3의 법칙 비례 계산기

세 개의 알려진 값을 입력하면 네 번째 값이 자동으로 계산됩니다.

입력값

실시간 계산 활성화
Quantity 1
A
B
=
Quantity 2
C
X

Proportion Result

X = 60
10/20 = 30/X -> X = 60 (Direta)

Formula & Solution

X = (B x C) / A
X = (20 x 30) / 10
X = 600 / 10
X = 60
Ratio A/B
0.50
Ratio C/X
0.50
Factor
x2.00
Type
Direta
How It Works

How the Rule of Three Calculator Works

Solve any proportion problem in 3 simple steps — no formulas to memorize.

The Rule of Three Calculator is a free online tool that solves proportions automatically and instantly. Just enter three known values to find the fourth unknown value using proportionality rules.

1
?

Enter 3 Values

Type your three known values in fields A, B, and C. The fourth value (X) is calculated automatically.

2
A B C X

Cross Multiplication

The calculator applies cross multiplication — A × X = B × C (direct) or A × B = C × X (inverse).

3
X

Instant Result

The value of X appears in real-time with step-by-step formulas, ratios, and multiplication factors.

Practical Example

If 5 workers complete a job in 12 days, how many days will 8 workers take?

Type: Inverse (more workers → fewer days)
Formula: X = (A × B) ÷ C = (5 × 12) ÷ 8
Result: X = 7.5 days

Real-Time Calculation

Updates instantly as you type without clicking any button.

Step-by-Step Formula

Shows complete formulas, ratios, and scaling factors for easy learning.

Direct & Inverse

Switch between direct proportion (grows together) and inverse proportion (grows oppositely).

Custom Precision

Adjust decimal precision from 0 to 6 places or full precision.

Formula

Rule of Three Formulas

Master direct and inverse Rule of Three formulas step by step.

Direct

Simple Direct Rule of Three

When quantities change in the same direction

A B
=
C X
Result
X =
B × C A
1 Set proportion: A is to B as C is to X
2 Cross multiply: A × X = B × C
3 Solve X: divide both sides by A
Inverse

Inverse Rule of Three

When quantities change in opposite directions

A B
=
X C
Result
X =
A × B C
1 Identify inverse relationship between quantities
2 Invert second ratio: A/B = X/C
3 Solve X: X = (A × B) ÷ C

Proportion

Equality between two ratios is called a proportion

Cross Multiplication

Multiply means and extremes to solve equations

Quantities

Check whether relation is direct or inverse before solving

Calculators

Specialized Calculators

Choose the perfect calculator optimized for your specific math problem.

Simple Rule of Three

Direct proportion between two quantities

Inverse Rule of Three

When one quantity increases as the other decreases

Simple & Compound

Compare both rule of three calculators side-by-side

Compound Rule of Three

Proportions involving multiple quantities

Percentage Calculator

Calculate percentages using Rule of Three

Nursing Math

Dosage, drip rate, and microdrip calculations
FAQ

Frequently Asked Questions

The Rule of Three is a mathematical method to find an unknown fourth value from three known values using proportion.

In direct rule of three, when one quantity increases, the other increases. In inverse rule of three, when one increases, the other decreases proportionally.

Set up the proportion, determine if it is direct or inverse, and apply cross multiplication to solve for X.

If both quantities grow or shrink together, it is direct. If one grows while the other shrinks, it is inverse.