Basic Calculator

A basic calculator does the four operations of arithmetic plus the few extra keys found on most desk models: percent, square root, sign change and memory. Type 84.17 + 23.95 + 112.40 + 9.99 + 45.00 = and this one shows a total of 275.51, with each amount printed on the tape above the ruled-off total, the way an adding machine prints on its paper roll.

It is a simple calculator on purpose: a free calculator online that does the everyday work of a multiplication calculator, a division calculator and a decimal calculator on one keypad, with nothing to install. When a problem needs more, a focused tool is faster: the Scientific Calculator evaluates whole formulas with parentheses, powers and trigonometry, the Fraction Calculator keeps answers as exact fractions, and the Percentage Calculator answers percent questions asked in words.

How the keys work: left to right, like a desk calculator

Basic pocket and desk calculators use immediate execution: each operator is applied to the running total as soon as you press the next operator or the equals key. The display therefore always shows a usable subtotal, which is exactly what you want when you add up a list. In formula form, every operator key updates one stored number:

The two methods only disagree when one chain mixes + or − with × or ÷. The table shows six chains as you would key them in, with the left-to-right result this calculator gives and the result under the order of operations taught in algebra (PEMDAS).

To get the algebra answer here, do the multiplication or division first and add afterward: 3 × 4 = 12, then + 2 = 14. For long formulas, the Order of Operations Calculator solves them step by step.

Worked example 1: adding a stack of receipts

Say you are totaling five receipts for an expense report: $84.17, $23.95, $112.40, $9.99 and $45.00. Type the first amount and press +, enter the next three the same way, and finish the last one with =. After each + the display shows the running total: 84.17, 108.12, 220.52, 230.51, and finally 275.51 when you press =. The tape lists all five amounts as you typed them, trailing zeros in 112.40 and 45.00 included, with 275.51 ruled off underneath.

If you mistype an amount, say 112.04 instead of 112.40, press CE before the next + and type it again. CE removes only that entry, the pending + survives, and the wrong number never reaches the tape. If you only notice after the total (which would read 275.15), the tape shows which line is off, and + 0.36 = brings the total to the correct 275.51.

If the total goes on a tax form, the IRS instructions for Form 1040 let you round to whole dollars, but they tell you to include cents while adding several amounts and to round off only the total. Rounding 275.51 gives $276. Rounding each receipt first (84 + 24 + 112 + 10 + 45) would give $275, a dollar short.

Worked example 2: an invoice total with the memory keys

Memory helps when a total is built from several products. An office supply order of 3 boxes of paper at $42.99, 2 toner cartridges at $89.50 and 12 pens at $1.25 takes three multiplications and one sum. Press MC to empty memory, then type 3 × 42.99 and press M+: the multiplication finishes (128.97) and lands in memory. Repeat with 2 × 89.50 M+ (179) and 12 × 1.25 M+ (15). MR recalls the subtotal, 322.97, and the M badge on the display has shown the memory value after every step.

To add a 6% sales tax, continue from the recalled subtotal: + 6 % shows the tax itself, 19.3782, and = gives 342.3482, which is $342.35 rounded to the cent. Use your own local rate; 6% is only an example. The tape keeps the whole story (each line item, each M+ amount, the recalled 322.97 and the taxed total), so the bill can be checked line by line.

How the percent key works

The % key confuses more people than any other, because its meaning depends on the operator in front of it. This calculator follows the convention found on many basic desk models. Every row was computed by the calculator itself:

After + or −, the percent is taken of the running total, so 10 % turns into 20 when the total is 200. After × or ÷, or with no operator, % divides the number by 100. The equals key then completes the operation as usual, and the tape prints the line as + 10% → 20 so you can see what was added.

Percent keys are not standardized, which is why two calculators can disagree. The manual for Casio's fx-82MS scientific models, for example, increases 2,500 by 15% with the keys 2500 × 15 SHIFT % + (result 2,875), and its 46 − 40 SHIFT % returns 15, the percent change from 40 to 46. Here the same increase is keyed 2500 + 15 % = (also 2,875), while 46 − 40 % = means 46 minus 40% of 46 and gives 27.6. For the change between two values, the Percentage Change Calculator spells out the formula.

