Invoice Calculator

An accurate invoice starts with correct line items.

• Multiply each item's quantity by its unit price, then sum all line amounts to get the subtotal.
• If you offer a discount, apply it to the subtotal — never to individual items after tax.
• Then compute tax on the discounted amount (not the original subtotal).
• Finally, subtract any deposit to arrive at the balance due.

This order — subtotal, discount, tax, deposit — is the standard accounting sequence used by invoicing software worldwide.

Late payment interest compensates you for the time value of money when a client pays after the due date.

The calculation is straightforward: multiply the outstanding balance by the daily interest rate (annual rate divided by 365) and the number of days overdue.

For example, a $5,000 balance at 12% annual interest that is 45 days late accrues $5,000 × 0.0329% × 45 = $74.05 in interest.

Always state your late payment terms on the invoice itself — courts generally enforce interest charges only when the debtor was informed of the rate in advance.

An invoice total builds up in steps:

• sum the line items (quantity × unit price) for the subtotal
• subtract any discount
• add applicable sales tax on the taxable amount
• then add other charges like shipping

For example, a $1,000 subtotal with a 10% discount ($100) and 8% sales tax on the $900 net = $72 tax, for a $972 total.

This calculator applies discounts, tax, and extras in the correct order so the total is accurate.

A complete invoice contains:

• your business name and contact details
• the client's details
• a unique invoice number
• issue and due dates
• an itemized list of goods/services with quantities and prices
• subtotal, discounts, tax, total due
• accepted payment methods
• payment terms

The SBA notes clear, complete invoices get paid faster and support clean records.

Including a unique number and due date is essential for tracking and for tax documentation.

If you sell taxable goods or services, you generally must collect sales tax based on the applicable rate for the transaction, shown as a separate line on the invoice.

Tax applies to the taxable subtotal (after discounts, typically). Rules on what's taxable and which rate applies vary by state and locality.

The IRS and state tax authorities require accurate collection and remittance, so use the correct combined rate for the customer's location.

Payment terms state when payment is due:

• 'Net 30' means within 30 days of the invoice date
• 'Due on receipt' means immediately
• '2/10 Net 30' offers a 2% discount if paid within 10 days

Clear terms set expectations and support collections.

The SBA recommends terms that balance client relationships with your cash-flow needs — shorter terms or early-payment discounts speed payment, which is critical for small-business liquidity.

You can charge a late fee — a flat amount or a monthly interest percentage on the overdue balance — if stated in your terms and permitted by state law, which caps allowable interest rates.

For example, a 1.5% monthly charge on a $1,000 overdue invoice adds $15 per month.

The SBA advises defining late fees upfront on the invoice. State usury laws limit the maximum rate, so set fees within legal limits and communicate them clearly.

For large projects, invoicing a deposit upfront (e.g., 30-50%) and the balance on completion protects cash flow and reduces non-payment risk.

Progress invoices bill in stages as milestones are met. Show deposits and prior payments as credits against the total so the balance due is clear.

The SBA notes deposits are common in service and custom work, aligning payment with the cost and risk you incur over the project.

These documents serve different stages:

• an estimate (or quote) proposes a price before work
• an invoice requests payment for delivered goods/services
• a receipt confirms payment was made

Confusing them causes bookkeeping and tax errors. The IRS relies on invoices and receipts for income documentation.

Issue an estimate to win the job, an invoice to request payment, and a receipt once paid — each with matching reference numbers.

To speed payment:

• invoice promptly
• state clear due dates and terms
• offer convenient payment methods (card, ACH, online)
• send reminders before and after the due date
• consider early-payment discounts

The SBA emphasizes that fast, professional invoicing is one of the biggest levers on small-business cash flow.

Automating invoices and follow-ups reduces the delay between delivering work and getting paid.

Frequent mistakes include:

• math errors in the subtotal or tax
• missing invoice numbers or due dates
• applying tax before discounts (or to non-taxable items)
• vague payment terms
• not following up on overdue invoices
• confusing invoices with receipts

Double-check the total, number every invoice, apply tax correctly, state clear terms and late fees, follow up promptly, and keep copies for taxes — the IRS requires accurate income records.