How to figure out the diameter of a circle by its circumference?

Question

The formula I have to go by with the ABYC is normally 3.142xR² to know the circle's circumference, but what I am having a hard time with is this, doing it in reverse. Ok, here's where I come into the Land of Confusion (a great Phil Collins/Genesis song btw). Let's say to find the circumference of an 8 inch circle I go with (R)4² = 16 * 3.142 = 50.27 inches. But in reverse by dividing 16 by 3.142 I get 5.1 as my radius.

The reason why I need to know this madness, is that I am trying to find out what a non-circular geometrical shape's circular diameter. For example, a triangle with three sides of equal size (3 inches) would give me the circumference reference of the circle, but not its diameter.

I must have slept in my Geometry class back in 1985 and missed this one. Proofs and Theorims were my bane of existence in those horrible days of mullets running amok!

If you can show me the correct formula, you will earn 10 brownie points. :) Everyone loves brownies right?

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The formula is for area (well the first part of a longer formula). Basically I am trying to figure out the accurate sized cirlce a non-circular shape would be, diameter-wise, then later extrude that diameter to get my volume of a tube. That way I know how much volume a wierd geometric shape has. Does that make more sense?

Ellee, if I did what you suggested, I get 50.272 / 3.142 = 16. Which is 4 squared.

So let's see if I can make this work with your formula. The circumference of a chevron let's say is a total of 150 inches.

C = 150/3.142 = 47.74 (So now how am I supposed to see what 47.74 is the square of?)

In

Yipeee! Thanks to Geezah's Dr. House like mannerisms and level headed temperment of the rest of you all, I finally figured it out. Well that, and I looked up your answers in some books since I am writing from the library. :) Thanks, I wish I could vote for you all for the best answer. Man, I have a lot brownies to bake tonight! :)

Answer 1

The formula for circumference is C = πd = 2πr. What you're remembering is the formula for area, A = πr².

If the circumference is 50, solve 50 = πd for diameter by dividing by π, d = 50/π = 15.915.

If the AREA is 50, solve 50 = πr² for r by dividing by π and then taking the square root:

50 = πr²
50/π = r²
√(50/π) = r = 3.99

You can find the circumcenter of a triangle (center of the circle the triangle's 3 corners would sit on) by constructing the perpendicular bisectors of 2 of the sides. The center is the point of intersection. See link.

Answer 2

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RE:
How to figure out the diameter of a circle by its circumference?
The formula I have to go by with the ABYC is normally 3.142xR² to know the circle's circumference, but what I am having a hard time with is this, doing it in reverse. Ok, here's where I come into the Land of Confusion (a great Phil Collins/Genesis song btw). Let's say to find the...

Answer 3

(EDIT: No, you're still not making any sense.)

First of all, you're confusing the formulas for a circle's area and a cirlce's circumference. The AREA of a circle of radius r is
A = π r^2.

The CIRCUMFERENCE for a circle of radius r is
C = 2 π r

Remember that the diameter is just twice the radius, so you could say C = π d too. Dividing both sides by π gives C/π = d. So if you have the circumference of a circle and want to know the diameter, just divide by π (using an approximation like 3.14, or perhaps the better one you give of 3.142).

Answer 4

How to figure out the diameter of a circle by its circumference?

Circumference = 2 pi r
r = Circumference/2 pi
diameter = 2r
diameter = Circumference/pi

Answer 5

It looks like you're using the formula for area, rather than circumference.

A = 3.14r^2
C = 3.14d, or 2(3.14)r

I think that should help.
Just divide the circumference by pi, and you have your diameter.

Answer 6

There are 2 key formulas for a circle:
Area = 3.14 R^2
Circumference =3.14D. or 3.14(2R)

In your work, 16(3.142)=50.27 is the AREA of the circle, not its circumference
To get the correct circumference, C = 2pi)R
=2(3.142)(4)
=25.136

I hope this helps

Answer 7

Circumference = diameter x pi; diameter = circumference / pi QED. Your formula is for area, not circumference.

Answer 8

If u r trying to calculate a circunference, maybe the problem is u r using the formula for calculating an area. U should try using 3.14 x diameter
G. luck

Answer 9

Why are you dividing 16 by pi? You should be dividing 50.27 by pi to get back to 16, then take the square root of 16 to get back to 4.

Answer 10

To get to Circum. you need


C=pi(d)

Just plug in the values and just finish the simple equation...