How the totals are worked out

The order matters, and it is the order most tax rules expect:

  1. Each line is quantity multiplied by the unit price.
  2. The subtotal is the sum of the lines.
  3. The discount comes off the subtotal. It is capped at the subtotal, so a discount can never make the invoice negative.
  4. The tax is charged on what is left after the discount. A percentage tax is calculated on the discounted amount, not on the original subtotal.
  5. The total is that discounted amount plus the tax.

Worked example: three lines of 120.00 make a subtotal of 360.00. A discount of 10% leaves 324.00. A tax of 20% on 324.00 is 64.80, and the total is 388.80. Applying the tax to the 360.00 instead would give a different answer, which is why the order is fixed rather than left to the person filling the form in.

Why the arithmetic is kept in cents

Every amount is stored as a whole number of cents and converted to a decimal only when it is printed. Decimal arithmetic on rounded values is where invoice errors come from: a quantity of three at 19.99 is 59.97, but a percentage of 19.5 on 59.97 can land on a half-cent that rounds differently depending on which way the rounding goes.

  • Working in integers means the totals are exact. There is no drift over many lines.
  • A percentage is rounded to the nearest cent once, at the point where it becomes an amount.
  • The total never goes below zero, whatever combination of discount, tax and negative quantity is entered.
  • A negative quantity is read as zero rather than subtracted from the invoice.

Currencies

Sixteen currencies are available, and each one brings its own symbol, its own position and its own number of decimals. It is the decimal count that catches people out most often.

  • Yen is shown without decimals, because the currency has no minor unit. The same quantity of work costs 12,000 rather than 12,000.00.
  • Euro puts the symbol after the amount, with a space: 1,250.00 €.
  • Dollars, pounds and dirhams put the symbol first.
  • The unit price, the line amounts and the totals all follow the currency, so switching it moves every number on the page at once.

The parts of an invoice that people forget

  • The tax ID of your business, which in several countries is a legal requirement on every invoice you issue.
  • The due date, and a reference such as the client's purchase order number so the invoice can be matched against their records.
  • The payment terms, with the bank details. A client cannot pay an invoice they cannot act on, and "payable by bank transfer" is not enough on its own.
  • A logo, placed in the header of the preview and the PDF. A PNG, JPEG or WebP file works, and it is read in the page rather than uploaded.
  • Notes for anything the client would otherwise have to ask about, such as a late payment arrangement or a rounding difference.

How to use it

  1. Fill in your business name, address, email, phone and tax ID, and add a logo if you have one.
  2. Fill in the client: name, address and email.
  3. Set the invoice number, the issue date and the due date. The dates pick up the format your country uses.
  4. Add each item with + Add an item, entering a description, a quantity and a unit price.
  5. Choose the currency, then set the tax and the discount in the options column. The totals under the table update as you type.
  6. Add notes and payment terms.
  7. Read the preview, check the subtotal, the discount, the tax and the total against your own figures, then press Download PDF.

Good to know

  • Your client details, prices and tax numbers never leave the browser. The PDF is built on your machine.
  • The exported PDF is a normal document that opens in any reader, with selectable text rather than a picture of a page.
  • The preview is a faithful drawing of the PDF, so what you check on screen is what the client receives.
  • To send a contract or a quote alongside the invoice, the PDF Merger & Splitter combines them into one file. The Images to PDF converter turns scans or signed pages into a PDF you can append.