To find the radius of a circle you need the following formula:

Circumference ÷ 2π

Let’s use a 110cm waist as an example,

Circumference = 110cm 2π is: 2×3.14 which is = 6.28

110cm ÷ 6.28 = 17.52cm

Lets do the same but in inches

• Circumference = 44 inches
• 2π is: 2×3.14 which is = 6.28
• 44 ÷ 6.28 = 7 inches

### Waist for a full circle skirt = C (+SA) ÷ 2π

If I were making a full circle skirt I would use the above formula (as shown in the example further up) but would also need to factor in the number of seams and the seam allowance amount (I usually use 1cm per seam, and I have two seams with one on each side of the skirt, which would be a total of 4 raw edges) before running the formula:

• 110cm (C) + 4cm (4xSA) = 114cm
• 2π = 2×3.14 = 6.28
• 114 ÷ 6.28 = 18.15cm

You’ll also want to subtract an amount for the waistline seam allowance, for finishing the waist with a facing, or a waistband of some sort:

• 18.15cm – 1cm = 17.15cm

This is how it looks for me in inches:

• 44 inches (C) + 2 inches (4xSA) = 46 inches
• 2π = 2×3.14 = 6.28
• 46 ÷ 6.28 = 7.3 inches (r)
• 7.3 – 0.5 (waist SA) = 6.8 inches

## Half Circle Skirt Formula (180º)

The half circle skirt uses our waist circumference in a semi-circle rather than the full circle we used in the formula above! This means we need to double our waist measurement to find the right radius! Here’s the basic formula for a half circle skirt:

### Waist for a half circle skirt = 2C (+SA) ÷ 2π

Lets use me as an example again:

• C = 110cm
• 2C = 2x 110cm = 220cm
• Number of seams = 2
• Amount for seam allowance = 1cm
• Total raw seam edges = 4
• SA = 4x 1cm = 4cm
• 220cm + 4cm = 224cm
• 2π = 2x 3.14 = 6.28
• R = 224 ÷ 6.28 = 35.67cm
• Finally, subtract the waistband seam allowance amount (1cm) = 34.67cm

## Three Quarter Circle Skirt Formula (270º)

Your formula for a three quarter circle skirt is:

### Waist for a three quarter circle skirt = 1.3C (+SA) ÷ 2π

Lets use the same measurements as before so you see the continuity in the changes of numbers!

• C = 110cm
• 1.3C = 1.3 x 110cm = 143cm
• Number of seams = 2
• Amount for seam allowance = 1cm
• Total raw seam edges = 4
• SA = 4x 1cm = 4cm
• 143cm + 4cm = 147cm
• 2π = 2x 3.14 = 6.28
• R = 147 ÷ 6.28 = 23.4cm
• Finally, subtract the waistband seam allowance amount (1cm) = 22.4cm

## One Third Circle Skirt Formula (120º)

Your formula for a one third circle skirt is:

### Waist for a one third circle skirt = 3C (+SA) ÷ 2π

I feel like this is all probably making sense for you now?

• C = 110cm
• 3C = 3x 110cm = 330cm
• Number of seams = 2
• Amount for seam allowance = 1cm
• Total raw seam edges = 4
• SA = 4x 1cm = 4cm
• 330cm + 4cm = 334cm
• 2π = 2x 3.14 = 6.28
• R = 334 ÷ 6.28 = 53.18cm
• Finally, subtract the waistband seam allowance amount (1cm) = 52.18cm

## Quarter Circle Skirt Formula (90º)

Finally, your formula for a one quarter circle skirt is:

### Waist for a quarter circle skirt = 4C (+SA) ÷ 2π

• C = 110cm
• 4C = 4x 110cm = 440cm
• Number of seams = 2
• Amount for seam allowance = 1cm
• Total raw seam edges = 4
• SA = 4x 1cm = 4cm
• 440cm + 4cm = 444cm
• 2π = 2x 3.14 = 6.28
• R = 444 ÷ 6.28 = 70.70cm
• Finally, subtract the waistband seam allowance amount (1cm) = 69.70cm