invoices go past due, commercial interest may accrue. The rate depends on what the contract specifies. If the contract is silent, state law fills the gap, and statutory interest rates for past-due commercial accounts typically range from about 5% to 9% depending on the state. Spelling out late-fee terms on the invoice itself helps avoid disputes later.
Credit Memos and Debit Memos
Invoices sometimes contain errors, or circumstances change after the fact. Rather than altering the original document, accounting practice uses credit memos and debit memos to make corrections while preserving the audit trail.
A credit memo reduces the amount a customer owes. You issue one when a customer returns defective goods, when the original invoice overcharged, or when you grant a discount after the fact. On the seller’s books, it debits Sales Revenue (or a returns account) and credits Accounts Receivable.
A debit memo does the opposite. It increases the amount owed. If you undercharged or forgot a line item, a debit memo adds the difference by debiting Accounts Receivable and crediting Revenue. Both documents should reference the original invoice number so anyone reviewing the books can trace the adjustment back to its source.
When an Invoice Goes Unpaid
Sometimes an invoice never gets paid, and the balance has to come off the books as a bad debt. The IRS allows a deduction for business bad debts under IRC Section 166, but only if the amount was previously included in your gross income.7Office of the Law Revision Counsel. 26 USC 166 – Bad Debts Cash-basis taxpayers generally cannot deduct unpaid invoices as bad debts because they never reported the income in the first place.8Internal Revenue Service. Topic No. 453, Bad Debt Deduction
For accrual-basis businesses that did record the revenue, you must show the debt is genuinely worthless before writing it off. That means demonstrating you took reasonable steps to collect: demand letters, phone calls, or working with a collection agency. A court judgment is not required, but you do need to show that obtaining one would be pointless because the debtor cannot pay.8Internal Revenue Service. Topic No. 453, Bad Debt Deduction The deduction must be taken in the year the debt becomes worthless.
On the accounting side, the entry debits Bad Debt Expense and credits Accounts Receivable under the direct write-off method, or credits an Allowance for Doubtful Accounts under the allowance method. The allowance method is more common for larger businesses because it estimates uncollectible amounts in advance and matches the expense to the period when revenue was earned.
How Long to Keep Invoices
The IRS requires businesses to keep records supporting every item of income, deduction, or credit on a return until the applicable limitations period expires. For most businesses, that means holding invoices for at least three years from the filing date. The period extends to six years if you fail to report more than 25% of your gross income, and to seven years if you claim a deduction for bad debts or worthless securities. If you never file a return or file a fraudulent one, there is no expiration.9Internal Revenue Service. How Long Should I Keep Records?
Retention alone is not enough. The IRS expects records to tell a coherent story, so voided or adjusted invoices should be preserved alongside the credit or debit memos that explain the change. Any document that supports income, a deduction, or a credit must be kept as long as it may be relevant to tax administration.10Internal Revenue Service. Topic No. 305, Recordkeeping Failing to produce adequate records during an audit can lead to disallowed deductions, meaning tax owed on income you thought was offset by legitimate expenses.
A financial analyst reviewing your books will measure collection efficiency using the accounts receivable turnover ratio, which divides net credit sales by average accounts receivable. A high ratio means you are collecting quickly. A low one suggests invoices are sitting unpaid too long and may signal deeper problems with credit policies or customer quality.