Repeat equals and constant calculations

Pressing = again repeats the last operation on the new result: 2 × 3 = shows 6, a second = shows 18 and a third shows 54. Typing a new number before = applies the same operation to it, which turns the calculator into a quick price list. 19.99 × 1.07 = gives 21.3893 (a $19.99 item with 7% tax); typing 24.50 = then gives 26.215 and 8.75 = gives 9.3625, each multiplied by 1.07 without retyping it.

Repeat equals also shows compound growth one period at a time. Enter 1000 × 1.05 = and keep pressing =; each press adds 5% to the previous balance:

From the sixth press on, the exact value has more than 12 significant digits, so the display rounds it while the calculator keeps working with the full value. For savings with regular deposits or monthly compounding, the Compound Interest Calculator builds the whole schedule.

Exact decimals and 12-digit rounding

Most software stores numbers in binary floating point. JavaScript, for instance, represents every number as an IEEE 754 binary64 value, and in base 2 the decimals 0.1, 0.2 and 0.3 are endless fractions that have to be cut off. The leftover errors surface in ordinary money math. This calculator keeps exact decimal digits instead, as the comparison shows:

Division and square roots can still produce endless decimals, and those are rounded. The display shows up to 12 significant digits, the width of a 12-digit desk calculator: 1 ÷ 3 shows 0.333333333333, 2 ÷ 3 shows 0.666666666667, 10 ÷ 7 shows 1.42857142857 and √2 shows 1.41421356237. Behind the display the calculator keeps 34 significant digits, so 1 ÷ 3 × 3 comes back as exactly 1. Results of one trillion or more switch to scientific notation, so 999,999,999,999 + 1 shows 1e+12.

Using the tape like an adding machine

Adding machines printed every entry on a paper roll so a bookkeeper could check a column of figures afterward. The tape here does the same job on screen. Each line shows the operator and the number exactly as you typed it, totals are ruled off, and memory steps and cleared calculations are printed too. Typical uses:

• Receipts and expense reports. Add the receipts, then compare each tape line with its paper slip. IRS Publication 583 lists sales slips, paid bills, invoices, receipts and deposit slips among the supporting documents a business keeps, and the tape makes it quick to confirm that a total matches them.
• Checkbook register. Start with the balance and work through the entries: 1,200 − 349.99 − 89.50 + 25 = leaves 785.51, with the $25 deposit marked + on its own line.
• Budgets and shopping. Subtract planned bills from a paycheck to see what is left, or add item prices as they go into the cart.
• Counts. Add the counts from each shelf or bin and keep the tape next to the count sheet.

When you are done, select the tape lines and copy them into an email or spreadsheet. The tape is plain text, holds the last 500 lines, and empties when you press Clear tape or reload the page.

How to use this calculator

1. Type a number with the on-screen keys or your keyboard. Each number can have up to 12 digits, and starting with the decimal point adds the leading zero for you (0.5).
2. Press an operator. The display switches to the running total, and the small line above it shows the calculation so far. Pressing a different operator right away replaces the first one.
3. Finish with =, or keep chaining operators. A second = repeats the last operation.
4. Correct mistakes with ⌫ (last digit), CE (the number you are typing) or C (the whole calculation).
5. Use the extra keys: % as described above, √ for the square root of the displayed number and ± to change its sign. Pressed at the start or right after an operator, ± begins a negative number, so 200 + ± 5 = gives 195.
6. Keep subtotals with M+ and M−, bring them back with MR and reset memory with MC.

You can type without clicking the calculator first. These keys work anywhere on the page:

The memory keys have no shortcut so they cannot be pressed by accident. Reach them with a click or tap, or with Tab and then Enter.

