🧮 Calculator — for the everyday sums
A basic calculator with a keypad to tap and a keyboard that works without being asked. The sum stays on screen as it is typed, the running answer sits under it, and = settles it. Times and divide are worked out before plus and minus — because the whole sum is written in front of you rather than hidden behind each key press.
The calculator
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The keyboard works as well: the digits themselves, + − * / and %, Enter for =, Backspace to take a key back, Esc to clear. The % key divides by 100, so 50% is a half — to add 10% to 200, multiply by 1.1, or use the percentage calculator.
The answer
Arithmetic in double precision — the same numbers every other calculator uses, good for about fifteen significant digits — and the display shows fifteen of them, which is why 0.1 + 0.2 reads as 0.3 here rather than as 0.30000000000000004. The figure the sum uses is not rounded, only the way it is written. A number too big or too small for digits comes out in the e form: 1.234e+21 means 1.234 × 10²¹. Nothing is sent anywhere or saved: the sum is worked out in this browser and is gone when the page is closed.
Why use this calculator?
It is the calculator for everything that is not a mortgage: splitting a bill, working out a tip, checking a number off a receipt, adding up the day. The reason to do it here rather than on a phone is the sum on the screen — you can see the whole thing, check it and take a key back, where a phone calculator shows only the last figure pressed and keeps the order of operations to itself.
Where the keypad comes from
The word is older than the machine, and it did not always mean one:
- “Calculate” comes from calculus, the Latin for a pebble — Romans counted with stones on a board. Until the twentieth century a “calculator” was a person: a human computer, paid to do exactly what this page does.
- Blaise Pascal built the Pascaline in 1642, at nineteen, to help his father with tax sums. Its gears could add and subtract, and the hard part was the carry — the 1 that has to travel along when 199 becomes 200. Leibniz got it multiplying and dividing in 1673.
- The pocket scientific calculator is younger than the Moon landing. The HP-35 arrived in 1972, and it is the machine that finally ended the slide rule's 350 years.
How does this calculator work?
The sum is read the way it is written:
× and ÷ first, then + and −, with the display showing the running answer as you go
- The figure under the sum is a running answer, worked out again on every key. A half-typed sum that cannot be worked out yet simply keeps the last figure it could — nothing jumps about.
- Pressing = settles it: the sum moves to the top line, the answer goes on the card, and the next key either starts a new sum (a digit or a point) or carries on from the answer (an operator).
- × and ÷ are worked out before + and −, so 2+3×4 is 14. A phone calculator usually runs each press as it comes — which is why the same four presses give 20 there. Here the sum is on the screen, so there is nothing to remember.
- A minus in front of a power takes the power first: −2² is −4, while (−2)² is 4. There is no bracket key on this pad, which is the one thing the scientific calculator adds.
- The display is rounded to fifteen significant digits, dropping the last digits of a binary rounding error — 0.1 + 0.2 reads as 0.3 — while the figure the sum itself uses is left alone.
- Nothing is stored and nothing is sent: the sum lives in this tab, and closing it takes the sum with it.
Frequently asked questions
Why is 2+3×4 = 14 here and 20 on my phone?
Because the two machines are answering different questions. This one reads the sum as written, so the × comes first: 3×4 is 12, plus 2 is 14. A phone calculator usually works out each press as it arrives, so it adds 2 and 3 to get 5 and then multiplies by 4 — and its display never shows the whole sum, so nothing gives the order away. Here the sum is on the top line, so what it is about to do is always visible.
What does the % key do?
It divides by 100: 50% is 0.5, and 250% is 2.5. That is the plain meaning of the symbol, and it makes the key predictable — but it does mean 200+10% is 200.1, not 220. For “add 10% to 200” type 200×1.1, and for anything more tangled than that the percentage calculator does the asking for you.
Where are the brackets?
On the scientific calculator, which is the page next to this one. They are the one thing a basic pad cannot do, and there is a workaround if you would rather stay here: work the bracketed part out first, press =, and then carry on from the answer — because an operator after = carries on from the figure on the card.
Why does 1 ÷ 3 stop at fifteen threes?
Because that is how much room a number has. A calculator works in double precision — about fifteen to seventeen significant digits — and the display shows fifteen, which is more than any everyday sum needs. The digits it drops are the tail of a binary rounding error: 0.1 has no exact form in binary, so 0.1 + 0.2 lands a hair over 0.3, and the fifteen-digit display is what cleans that up.
What is 1.234e+21?
Scientific notation, which is what the display falls back on when a number will not fit as digits — 1.234e+21 is 1.234 × 10²¹, and 5e-1 is 0.5. JavaScript writes the e form itself for anything at least 10²¹ or smaller than 10⁻⁶, and this page leaves it that way rather than inventing its own rule.
What does ± do?
It changes the sign of the number being typed, wrapping it in brackets so the sum still reads correctly: 12+3 becomes 12+(−3). Press it again and the brackets come off, so it is never a key you are stuck under.
What happens if I divide by zero?
The page says “Cannot divide by zero.” rather than showing Infinity, which is what the machine would actually produce. Infinity is not a number you can do anything with — everything after it is infinity too — so a calculator that shows it is telling you the arithmetic ran out rather than that the answer is big.