サイン比計算機

How This Sine Ratio Calculator Works

サイン比計算機は、直角三角形の対辺と斜辺の長さからサイン（正弦）の値を簡単に求めるための無料オンラインツールです。直角三角形における sin(A) は、角度 A の対辺の長さを斜辺の長さで割ることで計算されます。数値を入力するだけで、即座に結果と計算過程が表示されます。

Formula

The sine of angle A equals the length of the opposite side divided by the length of the hypotenuse. This is often remembered with the SOH part of SOH-CAH-TOA.

In a right triangle, the opposite side is the one directly across from angle A. The hypotenuse is always the longest side — the one facing the 90° angle. Since the hypotenuse is always longer than any leg, sin(A) always falls between 0 and 1 for acute angles.

How to Use

1. Identify angle A in your right triangle. It must be one of the two acute angles, not the 90° angle.
2. Find the side opposite angle A. This is side a.
3. Find the hypotenuse. This is side c, the longest side, opposite the right angle.
4. Enter the value of a in the first input field.
5. Enter the value of c in the second input field.
6. Click Calculate to see sin(A).
7. Review the result. It should be a decimal between 0 and 1.

Step-by-Step Example

Suppose you have a right triangle where the opposite side a = 3 and the hypotenuse c = 5.

The sine of angle A is 0.6. This means the opposite side is 60% of the hypotenuse length. If you need the actual angle, take the inverse sine: A = arcsin(0.6) ≈ 36.87°.

What the Result Means

The output is a unitless ratio. It tells you how the opposite side compares to the hypotenuse in size. A result of 0.5 means the opposite side is exactly half the hypotenuse — which happens in a 30-60-90 triangle where A = 30°.

Small values (close to 0) mean angle A is small and the opposite side is short relative to the hypotenuse. Values closer to 1 mean angle A is large (approaching 90°) and the opposite side is nearly as long as the hypotenuse.

Since sin(A) is a ratio of two lengths, it has no units. Whether your triangle is measured in centimeters, inches, or meters, the sine value stays the same as long as both sides use the same unit.

When to Use This Ratio

Use this sine ratio calculator in any of these situations:

• You know the opposite side and hypotenuse and want the sine value.
• You are preparing to find angle A using the inverse sine function.
• You need to verify a textbook answer or check your own trigonometry work.
• You are solving physics problems involving vertical components, like the height of a ramp or the vertical force on an incline.
• You are breaking a vector into its vertical and horizontal parts.

Common Mistakes

Watch out for these common errors when calculating sine:

• Using the adjacent side instead of the opposite side. Sine uses the side across from angle A, not the side next to it.
• Swapping a and c in the formula. The opposite side goes in the numerator, and the hypotenuse goes in the denominator. Writing c / a gives the cosecant, not the sine.
• Using sides measured in different units. If a is in centimeters and c is in inches, the ratio is meaningless. Convert to the same unit first.
• Forgetting that c must be greater than a. In a valid right triangle, the hypotenuse is always the longest side.
• Entering the right angle (90°) as angle A. Sine applies to the acute angles only. The 90° angle is at vertex B.