I typed the rate as 0.15 instead of 15 and the tax came out tiny; how should I enter the rate?

Where the habit comes from

In spreadsheets, 15% is stored as 0.15 and formulas multiply by it directly. If you write formulas a lot, you type 0.15 into any rate box on autopilot. The VAT calculator works like an ordinary calculator instead: the field is labelled Tax rate (%), and the number you type is the number of percentage points.

What the mistake looks like in numbers

• In add mode, 100 at a rate of 15: tax 15, gross 115.
• The same amount at 0.15: tax 0.15, gross 100.15.
• In extract mode, a gross of 115 at 0.15: net 114.83 and tax of only 0.17, instead of 100 and 15.

The last case is the dangerous one, because 114.83 looks like a plausible pre-tax price if you are not paying attention, and it can end up on an invoice or in a sheet before anyone notices that nearly all the tax has vanished.

Three signs that give it away

1. The Rate you entered line in the result shows the rate as the calculator understood it, such as 0.15%. If that is not what you meant, fix it.
2. Tax below 1 on an amount in the hundreds or thousands. That is rarely right.
3. In add mode, a gross total barely different from the amount you entered.

Related slips worth watching

The opposite mistake happens too: typing 150 for 15 gives more tax than the amount itself. The calculator accepts rates up to 1000% as a guard against very large typos; type anything higher and it asks you to correct the field instead of showing a result. Negative rates are not accepted at all.

And typing the number correctly does not make the rate itself correct. The rate for your product is for your tax authority or accountant to confirm; the tool only calculates with what you give it.