Adding sales tax to a pre-tax price
In add mode, the calculator treats the amount you enter as the net, pre-tax price. Tax equals net multiplied by the rate as a decimal, and the gross total is net plus tax. Written out: tax equals amount times rate divided by 100, and total equals amount times (1 plus rate divided by 100).
A 100 item at a 10 percent rate produces 10 of tax and a total of 110. A more realistic combined US rate makes the arithmetic less tidy: a 241.80 item at 8.25 percent gives tax of 19.95 and a total of 261.75. The result panel shows all three figures, net, tax, and gross, so you can quote whichever one the situation calls for.
This is the direction you need when pricing something. If you want a specific amount in your pocket and the tax is added at checkout, enter your target as the amount and read the total as what the customer pays.
Removing sales tax from a total, and why it is not a subtraction
In remove mode, the amount you enter is treated as the gross, tax-inclusive total. The net is the total divided by (1 plus the rate as a decimal), and the tax is the total minus that net. This is the calculation you need when a receipt shows only a final price and you have to report the tax component, for example to reclaim VAT or to record a net expense.
The reason division is required is that the tax was calculated on the smaller net figure, not on the total. Take 261.75 at 8.25 percent. The net is 261.75 divided by 1.0825, which is 241.80, and the tax is 261.75 minus 241.80, or 19.95. Check it: 241.80 times 0.0825 is 19.95, and 241.80 plus 19.95 is 261.75.
Subtracting 8.25 percent of 261.75 instead would give 21.60 of tax and a net of 240.15, which is wrong by more than a pound and a half on this transaction. The error grows with the rate. At a 20 percent VAT rate, subtracting 20 percent of the gross rather than dividing by 1.2 overstates the tax by a sixth.
Choosing the right rate
The tool does not look up rates. You enter one, which makes it usable anywhere but puts the burden of finding the correct rate on you. In the United States, sales tax is set at state level with county, city, and special district add-ons layered on top, so the applicable combined rate depends on the exact delivery or point-of-sale address rather than the state alone. Five states levy no statewide sales tax at all.
In most other countries the equivalent is a value added tax or goods and services tax with a single national standard rate, plus reduced or zero rates for categories such as food, books, children's clothing, and medicines. If the item you are calculating falls into a reduced-rate category, entering the standard rate will overstate the tax.
The rate field accepts decimals, which matters because combined US rates are rarely round numbers. Entering 8.25 rather than rounding to 8 changes the tax on a 1,000 purchase by 2.50.
Sales tax compared with VAT and GST
US sales tax is a single-stage tax collected from the end consumer at the final retail sale. Businesses buying for resale present an exemption certificate and pay no tax, so the tax appears exactly once, at the end of the chain.
VAT and GST are multi-stage. Tax is charged at every step, but registered businesses reclaim the tax they paid on inputs, so the net amount reaching the tax authority is the same as if the full rate had been charged once at the end. The practical difference for a calculator user is presentation: US prices are almost always shown pre-tax with tax added at checkout, while VAT and GST prices are usually displayed tax-inclusive.
That display convention determines which mode you need. A shelf price in London is a gross figure and needs remove mode to find the VAT. A shelf price in Chicago is a net figure and needs add mode to find the total. Using the wrong mode does not produce an error message, only a wrong number, which is the most common mistake made with this calculation.
Rounding and multi-item totals
The calculator computes on the single amount you enter and displays a currency-rounded result. Real point-of-sale systems differ in where they round: some compute tax per line item and round each one, others compute tax on the order subtotal and round once. Those two approaches can differ by a cent or two on a basket of many items, which is normal and not an error in either system.
For checking an invoice, enter the subtotal rather than each line separately. This matches the more common practice of taxing the order total and avoids accumulating rounding differences across lines.
When a discount applies, apply it before tax. Sales tax is generally charged on the discounted price a customer actually pays, not the list price, so enter the post-discount figure as the amount.