How to Use the Reverse Calculation Tool
The Reverse Calculation tool finds the original input when you know the result and one of the values in a basic arithmetic operation. It is useful for back-calculating costs, reversing tax, finding unknowns in formulas, and more.
Select the operation, enter the result and known value, and the original is calculated immediately. A verification value is also shown to confirm the answer is correct.
Reverse Formulas by Operation
Addition: original = result − known. Subtraction: original = result + known. Multiplication: original = result ÷ known. Division: original = result × known. Example: price after 10% tax is $110 → original = 110 ÷ 1.1 = $100.
Frequently Asked Questions
Dividing by zero is mathematically undefined, making reverse multiplication impossible when the known value is zero.
The verification re-applies the operation using the computed original and known values to confirm they equal the result.
Yes. Convert the percentage to a decimal first (e.g., 10% = 0.1, so add 10% = × 1.1), then use multiplication reverse.