⚡ConvertNinjas

    Proportion Calculator

    Solve A/B = C/D for any unknown using cross multiplication

    Enter any three values in the proportion A / B = C / D and leave the unknown blank. We solve it using cross multiplication.

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    Quick Facts

    • •A proportion is any statement of the form A/B = C/D
    • •Cross multiplication: A/B = C/D means A × D = B × C
    • •Leave exactly one field blank to solve for that unknown
    • •Works with decimals, fractions expressed as decimals, and negative numbers

    What Is a Proportion?

    A proportion is an equation that states two ratios are equal, written as A/B = C/D. It appears constantly in real life: scaling a recipe, resizing an image, converting between units, adjusting a medication dose, or working out a map distance. If you know any three of the four values, you can always solve for the fourth.

    How to Solve a Proportion

    The core trick is cross multiplication. Because A/B = C/D, multiplying both sides by B and D gives A × D = B × C. From there you divide to isolate the unknown. For example, if 3/4 = x/20, then 3 × 20 = 4 × x, so x = 60 / 4 = 15. This calculator does the cross multiplication and division for you and shows the exact formula it used.

    Common Uses for Proportions

    Cooks scale recipes up or down by keeping ingredient ratios constant. Designers resize photos and videos by preserving the width-to-height ratio. Nurses convert doses when the concentration of a medicine differs from the ordered amount. Students use proportions in similar triangles, map scales, and percentage problems. Anywhere two things scale together, a proportion is the fastest way to find the missing value.

    Watch Out for Zeros

    A proportion breaks down if the denominator you would divide by is zero. If you are solving for A and D is zero, or solving for B and C is zero, the equation has no valid solution. The calculator flags this so you do not get a misleading result.

    Frequently Asked Questions

    This calculator is for general educational and everyday use. For medical dosing, engineering tolerances, or financial contracts, verify results with a qualified professional.