Calculator
The keyboard works: digits, . , + - * /, Enter or = to finish, Backspace to delete, Escape to clear. Percent follows the phone convention — with a pending + or − it is a percentage OF the first number, so 200 + 10 % is 220, while 200 × 10 % is 20. Arithmetic is double precision and results are trimmed to 12 significant digits, which is what stops 0.1 + 0.2 showing its binary tail; sums beyond that lose accuracy silently, as they do on every calculator built this way.
The worked examples below are in US dollars. The tool itself works in whichever currency you pick above, and never converts between them — what you type is what it does the arithmetic on.
A plain calculator that behaves predictably
This is the four-function calculator: add, subtract, multiply, divide, percent, sign change and a decimal point, with a twelve-digit display. It is deliberately not a scientific calculator — there are no brackets and no operator precedence, because a pocket calculator has never had them and adding them silently would change what the same keystrokes mean.
It takes the keyboard as well as the buttons.
| Key | What it does |
|---|---|
| 0 to 9 | Enters a digit |
| . or , | Decimal point, whichever your keyboard offers |
| + − * / | The four operators |
| Enter or = | Evaluates |
| Backspace | Deletes the last digit of the current entry |
| Escape | Clears |
How chained operations are evaluated
Each operator key evaluates whatever is already pending before storing the new one. So 2 + 3 + 4 shows 5 as soon as the second plus is pressed and 9 on equals, rather than dropping the first pair. Because there is no precedence, 2 + 3 × 4 is evaluated strictly left to right and gives 20, not 14. If you want 14, you want the scientific calculator, where multiplication binds tighter than addition.
Results are trimmed to twelve significant digits before display, which removes the binary floating-point artefacts that otherwise leak through: 0.1 + 0.2 shows 0.3 rather than 0.30000000000000004. Anything below 1e−9 or at or above 1e12 switches to exponential notation instead of filling the display with zeros.
The percent key, which is not what most people expect
The percent key follows the iOS convention, and it does two different things depending on the pending operation.
| Pending operation | What % does | Worked keystrokes |
|---|---|---|
| A pending + or − | Takes the percentage of the previous number | 200 + 10 % displays 20, and equals gives 220 |
| A pending × or ÷, or nothing pending | Plain division by a hundred | 200 × 10 % displays 0.1, and equals gives 20 |
The first row is what you want for adding a tip or a tax. Both behaviours are defensible and neither is universal — several desktop calculators do the opposite for at least one of these cases.
If a percentage result looks wrong, this is almost always why. Check which operator was pending when you pressed the key.
Limits and error handling
Entry is capped at twelve digits, so a thirteenth keystroke is ignored rather than producing a number the display cannot show honestly. Division by zero produces "Error" rather than infinity, and the next digit you type starts a fresh calculation — the error state cannot be operated on.
There is no memory, no history tape, and no way to edit an expression once entered; Backspace removes the last digit of the current entry only. Underneath, the arithmetic is IEEE 754 double-precision floating point, which is exact for integers up to about 9 quadrillion and approximate for most decimal fractions. The twelve-digit trim hides that in ordinary use, but it does not remove it: money calculations that must be exact to the cent belong in a system that works in integer cents.
Left to right, adding tax, and twelve digits
Why does 2 + 3 × 4 give 20 here and 14 elsewhere?
Because a basic calculator evaluates in the order you press the keys, while a scientific one applies operator precedence. Both are correct for what they are. Use the scientific calculator on this site if you need brackets and precedence.
How do I add tax to a price?
Type the price, press +, type the tax rate, then %, then equals. For 8% on $50: 50 + 8 % = gives 54, because the percent key takes 8% of 50 — that is 4 — while an addition is pending.
Why did my long decimal get shortened?
The display trims to twelve significant digits to hide floating-point noise. If you need more precision than that, you need a calculator working in a different number representation, not a longer display.
Last reviewed 27 August 2026