Missing Number Calculator

Work out missing number instantly with clear inputs, formula shown and shareable results.

About Missing Number Calculator

Each missing-number puzzle is solved by applying the inverse operation to both sides, then substituting the answer back as a check. Missing-number sentences are the first algebra a child meets: the box is a variable, and the inverse operation is the rule for isolating it.

Formulas

Inverses

k + x = r gives x = r - k; k - x = r gives x = k - r; k x X = r gives x = r / k; k / x = r gives x = k / r

Check

Substitute the answer back into the original sentence and confirm it equals the result

Worked example

With the default values below, Missing Number Calculator returns Missing number of 27.0000. Change any input above and every figure here is recalculated in your browser.

Inputs used

Operationone of known + missing = result, known - missing = result, known x missing = result, known / missing = result: known + missing = result
Known number: 15
Result: 42

Results

Missing number: 27.0000
Completed number sentence: 15 + 27 = 42
Check by substituting back: 42.0000
Inverse operation used: Subtract to undo

Step by step

1Values used
2Inverses
3Check
4Missing number= 27.0000
5Completed number sentence= 15 + 27 = 42
6Check by substituting back= 42.0000
7Inverse operation used= Subtract to undo

Assumptions and limits

The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
Figures are rounded for display to 4 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

How is the Missing Number figure worked out?

k + x = r gives x = r - k; k - x = r gives x = k - r; k x X = r gives x = r / k; k / x = r gives x = k / r. Each missing-number puzzle is solved by applying the inverse operation to both sides, then substituting the answer back as a check.

Why does this matter?

Missing-number sentences are the first algebra a child meets: the box is a variable, and the inverse operation is the rule for isolating it.

What do I need to enter?

Enter operation, known number, result. The defaults shown are a realistic worked example — swap in your own figures to get a result you can use.