THE NUMORIX GUIDE
How to use the Invoice Calculator
Last reviewed September 14, 2026
What this calculator does
The engine rounds each line amount to cents as quantity x unit price, sums those rounded line amounts into subtotal, and applies either a percentage discount or a fixed discount capped at the subtotal.
Formula and method
The engine rounds each line amount to cents as quantity x unit price, sums those rounded line amounts into subtotal, and applies either a percentage discount or a fixed discount capped at the subtotal. Taxable amount is subtotal minus discount, tax is taxable amount x tax rate/100, and total is taxable amount plus tax.
Variables and inputs
Add line items with Description, Quantity, and Unit Price. Then enter Tax Rate and choose a percentage or fixed-dollar Discount. Quantities and prices are dollars or units as labeled; tax and percentage discount are percentages. The UI allows items to be added or removed before calculation.
Worked example
For Item 1 at 1 x $100 and Item 2 at 2 x $50, subtotal = $100 + $100 = $200. With a 10% discount, discount = $20, taxable amount = $180, tax at 10% = $18, and invoice total = $198.
How to interpret the result
The result separates product or service charges, discount, taxable base, tax, and total so the arithmetic can be checked. It is a calculation of the entered invoice, not a determination that every line, delivery charge, or discount is legally taxable.
Common mistakes to avoid
Check quantity and unit price separately, use 10 for a 10% tax rate, and choose the correct discount type. Do not assume a fixed discount can exceed the subtotal; the engine caps it. Preserve the unrounded source amounts when a jurisdiction requires line-level tax rounding.
Assumptions and limitations
The route applies one tax rate to the post-discount aggregate and does not model jurisdictional taxability, shipping, exempt lines, deposits, payment terms, late fees, currency, or line-by-line tax rounding. The engine rounds line amounts before summing, which can differ by a cent from an invoice that totals unrounded lines first.