Invoice Calculator — Methodology

Step-by-step calculation

Build an invoice total from multiple line items with automatic VAT, GST, or sales tax based on your region.

1. 1

   Add each line item

   Enter a description, quantity, and unit rate for each line. Add up to 5 lines.

2. 2

   Set the tax rate

   Pre-filled at 20% (UK VAT), 15% (SA VAT), or 0% (US — sales tax added at checkout). Override if needed.

3. 3

   Optionally add a discount

   A percentage discount applied to the subtotal before tax is calculated.

4. 4

   Read subtotal, tax, and invoice total

   Copy the result block to paste into your invoice template.

Assumptions and overrides

Tax pre-fills at headline rates (UK VAT 20%, SA VAT 15%, US 0%). UK businesses must register for VAT above £90,000 turnover; SA threshold is R1M. Verify your registration status before issuing VAT invoices.

Every region default on the calculator is editable. If your effective rate, fee, or threshold differs from the headline figure shown, type your own number into the field — the calculator recomputes instantly without leaving this page.

Primary sources

Region-tagged primary sources for the default rates, thresholds, and benchmarks used in this calculator.

• USAIRS — Sales and Use Tax (US varies by state)
• UKGOV.UK — VAT rates (UK standard 20%)
• SASARS — Value-Added Tax (SA standard 15%)

Review cadence and corrections

This methodology and the underlying rate defaults are reviewed at least annually, and immediately following any change to the headline rate from IRS, HMRC, or SARS. Every substantive update is recorded on the public changelog. Spotted an error? See the corrections policy for how to report it.

See the full editorial policy for the standards every page on BusCalcTools is held to.