Important: The calculator is no substitute for trying on the garment

The calculator does not determine the basic length of your elastic. You determine this length beforehand by trying it on.

Enter the planned finished length of the elastic into the calculator – in other words, the length that fits comfortably before adding the rows of lengthwise topstitching. Do not include any seam allowances or the overlap needed to join the ends of the elastic.

The calculator simply checks whether the lengthwise topstitching requires the elastic to be shortened further.

Why does the elastic get wider after topstitching?

When topstitching the waistband lengthwise, the fabric is stretched so that it lies smoothly over the elastic. The stitching secures the fabric to the elastic. Depending on the fabric, the elastic and your sewing method, the waistband may relax differently afterwards, which can affect the finished waistband length.

This can happen especially in the following situations:

• soft elastic band
• sturdy or thick fabric
• several longitudinal stitching lines
• short stitch length
• strong presser foot pressure
• significant stretching during sewing

It’s impossible to calculate exactly how much the waistband will stretch. That’s why it’s a good idea to make a test piece using the exact same fabric and elastic as you’ll be using.

How the test works

1. Enter required measurements

First, enter the finished circumference of the outer waistband and the planned finished length of the elastic.

If your waistband isn’t topstitched evenly along its entire length, please also indicate whether there are smooth areas or areas that haven’t been topstitched, and how long these areas are in total.

This information is important because it affects how much the elastic is stretched in the sections that will later be topstitched.

2. Prepare the test sample

The calculator suggests a length for the test sample. You can accept this length or adjust it.

A longer sample generally produces more accurate results. If you have enough fabric available, I recommend a length of at least 20 cm.

The calculator then calculates the required dimensions for the test sample:

• cutting length for the fabric strip
• required length of elastic
• the distance between the two securing seams

Cut the fabric and elastic according to these instructions.

3. Sew a sample

Iron the fabric strip in half lengthwise, just as you will do later for the waistband.

Place the elastic band along the inside of one of the short edges and sew it in place, leaving a 1 cm seam allowance from the edge. Then close the open long edge of the fabric strip.

Now measure out the calculated distance from the first securing seam. Stretch the elastic  to this point and stitch it in place.

The distance between the two securing seams is your starting measurement before topstitching the sample lengthwise.

4. Topstitch the sample and measure

Stitch the test piece in the same way you’ll want to stitch the finished waistband later:

• same stitch length
• the same number of stitching lines
• the same spacing between the rows
• as even a stretch as possible while sewing

Then measure the distance between the two securing seams again and enter the result into the calculator.

If the measurement hasn’t changed, no additional shortening is required. If it has increased, the calculator will calculate the recommended amount by which the elastic should be shortened.

Here’s how to apply the recommended shortening

Subtract the recommended shortening from your planned finished elastic length.

Example:

Planned finished elastic length: 75 cm
Recommended additional shortening: 2 cm

New finished elastic length:

75 cm – 2 cm = 73 cm

When you cut the elastic to size, you’ll need to add back the overlap or seam allowance needed to close it.

With a 2 cm overlap, the cutting length would be:

73 cm + 2 cm = 75 cm

This helps distinguish between the finished elastic length and the cutting length.