Invoice Calculations Explained

Creating an invoice seems simple until you have to apply multiple line-items, a client discount, and local sales tax. If you do the math in the wrong order, you will calculate the wrong tax amount, which can cause accounting headaches and upset clients.

Anatomy of an Invoice

Every professional invoice follows a strict mathematical order of operations:

1. Calculate Line Items
2. Calculate Initial Subtotal
3. Apply Discounts (if any)
4. Calculate Final Subtotal
5. Add Sales Tax (calculated on the Final Subtotal)
6. Add Shipping/Handling (usually non-taxable, depending on state)
7. Calculate Final Total

1. Calculating Line Items

A line item is a single row on your invoice detailing one product or service. You calculate the line total by multiplying the Quantity by the Unit Price.

• Item 1: 5 hours of Web Design × $100/hr = $500
• Item 2: 1 Website Theme License × $60/each = $60

Initial Subtotal: $560

2. Applying Discounts

Discounts must almost always be applied beforecalculating sales tax. If you calculate tax first, and then apply a discount to the grand total, you will end up charging the customer tax on money they didn't actually spend (which is illegal in most tax jurisdictions).

Let's apply a 10% "New Client Discount" to our $560 subtotal.

• 10% of $560 = $56
• $560 - $56 = $504

Final Subtotal: $504

3. Adding Sales Tax

Now we calculate the sales tax based on the Final Subtotal($504). Let's assume your local sales tax rate is 8%.

• 8% of $504 = $40.32
• $504 + $40.32 = $544.32

4. The Final Total

If there are no shipping charges to add, $544.32 is your final Total Amount Due.

If you need to quickly check the math on a discount before adding it to an invoice, you can use our Percentage Calculator.