What is the difference between calories and kilocalories?

In nutrition, calorie commonly refers to a kilocalorie (kcal), which equals 1,000 small calories and is approximately 4.184 kilojoules.

Is TDEE the same as maintenance calories?

TDEE estimates daily energy expenditure. Maintenance calorie intake is often set near TDEE, but actual needs vary.

How accurate is a calorie calculator?

It is an estimate rather than a laboratory measurement. Individual energy expenditure can differ from equation-based predictions.

Can I use this calculator to plan a diet?

It can provide an estimated energy target, but it does not replace individualized nutrition advice or a complete meal plan.

Geometry · Measurement · Circle Properties

Circle Calculator: Area, Circumference, Radius, and Diameter

A circle can be described by its radius, diameter, circumference, or area. Give the calculator any one of these measurements and it can determine the others, along with arc length and sector area when a central angle is supplied.

The core relationships are simple, but the choice of measurement matters: circumference is a length, area is measured in square units, and radius and diameter are directly related by a factor of two.

Quick reference: diameter = 2 × radius, circumference = 2π × radius, and area = π × radius². The calculator keeps the units consistent throughout the result.

01 · The basicsParts of a Circle

The terminology below is the foundation for almost every circle calculation. The center is the fixed point, the radius runs from the center to the circle, and the diameter passes through the center from one side to the other.

Part	What it means	Key relationship
Center	Fixed point equidistant from the circle	Reference point
Radius (r)	Center to any point on the circle	r = d / 2
Diameter (d)	Longest chord through the center	d = 2r
Circumference (C)	Distance around the circle	C = 2πr
Area (A)	Region enclosed by the circle	A = πr²

Circle anatomy

RadiusCenter → edge
r

→

DiameterAcross the center
d = 2r

→

AreaInside the circle
A = πr²

02 · Core formulasCircle Formulas

The three primary formulas are enough to move between the most common circle measurements. Once the radius is known, the diameter, circumference, and area follow directly.

Diameter: d = 2r

Circumference: C = 2πr = πd

Area: A = πr² = πd² / 4

If you know	Find radius	Find diameter	Find area
Radius r	r	2r	πr²
Diameter d	d/2	d	πd²/4
Circumference C	C/(2π)	C/π	C²/(4π)
Area A	√(A/π)	2√(A/π)	A

03 · Area & circumferenceArea vs. Circumference

Area tells you how much two-dimensional space is enclosed by the circle. Circumference tells you the distance around its boundary. This difference is why area uses square units while circumference uses ordinary length units.

Changing the radius changes area faster than circumference

Area → ×4

Circumference → ×2

Original radiusRadius doubledDifferent scaling laws

If the radius doubles, circumference doubles because it is proportional to r. Area becomes four times larger because it is proportional to r².

04 · Partial circlesArc Length and Sector Area

A central angle determines what fraction of a full circle is being considered. With the angle measured in degrees, the fraction is θ / 360.

Arc length = (θ / 360) × 2πr

Sector area = (θ / 360) × πr²

Central angle	Fraction of circle	Arc	Sector area
90°	1/4	C / 4	A / 4
180°	1/2	C / 2	A / 2
270°	3/4	3C / 4	3A / 4
360°	1	C	A

A central angle selects a fraction of the circle

Full circle360°
C and A

→

90° sector¼ of the circle
C/4 and A/4

→

180° sector½ of the circle
C/2 and A/2

05 · ChordsChord Length

A chord is a straight segment connecting two points on a circle. The diameter is the longest possible chord because it passes through the center.

If the radius and central angle are known, the chord length can be calculated with:

Chord = 2r sin(θ / 2)

Here θ must be expressed in the angle convention used by the formula. For a 180° central angle, the chord becomes 2r, which is exactly the diameter.

06 · The constantWhat Is π?

Pi, written as π, is the constant ratio of a circle's circumference to its diameter. The ratio is the same for every circle, regardless of size.

π = C / d ≈ 3.14159265359

Using more digits of π gives a more precise numerical result. The calculator keeps π in the calculation and rounds the displayed value for readability.

07 · Worked exampleExample: Radius = 10 Units

Suppose a circle has a radius of 10 units. Its diameter is twice the radius, its circumference is 2π times the radius, and its area is π times the square of the radius.

Radius: r = 10
Diameter: d = 2 × 10 = 20
Circumference: C = 2π × 10 = 20π ≈ 62.832
Area: A = π × 10² = 100π ≈ 314.159

If the central angle is 90°, the corresponding arc is one quarter of the circumference and the sector is one quarter of the area:

Arc ≈ 15.708 units · Sector area ≈ 78.540 square units

08 · UnitsCircle Units and Squared Units

Use one consistent length unit for the input. Radius, diameter, circumference, and arc length use that same unit; area uses the squared version.

Input length	Length results	Area result
cm	cm	cm²
m	m	m²
in	in	in²
ft	ft	ft²

Common mistake: a radius of 5 cm gives an area in cm², not cm. The numerical value may look correct while the unit is wrong.

09 · FAQCircle Calculator FAQs

Can I calculate circumference from diameter?

Yes. Use C = πd. Because d = 2r, this is equivalent to C = 2πr.

Can I calculate area from circumference?

Yes. Rearrange the circumference formula to get A = C² / (4π).

Can I calculate radius from area?

Yes. Rearrange A = πr² to get r = √(A/π).

What is the difference between a circle and a disk?

Strictly speaking, a circle is the boundary curve. The disk is the region enclosed by that curve. In everyday geometry, “area of a circle” normally refers to the area of the disk.

What is the longest chord of a circle?

The diameter is the longest chord because it passes through the center.

Circle calculator

Known measurement