How to use the invoice arithmetic checker
- Paste the invoice lines as description, quantity, unit price and line total, one per line.
- Enter the subtotal, discount, tax, other charges and total shown on the invoice. Leave a box blank to skip that check.
- Enter the tax rate printed on the invoice if there is one. It starts at 0.
- Read the mistakes found, each with the amount it is off by, and the table of every line.
Formula
line total = quantity x unit price, rounded to the cent with halves rounded up
subtotal = the sum of the line totals as written
tax = (subtotal - discount) x rate, rounded to the cent
total = subtotal - discount + tax + other charges
Quantities may have up to four decimals and unit prices up to four decimals, so prices below a cent can be checked. Money is held in whole cents.
An invoice with a wrong line
The sample lists four lines. Design work is 2.5 x 80.00 = 200.00 and Printing is 100 x 0.35 = 35.00 and Delivery is 18.00, which are right. Stock photos is 4 x 12.50 = 50.00 but the line shows 52.00, so it is off by 2.00. The line totals as written add up to 305.00, but the subtotal shows 303.00, which is off by -2.00.
That is 2 mistakes. A second invoice with Chair 4 x 59.99 = 239.96 and Table 249.50 has a subtotal of 489.46. After a 25.00 discount, a tax rate of 6 (an example) gives (489.46 - 25.00) x 0.06 = 27.87, and with 15.00 of other charges the total is 507.33, so every check passes.
Rounding is where small differences come from
Invoices round in different ways: each line, each tax code, or the grand total only. A one cent difference often comes from rounding, not from a mistake, so look at how the issuer rounds before you dispute it. The tool rounds each line, the tax once on the subtotal after the discount, and then adds.
To catch where a running total goes wrong, use the Running Balance Break Finder. To compare an invoice against what was ordered and received, use the Three-Way Match Checker. To make a clean invoice, try the Invoice Generator, and to test a ledger, the Trial Balance Checker.
What it does not judge
The tool checks arithmetic and nothing else. It does not know whether the prices were agreed, whether the tax rate is the right one, or whether an item was delivered. A correct invoice can still be wrong about everything that matters, and a wrong total is only a prompt to ask.
Assumptions and limits
- Each line ends with a quantity, a unit price and a line total, in that order.
- The tax rate has at most two decimals, and it applies to the subtotal after the discount.
- The discount is a dollar amount, and no tax is charged on shipping or other charges.
- Up to 500 lines are read, and nothing is uploaded.
Frequently asked questions
Why is a tax rate not filled in for me?
Rates differ by place and by item and change over time. You enter the rate printed on the invoice, and it starts at 0, so the check never relies on a figure that may be out of date.
What if tax is charged on shipping?
Put the shipping into a normal line, so it is part of the subtotal, and leave the other charges box empty. The tax will then apply to it.
What does a difference in brackets mean?
It is negative: the amount shown is lower than the amount the arithmetic gives. A plain amount means the invoice shows more than it should.
Can I check an invoice with several tax rates?
Not in one run. Check the lines for each rate as separate invoices, or leave the tax boxes blank and verify the line totals and the subtotal.