Solve any proportion a/b = c/d for the missing value using cross-multiplication. Enter three values and find the fourth instantly.
The Proportion Solver finds the missing value in a proportional relationship. Given three of the four values in the equation a/b = c/d, it uses cross-multiplication to solve for the unknown.
Proportions appear constantly in real life: scaling recipes, converting units, resizing images, calculating distances on maps, and determining medication dosages. The fundamental principle is simple — if two ratios are equal, their cross-products are equal: a × d = b × c.
This calculator lets you enter any three values and leave one blank (or enter zero). It automatically identifies the missing value and computes it. The result is shown with high precision for accuracy in technical applications.
Integrating this calculation into regular planning habits ensures that work priorities reflect actual data about where time and energy produce the greatest results each week. Precise measurement of this value supports better personal and professional planning, helping you make informed decisions about how to prioritize tasks and manage competing demands.
Cross-multiplication is straightforward but error-prone with complex numbers. This tool solves any proportion instantly, perfect for students, cooks, engineers, and anyone working with proportional relationships. Precise quantification supports meaningful goal-setting and accountability, ensuring that improvement efforts are focused on areas with the greatest potential impact on output. Data-driven tracking enables proactive schedule management, helping professionals protect focused work time and reduce the cognitive overhead of constant task-switching throughout the day.
Cross-multiplication: a × d = b × c Solve for d: d = (b × c) / a Solve for c: c = (a × d) / b Solve for b: b = (a × d) / c Solve for a: a = (b × c) / d
Result: d = 28
In the proportion 3/7 = 12/d, cross-multiply: 3 × d = 7 × 12 = 84. Solve: d = 84 / 3 = 28. Verify: 3/7 = 0.4286, 12/28 = 0.4286. ✓
Scientific laws often express proportional relationships. Boyle's Law (pressure × volume = constant), Ohm's Law (V = IR), and concentration calculations all rely on proportional reasoning.
Map scales are proportions: 1:50,000 means 1 cm on the map represents 50,000 cm (500 m) in reality. Architects use similar scales on blueprints, making proportion solving essential.
Similar triangles and other similar figures have proportional corresponding sides. If triangle ABC is similar to DEF with sides 3, 4, 5 and 6, 8, ?, then ?/5 = 6/3, so ? = 10.
Cross-multiplication is a method to solve proportions. In a/b = c/d, multiply the diagonals: a × d = b × c. Then solve for the unknown variable.
Yes. The math works identically with negative values. If a/b = c/d and one value is negative, the solver computes correctly.
Medication dosages are often calculated proportionally based on body weight. If the standard dose is 10mg per 50kg, a 75kg patient needs (10 × 75)/50 = 15mg.
In direct proportion, as one value increases, the other increases proportionally (a/b = c/d). In inverse proportion, as one increases, the other decreases (a × b = c × d).
Set up original servings/desired servings = original ingredient amount/new amount. Solve for the new amount. For example, scaling from 4 to 6 servings: 4/6 = 2 cups/?; ? = 3 cups.
Yes. Enter any three values and leave the unknown as 0. The solver determines which value is missing and computes it using cross-multiplication.