Add any list of numbers — including decimals and negatives — and get a running sum and count as you edit the list.

How to Use the Addition Calculator

Start from the three prefilled rows, or use the plus button and the × next to each row to change how many numbers you're adding — every edit recalculates the sum immediately, with no separate calculate button to press.

Addition is the one operation most people can do in their head, right up until decimals or negative numbers show up and the mental math gets shakier. This Addition Calculator treats 12, -4.5, and 0.3 exactly the same way it treats 12, 45, and 8 — there's no special decimal mode and no negative-number toggle, because the underlying arithmetic doesn't need one.

sum = n1 + n2 + n3 + ... + nn

That's really the entire idea: however many rows are in the list, they all get added together, and the count field just reports how many rows that was. Delete a row and both numbers update; add one and they update again, live, as you type.

Where a mental shortcut usually trips up is the negative sign — subtracting 4.5 is the same as adding -4.5, and it's easy to lose track of a sign while carrying a running total across five or six numbers in your head. A calculator doesn't drop signs or misplace a decimal point partway through, which is most of the actual reason to reach for one even for something as basic as addition.