Leave one number blank in any equation and get the answer instantly. Works for addition, subtraction, multiplication and division — including decimals and negatives.
Most missing number tools only find the result. Ours finds whichever number is missing — first, second, or the result — across all four basic operations. Just leave one box blank. That's it.
Don't just get the answer — understand how it's solved. Every result shows the algebraic rearrangement used to find the missing value, written in plain language. Perfect for checking homework or learning the method.
Fill all three boxes and the calculator checks whether your equation is mathematically correct — useful for verifying your own calculations or spotting arithmetic errors. No blank required.
The key idea is that every basic operation has an inverse. To find a missing number, you apply the inverse operation to both sides of the equation.
Addition (a + b = c): if b is unknown, rearrange to b = c − a. You reverse addition with subtraction.
Subtraction (a − b = c): if a is unknown, a = b + c. If b is unknown, b = a − c.
Multiplication (a × b = c): if either factor is missing, the missing factor = c ÷ the known factor.
Division (a ÷ b = c): if a is missing, a = b × c. If b is missing, b = a ÷ c. This calculator handles all these cases automatically and shows you exactly which rearrangement was used.
a + b = c and b is missing, then b = c − a. For multiplication: if a × b = c and a is missing, then a = c ÷ b. For division: if a ÷ b = c and b is missing, then b = a ÷ c. Our calculator does all of this automatically and explains which rule it applied.3.14 or 0.5, and negative numbers like -7 or -0.25. The calculator works with any real number and will return the exact decimal result. Results are rounded to 10 decimal places to avoid floating-point display issues.12 ÷ ? = 0 — the calculator will display a clear error message explaining why it can't be solved.