Diameter of Circle Calculator
Enter the radius of a circle, or its circumference or area, to find its diameter.
Updated
Diameter of Circle Calculator
Our Diameter of Circle Calculator finds the distance straight across a circle through its centre. Enter the radius, the circumference or the area, and it gives the diameter with the working, plus the other circle measurements and the diameter in other units.
Quick answer
The diameter of a circle is twice the radius, d = 2r: 2 × 7 cm = 14 cm.
- From the circumference: d = C ÷ π, so 44 m gives ≈ 14.01 m
- From the area: d = 2√(A ÷ π), so 100 m² gives ≈ 11.28 m
- Radius from the diameter: r = d ÷ 2, so 14 cm gives 7 cm
- Pi: every circle is π ≈ 3.14159 diameters around
What Is the Diameter of a Circle?
The diameter is a straight line from one side of a circle to the other that passes through the centre. It is the widest distance across the circle: any other line joining two points on the edge (a chord) is shorter. The word also means the length of that line.
Diameter of a Circle Formulas
d = C ÷ π
d = 2√(A ÷ π)
Here r is the radius, C the circumference, A the area and π ≈ 3.14159. Use whichever measurement you have. Going the other way, r = d ÷ 2, C = πd and A = πd² ÷ 4.
Diameter From the Radius
The radius runs from the centre to the edge, and the diameter is two radii end to end, so double it:
This answer is exact. No π is involved.
Diameter From the Circumference
The circumference of every circle is π times its diameter, so divide the circumference by π:
44 ÷ π ≈ 14.01 m
This is the easy way to measure a tree trunk, pipe or post: wrap a tape around it. A trunk that measures 157 cm around is about 157 ÷ π ≈ 49.97 cm across. For this conversion on its own, see the Circumference to Diameter Calculator.
Diameter From the Area
The area is πr², so the radius is √(A ÷ π). Double that for the diameter:
- Divide the area by π. 100 ÷ π ≈ 31.831.
- Take the square root. √31.831 ≈ 5.642 m, the radius.
- Double it. 2 × 5.642 ≈ 11.28 m.
The same formula written another way is d = √(4A ÷ π). For the area from the diameter, use the Area of Circle Calculator.
Circle Diameter Examples
| You know | Working | Diameter |
|---|---|---|
| r = 7 cm | 2 × 7 | 14 cm |
| r = 2.5 in | 2 × 2.5 | 5 in |
| C = 44 m | 44 ÷ π | ≈ 14.01 m |
| C = 100 cm | 100 ÷ π | ≈ 31.83 cm |
| A = 154 cm² | 2√(154 ÷ π) | ≈ 14 cm |
| A = 100 m² | 2√(100 ÷ π) | ≈ 11.28 m |
A circle with a circumference of 1 has a diameter of 1 ÷ π ≈ 0.3183, and a circle with an area of 1 has a diameter of about 1.1284.
Common Mistakes
- Halving instead of doubling. The diameter is twice the radius, not half of it.
- Dividing the circumference by 2π. That gives the radius. For the diameter, divide by π.
- Forgetting the square root. From the area, A ÷ π is r², not r.
- Mixing units. An area in cm² gives a diameter in cm, not cm².
Key Terms
- Diameter
- Edge to edge through the centre, d = 2r.
- Radius
- Centre to edge, half the diameter.
- Chord
- Any line joining two edge points.
- Pi (π)
- Circumference ÷ diameter, ≈ 3.14159.
How to Use the Diameter of Circle Calculator
- Choose what you know. Pick Radius, Circumference or Area. Radius is the usual choice; the other two work the diameter out from the distance around or the space inside.
- Enter the value. Type it, such as 7, and pick a unit if you want one (mm, cm, m, km, in, ft, yd or mi). Decimals and fractions such as 3/4 work.
- Read and copy the result. The diameter appears as you type, with a drawing, the working, the radius, circumference and area, and the diameter in other units. Press Copy to copy it, or Clear or Esc to start again.
Example: a round pond covers 50 m². Choose Area, enter 50 and pick m. The diameter is about 7.978846 m, the radius about 3.989423 m and the circumference about 25.066283 m.
Who Can Use This Calculator?
Frequently Asked Questions (FAQs)
How do you find the diameter of a circle?
Double the radius. A circle with a 7 cm radius has a diameter of 2 × 7 = 14 cm.
What is the formula for the diameter of a circle?
d = 2r from the radius, d = C ÷ π from the circumference, and d = 2√(A ÷ π) from the area, with π ≈ 3.14159.
How do you find the diameter from the circumference?
Divide the circumference by π. A circumference of 44 m gives 44 ÷ π ≈ 14.01 m.
How do you find the diameter from the area?
Divide the area by π, take the square root to get the radius, then double it. An area of 100 m² gives 2√(100 ÷ π) ≈ 11.28 m.
Is the diameter twice the radius?
Yes. The diameter is two radii end to end through the centre, so d = 2r and r = d ÷ 2.
What is the diameter of a circle with a circumference of 100?
100 ÷ π ≈ 31.83, in the same unit as the circumference.
How do you measure the diameter of a tree or pipe?
Wrap a tape around it and divide by π. A trunk 157 cm around is about 49.97 cm across.
Is the diameter the longest line in a circle?
Yes. It is the longest chord, the only one that passes through the centre.
Is the Diameter of Circle Calculator free?
Yes. It is completely free, needs no sign-up, and can be used as many times as you like.
Try our Diameter of Circle Calculator to find the diameter of any circle from its radius, circumference or area, in any units.