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Proportion Calculator

a : b = c : x

x = --

How this proportion calculator works

Enter a, b, and c for the proportion a : b = c : x. The calculator finds the missing term with x = (b × c) ÷ a. For a=2, b=4, c=3 you get x = 6.0000, so 2 : 4 = 3 : 6. The result is shown to four decimal places.

Equal ratios mean a ÷ b = c ÷ x. Cross-multiplying gives a × x = b × c, which rearranges to the on-page formula. 1 : 2 = 5 : x yields 10.0000. 3 : 9 = 4 : x yields 12.0000. You can check by confirming both sides of the proportion match.

All three fields must be finite numbers, and a must not be 0 because the formula divides by a. An empty box or a=0 leaves x as “--.” Decimals are allowed: a=1.5, b=3, c=2 gives x = 4.0000.

This page only solves for x. It does not rearrange to find a, b, or c. Check by hand: multiply b by c, then divide by a. For 5, 10, and 7 that is 70 ÷ 5 = 14.0000.

Proportion examples

Each row is x = (b × c) ÷ a, rounded to four decimals as on the calculator.

abcxWhat it marks
2436.00002 : 4 = 3 : 6
12510.0000Both sides double
39412.0000Both ratios 1 : 3
510714.000070 ÷ 5
25820.000040 ÷ 2
461015.000060 ÷ 4
1.5324.0000Decimal a
1025410.0000100 ÷ 10

How to read the result

  • a : b = c : x

    2 : 4 = 3 : 6 when x = 6.0000.

  • x = (b × c) ÷ a

    Cross-multiply, then divide by a.

  • a cannot be 0

    Division by a would be undefined.

  • Four decimals

    15 shows as 15.0000.

Frequently asked questions

  • What does this proportion calculator find?

    The missing fourth term x in the proportion a : b = c : x. The formula is x = (b × c) ÷ a. For a=2, b=4, c=3 you get x = 6.0000, so 2 : 4 = 3 : 6. The result is shown to four decimal places.

  • Why is x = (b × c) ÷ a?

    If a : b = c : x, then a ÷ b = c ÷ x. Cross-multiplying gives a × x = b × c, so x = (b × c) ÷ a. That is the same rule as “product of means equals product of extremes” for a/b = c/x.

  • Why can a not be zero?

    The formula divides by a. If a is 0, x is undefined. The page needs a finite a that is not 0. An empty field or a=0 leaves the result as “--.”

  • What is 1 : 2 = 5 : x?

    x = (2 × 5) ÷ 1 = 10.0000. So 1 : 2 = 5 : 10. Each side doubles: 1 to 2, and 5 to 10.

  • What is 3 : 9 = 4 : x?

    x = (9 × 4) ÷ 3 = 12.0000. So 3 : 9 = 4 : 12. Both ratios equal 1 : 3.

  • Can the values be decimals?

    Yes. a=1.5, b=3, c=2 gives x = (3 × 2) ÷ 1.5 = 4.0000. Negatives are also accepted as long as a is not 0.

  • What inputs are required?

    All three boxes—a, b, and c—must hold finite numbers, and a must not be 0. Any empty field leaves x as “--.”

  • How precise is the answer?

    x is fixed at four decimal places. 2 : 5 = 8 : x gives 20.0000. 4 : 6 = 10 : x gives 15.0000.

  • How do I check the answer by hand?

    Multiply b by c, then divide by a. For 5, 10, and 7: 10 × 7 = 70, and 70 ÷ 5 = 14.0000. You can also confirm a ÷ b equals c ÷ x.

  • Is this the same as solving for a different missing term?

    This page only solves for x in a : b = c : x. It does not rearrange to find a, b, or c. Fill those three values and read x.