How to find a missing number
For a reference ratio of 3 : 2, a row starting with 12 needs 8 on the right: 12 × 2 ÷ 3 = 8. A row ending with 10 needs 15 on the left: 10 × 3 ÷ 2 = 15.
Missing Number Ratio Table Calculator
Fill the gaps in a ratio table. Enter one complete reference row, then the values you know in the other rows. Leave each unknown value blank.
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How to use this calculator
Why equivalent ratios work
Both sides of a ratio must scale by the same factor. The pairs 3 : 2, 6 : 4 and 15 : 10 all have the same relationship. Adding the same number to both sides usually changes the ratio.
What if a complete row does not match?
The tool checks all complete rows against the first positive reference pair. If you enter 12 : 9 after 3 : 2, it asks you to correct the row or clear the value you want to solve. It does not silently replace a known number.
When is a table unsolvable?
This calculator needs one complete row with both values greater than zero. Two half-filled rows alone do not establish a unique ratio. An all-zero row cannot establish a reference, although 0 : 0 can appear as the zero multiple of a positive reference pair.
Your calculations run in your browser. Entered numbers are not uploaded or saved by this tool. Changing an input clears the previous result; select Calculate to update it.
About this calculator
Created by Marchelle Cook at Roof & Ratio. Use the worked examples to check the method. If an answer seems incorrect, send us a calculation question or correction.
Guide updated October 16, 2026.