Basic Calculator - Add, Subtract, Multiply, Divide

A basic calculator (also called a four-function calculator) adds, subtracts, multiplies, and divides. This page also includes the extras that sit on almost every desk unit: percent, sign change (±), and square root. What it does not do is parse a typed algebraic sentence. There is no hidden stack of parentheses and no PEMDAS reorder of the keys you already pressed.

That design is intentional. Hardware from the 1970s onward stored a pending left operand in one register, an operator, and the number currently on the display. Pressing + after 12 parks 12 and waits for the next value. Pressing = (or another operator) finishes that pending pair. If you key 2 + 3 × 4, the machine computes (2 + 3) first, then multiplies by 4, and you get 20 - not 14. Students who grew up on phone “expression” calculators are often surprised. This free online basic calculator behaves like the plastic unit in a classroom supply closet so homework that says “use a four-function calculator” matches what you see.

Everything runs in the browser. Digits never leave your device. After the page loads, a dropped network does not stop arithmetic. The last display, pending operator, and recent history lines are stored locally for up to 30 days so a refresh does not wipe a chain you were checking.

Using this two-register desk calculator is the same muscle memory as a physical keypad:

1. Enter a number with the digit keys or your keyboard. One decimal point is allowed. ± flips the sign of whatever is on the display.
2. Press an operator (+, −, ×, or ÷). The current value is stored as the pending left operand.
3. Enter the second number and press =. To chain, press another operator instead of equals: the pending pair is finished first, then the new operator is armed.
4. Use % as a percent of 100, or as a percent of the pending register when an operator is already set. Use √ only on a non-negative display. C or Escape clears the registers. ⌫ or Backspace deletes the last digit while you are still typing a number.

Note: divide by zero is an error, not Infinity. Press C to recover. The display accepts at most 16 characters while you type.

Example - 12 + 3 = 15

• Type 12, press +, type 3, press =
• History records 12 + 3 = 15

Example - why 2 + 3 × 4 is 20 here

• 2 + stores 2 with addition pending
• 3 × finishes 2 + 3 = 5, then arms multiply
• 4 = finishes 5 × 4 = 20
• An algebraic engine would have done 2 + (3 × 4) = 14. Use a scientific or expression calculator when a worksheet requires that grouping.

Example - percent of a pending amount

• 200 + 10 % turns the 10 into 10% of 200, which is 20
• Then = yields 220 (a 10% increase, desk-calculator style)
• With no pending operator, 25 % simply becomes 0.25

Long division with a remainder, or rewriting a decimal as a × 10ⁿ, is a different job. Open the long division calculator or the scientific notation calculator when you need those formats instead of a running total.

Percent on four-function hardware is not a spreadsheet formula. If an operator and a left operand are already stored, the current display is treated as a percentage of that pending value: current ← acc × (current / 100). Otherwise the display is divided by 100. That matches the “tax” and “discount” sequences people learned on desktop machines, and it is why 200 + 10 % is not the same as typing 200 + 0.10 unless you meant a raw addend of a tenth.

± only negates the number you are looking at. It does not reverse the pending operator. √ uses Math.sqrt on the current display. A negative radicand is an error; this page does not invent imaginary results. Division by zero is also an error string - Cannot divide by zero - so you never copy Infinity into a lab notebook.

When you need the mean of a list rather than a running four-function total, switch to the average calculator. A desk calculator can add scores one at a time; it will not report median, mode, or range unless you do that extra work by hand.