Free · Arithmetic calculators ➗
Two-Number Sum & Missing Addend
Add two signed decimal numbers or find an unknown addend from the other number and the sum.
- Formula & worked example
- Private in your browser
- No signup
Calculator inputs
Loading calculator…
How to use this calculator
- Enter the known values in the units shown. Results update as you type.
- Where results are editable, change one to solve backwards. Lock a value to hold it fixed.
- Use the worked example to check the method. Reset restores the starting fields.
Private by default
Inputs and results stay in this browser tab. Bookify does not upload or store the values you enter.
Formula and method
12.75 plus −4.25 equals 8.5. If the sum is 8.5 and one addend is 12.75, the missing addend is −4.25.
A + B = sum; missing addend = sum − known addend
Worked example
Enter these known values and leave the other values blank.
- First addend
- 12.75
- Second addend
- -4.25
- Sum
- 8.5
Assumptions and limitations
- Enter plain real numbers, including negative values, zero and decimals.
- For measured quantities, both addends must use compatible units. No unit conversion is performed.
- The result uses decimal arithmetic with finite precision. This tool returns the sum or a missing number; it does not show a handwritten column-addition layout.
Common questions
Can I add a negative number?
Yes. Adding −4.25 has the same numerical effect as subtracting 4.25.
How do I find a missing number?
Enter the sum and the addend you know, leaving the other addend empty. The calculator subtracts the known addend from the sum.
References
The calculation equations, inverse formulas, units, and input rules were imported from this source. Bookify provides the interface and equation solver.
Related calculators
Explore arithmetic calculators ➗ or browse all math calculators.