Proportions Calculator
Cross-multiply to find the unknown. Click any i for detail.
Proportion
a
i
a
Leave blank if unknown. |
|
|---|---|
b
i
b
Leave blank if unknown. |
|
c
i
c
Leave blank if unknown. |
|
d
i
d
Leave blank if unknown. |
How to use this calculator
- Enter three known values in a/b = c/d.
- Leave exactly one blank for the unknown.
- Read the solved missing term.
- Check that denominators are not zero.
Results explained
A proportion states two ratios are equal. Cross-multiply to isolate the blank: a·d = b·c. This is the algebra behind scale drawings and unit prices.
Quick reference: proportions
If a/b = c/d, then a·d = b·c.
| Item | Detail |
|---|---|
| Solve a | a = b·c / d |
| Solve b | b = a·d / c |
| Solve c | c = a·d / b |
| Solve d | d = b·c / a |
Division by zero is rejected.
How the estimate is built
From a/b = c/d → ad = bc; isolate the unknown.
Example scenario
3/4 = ?/12 → cross-multiply → 9.
FAQ
Can I leave two blanks?
No. Exactly one unknown keeps the system defined.
How is this different from simplifying 8:12?
Simplifying reduces a ratio. Proportions solve for a missing fourth term.
Do units matter?
Keep units consistent on each side (miles with miles, dollars with dollars).