What this calculator does
Add line items with a quantity and a unit price, an optional tax rate, and get a correct subtotal, tax amount and total — the arithmetic that goes wrong most often when it is done by hand across several lines, especially once a discount or a tax rate that only applies to some lines gets involved.
Line total = quantity × unit priceSubtotal = sum of all line totalsTotal = subtotal × (1 + tax rate)Where tax is applied, and why it matters
Tax is calculated on the subtotal, after every line is summed, not on each line individually and then added up. For a clean flat tax rate the two approaches give the same answer, but they diverge the moment rounding enters the picture: rounding tax to the cent on each of ten lines and adding the results can land a cent or two away from rounding the tax once on the combined subtotal. Invoices, receipts and accounting software consistently use the second method — tax the subtotal once — because it is the one that reconciles cleanly, and this tool follows the same convention.
A worked example
| Item | Quantity | Unit price | Line total |
|---|---|---|---|
| Consulting hours | 12 | $85.00 | $1,020.00 |
| Software licence | 1 | $240.00 | $240.00 |
| Onboarding session | 3 | $60.00 | $180.00 |
Subtotal: $1,440.00. At an 8% tax rate, tax is $115.20, and the total due is $1,555.20.
When to leave tax at zero
- Business-to-business invoices in many countries where VAT or GST is reverse-charged to the buyer rather than added by the seller.
- Tax-exempt clients such as registered charities or government bodies in jurisdictions that exempt them.
- Quotes and estimates where the final tax treatment has not been determined yet — better to leave it blank than guess.
When tax genuinely does apply, check the correct rate for your jurisdiction and category of goods or services — rates commonly differ between standard goods, services, and specific categories like food or children's items. If the tax itself needs working out from a total that already includes it, rather than added on top, Sales Tax Calculator and VAT Calculator handle that reverse case directly.
What this tool does not replace
It produces the numbers, not a compliant document. A real invoice generally needs a sequential invoice number, both parties' details, an issue date and a payment due date, and in many jurisdictions a specific tax registration number and wording. Use this to get the totals right, then place them into whatever template or accounting software your business actually issues invoices from.