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Proportion Calculator solve for x

Fill in three of the four values and leave the fourth blank. The calculator cross-multiplies, divides and gives the missing number exactly.

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Answer

Enter your numbers or pick an example — the answer and the steps appear here instantly.

How to solve a proportion here

  1. Enter three values

    Fill A, B, C and D with the three numbers you know. Whole numbers, decimals, fractions such as 2/3 and negatives all work.

  2. Mark the unknown

    Leave exactly one box empty or type x in it. The unknown does not have to be D.

  3. Calculate

    You get the missing value as a reduced fraction and as a decimal, with the cross-multiplication written out. Nothing is sent anywhere and no account is needed.

  4. Check or explore

    Copy the answer, or let “Explain step by step” route it to the AI Math Solver. Its write-up is helpful but not infallible.

What the answer shows

  • The cross-products

    For 3/4 = 9/x, you see 3 × x = 4 × 9, which becomes 3x = 36.

  • The division

    Dividing both sides by the number beside x isolates it: x = 36 ÷ 3 = 12.

  • Exact and decimal forms

    When the answer is not whole, both appear. 3/7 = x/2 gives x = 6/7 ≈ 0.857143, with the decimal tagged Rounded.

Setting up and solving proportions by hand

A proportion states that two ratios are equal. If 3 cups of flour make 4 servings, then 10 servings need x cups, where 3/4 = x/10. Keep the same kind of quantity in the same position on both sides, with cups on top and servings below.

Cross-multiply: the product of the outer terms equals the product of the inner terms, so 3 × 10 = 4 × x, which is 30 = 4x. Divide by 4 and x = 30/4 = 15/2 = 7.5 cups.

The unknown can sit anywhere. For x/5 = 6/8, cross-multiplying gives 8x = 30, so x = 30/8 = 15/4 = 3.75. Check by putting the answer back in: 3.75 ÷ 5 = 0.75, and 6/8 = 0.75 as well.

Cross-multiplication only applies when each side is a single fraction. If a term like x + 2 appears, wrap it in parentheses before multiplying, or turn to the solve-for-x calculator for more general equations.

Frequently asked questions

You can leave any one of the four empty. The calculator needs exactly one unknown, so fill the other three boxes and leave the fourth blank or type x.

Yes. Enter 1/2, 0.75, 0,75 or 1 1/4 in any box. Each value is converted to an exact fraction before cross-multiplying, so decimal rounding never creeps into the result.

It is shown as an exact fraction first, such as 6/7, followed by a rounded decimal that has ≈ in front of it and a Rounded label.

A ratio is a single comparison, like 3:4. A proportion is an equation between two ratios, which is why it can be solved for a missing term; to reduce or split a ratio, the ratio calculator is the better fit.

The calculator's answer and outline are free, with no account and nothing sent off your device. The longer AI explanation costs 2 credits if you have no plan and comes with the Pro plans* (*fair use: up to 300 runs a day across all writing, study and code tools), and a free preview of how it starts is shown to signed-out visitors.

Only if your teacher allows it. It works best for practice: set up the proportion yourself, solve it on paper, then compare your answer here.

Keep solving

Missing one number?

Type the three you know and let cross-multiplication find the fourth.

Solve proportion
  • Unknown in any position
  • Exact fraction plus decimal
  • Cross-products written out