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What is the symbol for radius?


Dimensioning symbols replace text and are used to minimize language barriers. Many companies produce parts all over the world. A print made in the U.S.A. may be read in several different countries. The goal of using dimensioning symbols it to eliminate the need for language translation.

GD&T uses internationally accepted symbols that for the most part cross all language barriers. This enables the correct interpretation of drawing specifications without language translation. The following tables show the symbols that are used in GD&T. The symbols will be illustrated and explained as the learning material unfolds.


Spherical DiameterS n
Spherical RadiusSR
Reference Dimension(8)
Counterbore / Spotfacev
Number of Places4X
Depth / Deepx
Dimension not to scale10
Square (shape)o
Arc LengthArc length symbol
Conical Tapery


Tolerance TypeCharacteristicSymbol
For individual featuresFormStraightnessu
For individual or related featuresProfileProfile of a Linek
Profile of a Surfaced
For related featuresOrientationAngularitya
RunoutCircular Runouth
Total Runoutt


Maximum Material Conditionm
Least Material Conditionl
Regardless of Feature SizeNone


Basic DimensionBasic dimension symbol
BetweenBetween symbol
Dimension OriginDimension Origin symbol
All AroundAll around symbol
Controlled RadiusCR
Statistical ToleranceStatistical Tolerance symbol
Projected Tolerance Zonep
Regardless of feature sizes or None
Tangent PlaneDatum Feature Symbol
Free StateDatum Feature Symbol
Datum FeatureDatum Feature Symbol
Datum TargetDatum Target Symbol
Target PointTarget Point Symbol

Circle Math

In circle math the terms related to the circle are discussed here.

We have learned to draw a circle, by tracing the outlines of objects like a bangle or a bottle cap.

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Circle using a Bangle

We shall learn now how to draw a circle using a compass.

Compass is a handy drawing tool available in the geometry box. A compass has two arms which can be pulled apart easily to adjust to the size of the circle we want to draw.

Fix a sharp pencil into the holder on the compass.

Fix the needle of the compass on the point where the center of the circle would be.

Open and adjust the other arm which is holding the pencil and rotate the compass to draw a circle.

Circle using a Compass

The point O where the needle of the compass is placed in the centre of the circle. The distance of any point on the circle from the centre O is the radius of the circle. In the given figure OX is the radius.

Parts of a Circle

A line segment which joins any two points on the circle is called a chord. MN is a chord in the given figure.

The line passing through the centre which joins two points on the circle is the diameter. Diameter of the circle is twice its radius. YZ is the diameter of the circle in the given figure.

The length of the circle is called its circumference.

A circle is such a closed curve whose every point is equidistant from a fixed point called its center.

The symbol of circle is O .

Symbol of Circle

What are the terms related to the circle?

The terms related to the circle math are:

The center of a circle is a fixed point within the circle from which all the points of the closing curve are equidistant.

Centre of a Circle

O is the centre.

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(ii) Circumference:

The curve which closes a circle is called its circumference. The length of the circumference is called the length of the circle.

Circumference of a Circle

The distance from the centre to any point on the circumference of a circle is called the radius of the circle. The symbol of the radius is r .

Radius of a Circle

(iv) Diameter:

The line-segment passing through the centre and meeting the points on the circumference is called the diameter of the circle. Diameter is denoted by ‘ D ‘.

Diameter of a Circle

AB is a diameter of the circle.

Diameter is twice the length of the radius concerned.

Thus, D = 2r [ Diameter = 2radius ]

r = D/2 [ Radius = Diameter/2 ]

Any part of the circumference is called an arc of the circle.

Arc of a Circle

The line-segment joining the two ends of an arc is known as a chord. A diameter is the longest chord of a circle.

Chord of a Circle

O is the centre.

OP is one radius.

AB is a diameter.

MN is a chord. (line-segment)

OA and OB are also radii.

Related Concepts on Geometry — Simple Shapes & Circle

Pi ( π )

The circumference divided by the diameter of a circle is always π , no matter how large or small the circle is!

To help you remember what π is . just draw this diagram.

Finding Pi Yourself

Draw a circle, or use something circular like a plate.

Measure around the edge (the circumference):

plate circumference 82

I got 82 cm

Measure across the circle (the diameter):

plate diameter

I got 26 cm

82 cm / 26 cm = 3.1538.

That is pretty close to π . Maybe if I measured more accurately?

Using Pi

We can use π to find a Circumference when we know the Diameter

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Circumference = π × Diameter

Example: You walk around a circle which has a diameter of 100 m, how far have you walked?

Distance walked = Circumference
= π × 100 m
= 314.159. m
= 314 m (to the nearest m)

Also we can use π to find a Diameter when we know the Circumference

Diameter = Circumference / π

Example: Sam measured 94 mm around the outside of a pipe . what is its Diameter?

Diameter = Circumference / π
= 94 mm / π
= 29.92. mm
= 30 mm (to the nearest mm)


The radius is half of the diameter, so we can also say:

For a circle with a radius of 1

The distance half way around the circle is π = 3.14159265.


π is approximately equal to:


The digits go on and on with no pattern.

π has been calculated to over 100 trillion decimal places and still there is no pattern to the digits, see Pi Normal.


A quick and easy approximation for π is 22/7

22/7 = 3.1428571.

But as you can see, 22/7 is not exactly right. In fact π is not equal to the ratio of any two numbers, which makes it an irrational number.

A really good approximation, better than 1 part in 10 million, is:

355/113 = 3.1415929.
(think «113355», slash the middle «113/355», then flip «355/113»)


Remembering The Digits

I usually just remember «3.14159», but you can also count the letters of:

«May I have a large container of butter today»
3 1 4 1 5 9 2 6 5

To 100 Decimal Places

Here is π with the first 100 decimal places:


Calculating Pi Yourself

There are many special methods used to calculate π and here is one you can try yourself: it is called the Nilakantha series (after an Indian mathematician who lived in the years 1444–1544).

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It goes on for ever and has this pattern:

3 + 4 2×3×44 4×5×6 + 4 6×7×84 8×9×10 + .

(Notice the + and − pattern, and also the pattern of numbers below the lines.)

It gives these results:

TermResult (to 12 decimals)
.. etc! .

Get a calculator (or use a spreadsheet) and see if you can get better results.

Pi Day

Pi Day is celebrated on March 14. March is the 3rd month, so it looks like 3/14

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