Proportion calculator for equivalent ratios (a÷b = c÷d)

Toolsle's calculator keeps two fractions equal: whatever you put above the bar (numerators A and C) must scale with what is below (B and D). Leave one box empty to solve using the usual cross-products, or fill all four boxes to verify the numbers really match — useful for recipes, resizing plans, percentages, and school problems.

Cross-multiplication shortcut

If B and D are non-zero, the statement A÷B = C÷D is exactly the same as A×D = B×C. Multiply diagonals and compare — quick way to solve for one missing number without memorizing twelve different formulas.

Tip for word problems

Put matching units on top with each other and matching units below (grams per cup, kilometers per litre, currency per day, etc.). If you mix placements, algebra still works mathematically — but aligning units makes catching mistakes easier.

Related tools

Use Toolsle's percentage calculator, discount calculator, or Markup calculator when ratios turn into percents applied to money.

Frequently asked questions

Why do denominators matter?

Division by zero breaks the proportion model shown here — if B or D is zero, ratios A÷B and C÷D are not usable in this form and the tool will flag that for you instead of pretending an answer exists.

Does this allow negative numbers?

Negative values are allowed algebraically — they behave like signed scale factors or opposite directions — but denominators cannot be zero, and visually you should keep real-world interpretations (temperatures, deficits) consistent with signs.

Why does verifying say “floating-point tolerance”?

Computers store decimals in binary. Two expressions that ought to match algebraically sometimes differ slightly at the tenth decimal place — the verifier uses a tolerance so harmless rounding drift does not look like failure.