Segment Area Calculator
Use this segment area calculator to quickly compute the area of a segment. It can also be used to find chord length and arc length. If you're unsure what a segment of a circle is, or even what a chord of a circle is, don't feel embarrassed – just scroll down to find a few definitions and some selfexplanatory images.
What is a segment of a circle? ⌓
If you want to understand what the segment of a circle is, try to imagine cutting part of a circle off with a single, straight cut. And that's it! You've just created two parts of the circle, and the smaller one is called the circular segment. A more formal mathematical definition says that:
A circular segment is a region bounded by a chord and the arc of a circle (of less than 180°)
If it's equal to 180°, then it's simply a half circle – semicircle. According to some definitions, the central angle doesn't need to be smaller than 180° – in that case, you can say that cutting a circle with a line gives you two segments: a major segment and a minor segment.
Have a look at the picture below to help you visualize the difference between segment and sector, as those two names are sometimes confused:
What is a chord of a circle?
A chord is a line that connects two points on the circle – you can think of it as the bowstring of an ideally curved bow. The infinite line extension of a chord is called a secant. The special case of a chord is one that passes through the center of a circle – and that's the circle diameter, of course!
Formulas for a segment of a circle area
To find the circle segment area, you need to know at least two variables. In our segment area calculator, you'll find two popular formulas implemented:

Formula given radius and central angle
A_{segment} = 0.5 × r² × (α – sin(α))
Where does this formula come from? You can look at the segment area as the difference between the area of a sector and the area of an isosceles triangle formed by the two radii:
A_{segment} = A_{sector}  A_{triangle}
Knowing the sector area formula:
A_{sector} = 0.5 × r² × α
An equation for the area of an isosceles triangle, given arm and angle (or simply using law of cosines)
A_{isosceles triangle} = 0.5 × r² × sin(α)
You can find the final equation for the segment of a circle area:
A_{segment} = A_{sector}  A_{isosceles triangle} = (0.5 × r² × α)  (0.5 × r² × sin(α)) = 0.5 × r² × (α – sin(α))

Formula given radius and height
A_{segment} = r² × arccos((rh)/r)  (rh) × √(2 × r × h  h²)
where h is the height of a segment, also known as sagitta.
This formula may be useful when you need to calculate, e.g., the volume of fluid in a pipe or in a circular tank, which is not completely full.
Segment area calculator can work as a chord length calculator as well!
Let's find out how to use this segment area calculator. In our example, we want to find the area of the crosssection of a partially filled pipe:
 Input the circle radius. Assume our pipe radius is 5 in.
 Enter the second variable. Let's say that it's filled 3 inches high, so input that value into the
height
box.  There you go, that's it! Now we know that our segment area is equal to 19.8 in². Additionally, we determined the chord length (9.17 in), arc length (11.6 in) and central angle (132.84°).