On-Point Quilt Math Made Easy

On-point units within blocks need to measure perfectly across their diagonals in order to fit correctly with straight-set corners. The quilt math is done for you when you use the Omnigrip On Point Ruler™ by Omnigrid, designed by Donna Lynn Thomas. In this guest article, Donna Lynn explains why she designed the On Point Ruler and how to use it.  


The Making of the On Point Ruler

The rotary cutter took the quilting world by storm in the late 1970s and ’80s. It was an exciting time to be a quilter as ideas and new techniques grew exponentially and there was always something new on the horizon. In the process of moving almost exclusively from handmade templates to rotary cutting, we jettisoned an entire genre of patchwork blocks—those with patchwork units set on-point inside a block or border.

Rainbow Radiance, 55˝ x 65˝, 2013, by Donna Lynn Thomas

Rainbow Radiance, 55˝ x 65˝, 2013, by Donna Lynn Thomas

As you look at the blocks in Rainbow Radiance, you can see all kinds of patchwork units set on point: the block center, the rectangles framing it, the sawtooth bands around it. But notice the corner squares—they are set straight with the edges parallel to the outer edges of the block. The on-point units need to measure precisely across their diagonals in order to fit perfectly against the straight-set corners.

Messy Quilt Math

If you browse through Barbara Brackman’s Encyclopedia of Pieced Quilt Patterns, you will find many beautiful and interesting blocks with on-point units. The sad thing is we don’t make them very much anymore. The reason? Regular rotary rulers measure units from side to side. Since the on-point units need to measure evenly across their diagonals, side-to-side measurements are not what we want. You can use some pretty messy quilt math to calculate cut sizes that come close to what we need, but most quilters cringe at the thought of the math involved.

My Twisted Quilters DNA, 47˝ x 62˝, 2013, by Donna Lynn Thomas

My Twisted Quilters DNA, 47˝ x 62˝, 2013, by Donna Lynn Thomas

I designed the On Point Ruler to solve this problem. It allows us to cut squares, rectangles, triangles, and half-square triangle units that measure perfectly on their diagonals instead of from side to side.

Party Poppers, 56˝ x 65˝, 2013, by Donna Lynn Thomas

Party Poppers, 56˝ x 65˝, 2013, by Donna Lynn Thomas

How to use the On Point Ruler

Use the On Point Ruler just as you would a regular ruler to cut strips, squares, rectangles, and triangles. The process of cutting is no different than what you are used to—the only thing different is the dimension of the unit you are measuring. Keep in mind that the On Point Ruler grids don’t measure 1˝ from side to side but rather 1˝ across the diagonal from corner to corner. To help you remember the difference, the cutting dimensions using the On Point Ruler are called ‘cutting marks.’

The On Point Ruler by Omnigrid

The On Point Ruler by Omnigrid

The ½˝ margin around the edge of the ruler is for the ¼˝ seam allowances on the two sides of your patches. This means you don’t need to add anything for seam allowances. All you need to know is what you want the diagonal measurement to be. For squares, strips, and rectangles it’s that simple: Diagonal Measurement = Cutting Mark.

Using the Ruler for Half-Square Triangle Blocks

For half-square triangle units add ½˝ to the diagonal measurement and cut squares that size. Cut them once diagonally to make half-square triangles. If you prefer to make oversize half-square triangle units, add 1˝ to the desired diagonal size, cut squares this size, cut diagonally, sew and trim back to size. This is the method I prefer.

You can view a demonstration of the On Point Ruler in the “On Point Quilted Placemats and Table Runner” streaming video available on The Quilting Company website; a preview video is below.


From the Vault: This feature by Donna Lynn Thomas was originally published in the December/January 2014 issue of Quilters Newsletter.

Rainbow Radiance block

Rainbow Radiance block

Take the trouble out of on-point piecing with these great resources!


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