The circle is, in my humble opinion, the Queen of the geometric shapes. Don't become me wrong; I similar all those squares, rectangles, triangles, octagons, and whatnot; but the circle is the coolest of the agglomeration: smooth and pretty and incessantly useful. Yet, trying to describe a perfect circle without a pattern is a claiming, and figuring out the proper size of an opening into which a circle tin exist inserted requires working with Pi (or π), which is not the succulent kind you tin eat with a bit of ice cream. Nosotros're here today to assistance y'all with the steps yous've forgotten since high school geometry class (or maybe never learned because you were as well busy passing notes with Susan Ellery!). We'll show yous the parts of a circle, how broad to cut fabric to fit a circle, and how to draw a circle without a blueprint. We've likewise included a handy conversion from decimals to inches, which is necessary when working with Pi.

The parts of a circumvolve

Permit's start with remembering what all the parts of a circumvolve are called and how Pi (π) fits into the mix.

Radius: the distance from the center of the circumvolve to the outside edge

Diameter: the distance across a circle through its center signal

Circumference: the distance around the outer edge of a circle

π or Pi: the proper name given to the ratio of a circle's circumference to its diameter, expressed as the decimal iii.xiv

How wide to cutting fabric to fit a circle

If you know the diameter of your circle, you tin use a standard formula to figure out the width of the fabric cut needed to make a tube. That width is the circumference of the circle that will be inserted into the tube (we take a great step-by-pace tutorial on how to insert a circle into a tube).

The formula: 3.14 (π) x diameter = circumference

Example: You want a finished 12″ diameter base (a 12″ diameter circumvolve) in a duffle bag.

3.14 x 12 inches = 37.68 inches

(This works with the metric system as well: iii.xiv x 30 cm = 94.2 cm)

An of import footstep many people miss at this bespeak is forgetting to add actress (to both pieces) for the seam assart. If you lot utilise a standard ½" seam allowance, yous need to add ane″ to the diameter of your circle ( the diameter increases by double the seam allowance)and 1″ to the width of your fabric (½" for both sides of the seam allowance). In our instance, that means:

The circle should start every bit thirteen″ in diameter.

The fabric should exist 38.68″ in width

The summit of your fabric cut is variable and dependent on your project. For example, a tall duffle purse might exist 30″ in elevation whereas a shorter bucket might exist only x″.

Converting a Decimal to a U.s.a. Ruler Measurement

If y'all are using Pi, think information technology always returns a decimal number. If you already deal with the metric organization, you rock –  no conversion necessary.

For those of us in the world of inches, you need to notice a yardage conversion.

In our example we have 38.68 inches. Harumph! The table beneath will give you a shut-enough ruler friction match.

The decimal .68 is closest to .63 or ⅝". We can use 38⅝" equally the width of the fabric piece yous are cutting for your tube.

How to Describe a Circle

If yous have a supply of large compasses, you're in luck, and can easily draw yourself all sizes of circles. But you tin can also easily make your own compass to draw a circumvolve.

To start, you lot demand to know how big you want your circumvolve (the bore). For our ongoing instance, nosotros want a 13″ diameter circumvolve

To draw a circle y'all need to know its radius. Equally you learned above in the first department, the radius is one half of the diameter. In our instance, one half of 13″ is 6½".

The total circle method

  1. Apply a sheet of lightweight paper (graph or blueprint paper works well) that is at least ane″ larger all around than the circle you want to draw.
  2. Cutting a piece of string about 4″ – 5″ longer than your radius. We used a ten″ length of string.
  3. Tie ane end of the string to a brusk pencil.
  4. Place the bespeak of the pencil toward the outer edge of the newspaper with enough room from the edge to make a full sweep.
  5. Measure out from where the point of the pencil touches the newspaper backwards by the length of the radius (in this case 6½").
  6. Pin directly through the string into the paper at that verbal point.
  7. Keeping the string taut, draw a perfect circle using your homemade compass.

The folded quarters method

  1. Again, commencement with a square of lightweight paper at least 1″ larger than the circle you want to draw.
  2. Fold the paper into quarters. Make certain your original foursquare is fifty-fifty and true! Position the paper with its folded edges along the bottom and left side and the open edges along the top and right side.
  3. Place a run across-through ruler at the verbal center of the bottom left corner of your folded foursquare. Swing the ruler from the top to the bottom of the foursquare, like a pendulum or compass, measuring and marking a dot at the 6½" indicate in three to four spots. Y'all are creating a semi-circle arc. Make sure the terminate of the ruler at the corner point doesn't shift position.
  4. Cutting forth the arc through all the layers and unfold the finished 13″ circle. You can now use this paper pattern to cut your fabric circle.

With your spiffy new circumvolve, you can now sew the side seam in the main cloth cut. Then pin the base to the resulting tube and run up the tube to the circle using a ½" seam allowance. The result is a 12″ diameter finished base of operations.

As mentioned above, for more on this technique, see our tutorial: How to Insert a Flat Circle Into a Tube.