Invoice Calculator

Enter your line items, any discount, your sales tax rate, and what has already been paid. You'll get the invoice total and balance due with every step shown, plus the exact due date from your payment terms.

Enter up to six line items. Leave a row's quantity blank for a flat-fee line, and leave unused rows empty. A negative rate makes a credit line.

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How an invoice total is calculated

Every invoice is the same five moves in the same order: add up the line items, subtract the discount, apply sales tax to what's left, add shipping, then subtract anything already paid. The order is not cosmetic. The single most common invoicing mistake is applying tax to the pre-discount amount, which overcharges the customer and, less obviously, overstates the tax you owe. Tax is charged on what you actually billed, so the discount always comes off first.

This calculator runs that exact sequence with your numbers, shows every step, and, if you give it the invoice date and your payment terms, tells you the day payment is due, including the polite warning when Net 30 lands on a Sunday.

The formula

Total = (Line items − Discount) + Shipping + Tax rate × (Line items − Discount [+ Shipping])

Each line item is quantity × rate, rounded to the cent. The discount is a percentage of the subtotal or a flat dollar amount. Shipping joins the tax base only when your state taxes it (the toggle above). Balance due is the total minus any deposit or partial payment.

Worked example

A freelancer bills three lines: design work at 12 hours × $95 = $1,140.00, a flat $250 hosting setup, and 4 stock photos × $15 = $60.00. Subtotal: $1,450.00.

A 10% repeat-client discount takes off $145.00, leaving $1,305.00. Sales tax at 8.25% on $1,305.00 adds $107.66. Invoice total: $1,412.66.

The client paid a $500 deposit, so the balance due is $912.66. Invoiced on July 24, 2026 at Net 30, payment is due Sunday, August 23, 2026, which lands on a weekend, so expect the money Monday.

Net 30, taxed shipping, and the other fine print

Net 30 means 30 calendar days from the invoice date, not 30 business days and not 30 days from whenever the client happened to open the email. Weekends and holidays count. That is why the due date here names the day of the week: a due date you can say out loud ("due the 23rd, that's a Sunday") is a due date that gets paid.

Whether shipping is taxable depends on your state, and the rules are genuinely inconsistent: some states tax delivery charges whenever the goods are taxable, some exempt them if the charge is separately stated on the invoice, and a few split the difference. If you ship across state lines regularly, five minutes on your state revenue department's site is worth more than any general rule we could print here, which is why it is a toggle and not an assumption.

Two smaller conventions worth knowing. Late fees are enforceable only if your contract or invoice terms established them before the work happened, and typical practice runs 1% to 1.5% per month on the overdue balance, with some states capping it lower. And if you need to credit a customer on the same invoice (a returned item, a goodwill adjustment), enter it as a line with a negative rate; the math handles it and the tax base shrinks accordingly, which is exactly what you want.

Frequently asked questions

How do I calculate an invoice total?

Add up the line items (each is quantity times rate), subtract any discount, apply your sales tax rate to what remains, add shipping, then subtract any deposit already paid. The order matters: discount first, then tax, so you only charge tax on what you actually billed.

Does the discount come before or after sales tax?

Before. Tax is owed on the amount the customer actually pays, so a 10% discount reduces the tax base as well as the price. Applying tax first overcharges the customer and overstates the tax you remit, and it is the most common invoicing math mistake there is.

Is shipping taxable on an invoice?

It depends on your state. Some states tax delivery charges whenever the goods are taxable, some exempt shipping if it is listed as a separate line, and a few have rules that split the difference. That is why this calculator makes it a toggle; check your state revenue department's guidance if you are unsure.

What does Net 30 mean?

Payment is due 30 calendar days after the invoice date, weekends and holidays included. It is not 30 business days, and the clock starts on the invoice date, not the day the client reads it. Due on receipt means payment is expected immediately upon delivery of the invoice.

Can I charge a late fee on an overdue invoice?

Generally yes, but only if your contract or invoice terms established the fee before the work was done, and some states cap the rate. Typical practice is 1% to 1.5% per month on the overdue balance. A late fee that appears for the first time on a past-due notice is rarely enforceable.

How do I handle a deposit or partial payment?

Enter it in the already-paid field and the calculator subtracts it from the total to give the balance due. If the payments exceed the total, the balance shows zero and the result tells you the size of the customer's credit instead of showing a negative bill.

How do I add a credit or refund line?

Give the line a negative rate, for example a returned item at -25. The credit reduces the subtotal before the discount and tax are applied, which shrinks the tax base too, exactly as a proper credit memo would.

Can this print an invoice?

It prints a clean summary: line items, discount, tax, total, balance due, and the due date, with the site chrome stripped out. It is a calculator rather than a full invoice generator, so for a client-facing document you would still add your business details and payment instructions.

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