Invoice Calculator: Build a Clean Total from Subtotal, Tax, and Discount

Invoices go out with a subtotal, a tax line, sometimes a discount, and a grand total. Getting those numbers right matters: an undercharged invoice is money lost, an overcharged one can damage trust. An invoice calculator does the arithmetic for you so the math is consistent every time.

The tool applies a discount to the subtotal first, then applies tax to the discounted base, then shows discount amount, tax amount, and grand total. Use it before sending a client invoice, or to double-check an invoice you have already drafted. It is especially useful when you offer a discount and need to know whether tax applies before or after the discount in your jurisdiction.

What Does This Tool Do?

Three inputs: subtotal, tax %, and discount %. The calculator computes discount amount (subtotal × discount %), then a taxable base (subtotal − discount), then tax amount (taxable base × tax %), and finally grand total (taxable base + tax amount).

This order—discount first, then tax on the discounted base—is the most common convention, but tax rules vary by region. If your jurisdiction taxes the original subtotal before discounts, set discount to 0% and subtract the discount manually from the subtotal instead. The tool shows every line so you can verify or adjust.

How to Use It (Step-by-step)

1
Enter the subtotal. This is the sum of all line items before tax or discount, in your currency.
2
Enter the tax percentage. Use the sales tax, VAT, or GST rate that applies to your service or product in your jurisdiction.
3
Enter the discount percentage if you are offering one. Enter 0 if there is no discount.
4
Read the discount amount, tax amount, and grand total. Use these numbers on your invoice so the math is consistent and easy for the client to verify.
5
Use Reset to clear and start a new invoice. Keep your tax rate handy so you can reuse it across invoices.

Key Features

Three inputs: subtotal, tax %, discount %.

Discount applied before tax, with each line shown so you can verify the math.

Grand total shown clearly, ready to drop into an invoice template.

Currency-agnostic: enter amounts in your local currency.

Instant calculation: results update as you type, no submit button.

No storage: runs in your browser, no data sent to a server.

Use Cases

Drafting a client invoice

plug in the subtotal, your tax rate, and any discount to get the grand total.

Offering a discount

see how much a 10% or 20% discount costs you before you commit to the client.

Tax verification

double-check that an existing invoice calculates tax on the correct base.

Quoting with tax

include tax in your quote so the client sees the full amount they will pay.

Comparing discount vs. flat reduction

see whether a percentage discount or a flat amount off is better for the client and for you.

Onboarding new contractors

show them how invoice totals are built so their invoices match your format.

FAQ

It depends on your jurisdiction. The calculator applies the discount first, then taxes the discounted base, which is the most common convention. If your jurisdiction taxes the original subtotal, set discount to 0% and subtract the discount from the subtotal manually.

Use the sales tax, VAT, or GST rate that applies to your service or product in your jurisdiction and your client's location. Rates vary by country, state, and sometimes by service type. When in doubt, check with a tax professional.

The tool currently supports percentage discounts. For a flat discount, subtract it from the subtotal before entering it, or set the discount to 0% and enter the net subtotal manually.

Not directly. Add up the line items yourself to get the subtotal, then enter the subtotal in the tool. The tool focuses on the tax and discount arithmetic rather than line-item invoicing.

No. It is a quick arithmetic check, not a full invoicing or accounting system. For recurring invoices, tracking, and reporting, use dedicated accounting software and consult an accountant.

No. Everything runs in your browser and is cleared when you reset or close the page. Nothing is sent to or stored on our servers.