Quilt Math Calculator: Easy Fractions for Perfect Patchwork

Traditional pieced quilts are all based on math. There are exceptions to that statement: paper foundation methods, appliqué and crazy quilts can free you from the mathematical restraints. However, most traditionally pieced quilts fit into grids, and the math is needed to ensure all the pieces fit together properly. Adding whole numbers is an easy quilt math calculation, but there are times with those pesky fractions get involved! Adding and subtraction fractions can strike fear in our hearts. How do you figure yardage for quilt back? How do you calculate yardage for quilt borders? Have no fear! We have some basics to make you a quilt math specialist in no time.

Fractions on Quilting Rulers

Quilt designers tend to use a limited range of fractions when it comes to patchwork quilting. The fractions 1/4 inch, 1/2 inch and 3/4 inch being the most common. These are all measurements found on the acrylic quilting rulers we use today. They are represented in a variety of ways depending on the brand of ruler, but in the photo here the whole number is the heavy black line, the 1/2 inch is depicted as the broken vertical and horizontal lines. The 1/4 inch is the longer dash and the 1/8 inch is the shortest dash. Smaller fractions become very difficult to decipher and are not generally used to eliminate frustration on the part of everyone.

The 1/8 Inch Mystery

The 1/8 inch measurement comes into play when we create triangles quilt blocks. While terms like hypotenuse, right triangles, and Pythagorean Theorem might sound intimidating, understanding these concepts can significantly improve your quilting accuracy. (I never was a huge fan of math class either!)

Just know that 1/8 inch, 3/8 inch, 5/8 inch, and 7/8 inch fractions are essential for precise quilting. When constructing triangle quilt blocks, you’ll need that tiny bit of extra fabric to ensure accurate unit size. Most modern quilters opt to cut slightly larger units and trim them to the desired size, rather than working with the more challenging 1/8 inch fraction.

NOTE: “finished” refers to the size of the unit once it is pieced into the patchwork

Example: You want to create 4 inch finished half square triangle.

You are using the Half Square Two at a Time Method – what size squares would you need to cut in order to create these units?

The formula is finished size + 7/8 the size needed

4 inch + 7/8 inch = 4-7/8 inch squares

This would lead you to using the 7/8 inch mark on your rulers. Many quilters today may use a default method of rounding the fraction up to the whole number and cutting 5 inch squares and then trimming all the units to the required 4-1/2 inch unit size – this also eliminates the any inaccuracy or distortion. Others dislike the trimming step and will continue to use the fraction measurement in their unit creation.

Quilt Cutting Calculation

Another area where fractions come into play is the calculation of yardage and determining the sub-cutting of strips in the construction phase of patchwork. Here is the rationale. When determining yardage for a quilt whether online or in person at your local quilt shop you will be asked for the desired yardage off the bolt. (A conversion chart is available at the end of this post for your quick reference.)

A pattern may denote the following:

• • Fabric A – 1-3/8 yds or 1.375
• • Fabric B – 2-1/2 yds or 2.5
• • Border – 1-5/8 yds or 1.625
• • Binding – 1/2 yd or .5

You decide to use the same fabric for the border and binding. How to figure yardage for this quilt? Okay, maybe you are good at fractions and it’s a no brainer. If you decide to use a calculator – you will need to use decimals and so will the quilt shop.

Border 1.625 + Binding .5 = 2.125 or 2-1/8 yds for border and binding

Also, when determining the number of pieces you can cut from a width of fabric strip the decimal conversion is helpful. For example: You have a strip (approximately 42 inch usable width) and are cutting 3-1/2 inch squares.

42 inch divided by 3.5 = 12 squares

You will be able to cut 12 squares from each strip. If you need a total of 36 squares you will then need to cut 3 strips to cut the required pieces. This will enable you to efficiently cut the required number of units without wasting yardage. This is also helpful when designing a pattern and double checking the yardage requirements.

A Fraction to Decimal Conversion Chart

Here is the Fraction to Decimal Conversion Chart for quick reference that many quilters will commit to memory. Another tool to add to your quilt toolbox for your adventure in patchwork!

Happy Quilting,
Colleen Tauke Managing Editor NQC

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