Who this calculator is for

It suits anyone who needs arithmetic fast and wants to trust the result: shoppers comparing totals and discounts, bookkeepers and small business owners adding receipts and invoices, students checking long multiplication and division, parents helping with decimal homework, and DIYers working out materials. The side of a square room with 250 square feet of floor is √250 = 15.8113883008, about 15.81 feet. Because it runs in the browser, it behaves the same on a phone in a store and on a laptop at a desk, and the keyboard support keeps number-heavy work quick.

Key terms

• Immediate execution: each operation runs as soon as the next operator or = is pressed; also called chain or left-to-right calculation.
• Running total: the number carried from one operation to the next and shown after every operator.
• Operand: a number an operation acts on. In 200 + 10, both 200 and 10 are operands.
• Constant calculation: repeating the last operation with =, often shown as K on desk calculators.
• Memory register: a separate stored number controlled by MC, MR, M+ and M−.
• Significant digits: the digits from the first nonzero digit onward; the display shows up to 12.
• Scientific notation: a compact form for very large or small numbers; 1e+12 means 1 × 10¹², one trillion.
• Binary floating point: the base-2 number format most software uses, which cannot store most decimals exactly.

What changes the result

When two calculators give different answers for the same keys, one of these is almost always the reason:

• Order of evaluation: left to right here and on most basic desk models, algebraic order on scientific calculators and in spreadsheet formulas.
• Percent convention: the table above shows this calculator's rules; some models give % a different meaning after + or −.
• Repeat behavior: an extra = repeats the last operation here, so a stray press changes the total.
• Display width: 10 ÷ 7 shows 1.4285714 on an eight-digit pocket model and 1.42857142857 on this 12-digit display.
• Number format: software built on binary floating point can show tiny errors such as 0.30000000000000004 that decimal arithmetic avoids.

Tips for multiplication, division and decimals

• Estimate first. 19.99 × 3 is close to 20 × 3 = 60, so 59.97 is plausible and 599.7 means a slipped decimal point.
• Count decimal places. 0.25 × 0.4 has three decimal places in total, so the product 0.100 is shown as 0.1.
• Dividing by a number below 1 makes the result bigger: 7.5 ÷ 0.25 = 30 and 128 ÷ 0.5 = 256.
• Remainders: 17 ÷ 5 = 3.4, so 5 goes into 17 three times with 0.4 × 5 = 2 left over. The Long Division Calculator shows the steps and the Modulo Calculator returns just the remainder.
• Division by zero has no answer. 8 ÷ 0 shows Cannot divide by zero, 0 ÷ 0 shows Result is undefined, and any digit starts over.
• Round once, at the end. Continue from a displayed result instead of retyping it, and round only the final answer; the Rounding Calculator handles a set number of decimal places.

Limitations and when to use a different calculator

This is a four-function desk calculator by design. It does not read typed formulas, has no parentheses or powers and always works left to right; the Scientific Calculator handles those. Answers are decimals, never fractions, so for exact thirds and sevenths use the Fraction Calculator or turn a result into a fraction with the Decimal to Fraction Calculator. Typed numbers are limited to 12 digits, and very large results appear in scientific notation, which the Scientific Notation Calculator converts. The tape lives only in your browser tab. For everyday money questions, the Sales Tax Calculator, Discount Calculator and Tip Calculator ask for the inputs in plain words and show every intermediate amount.

Sources

Arithmetic itself needs no source: sums, products and quotients follow from the rules of the decimal number system, and every example on this page was computed with the same engine the calculator runs. The outside facts cited above come from these primary sources:

• IRS: 1040 and 1040-SR Instructions (2025), Rounding Off to Whole Dollars: include cents when adding two or more amounts and round off only the total.
• IRS: Publication 583 (Rev. December 2024), Starting a Business and Keeping Records: the supporting documents a business should keep.
• Ecma TC39: ECMAScript Language Specification, section 6.1.6.1 The Number Type: JavaScript numbers are IEEE 754-2019 binary64 values.
• Casio: fx-82MS/85MS/220PLUS/300MS/350MS User's Guide, Percent Calculations: the manufacturer's percent key sequences quoted above.