How to Calculate Fabric Stretch Percentage

If you’ve ever looked at a pattern that says “Pattern designed for knit fabrics with at least 60% horizontal and 40% vertical stretch” and wondered “How do I know if my fabric is stretchy enough for my project and does it even matter?” you’re not alone.

Understanding stretch percentage is one of the most important skills when sewing with knit fabrics, because it can significantly change the fit of your garment. Luckily, it’s incredibly simple to check once you know how to measure it!

Today we’re sharing a free printable stretch percentage ruler to make it easy to test your fabrics at home, as well as a way to measure it with just a tape measure or ruler if you don’t have a printer. This is especially helpful when you’re working with any of the knit fabrics from our preorders, as fabrics like French Terry will have a very different stretch percentage to say swim, bamboo lycra or modal.

Why Stretch Percentage Matters

Not all knit fabrics stretch the same, even if they have the same fibre content. Some projects (like swimwear) require high stretch both horizontally and vertically (otherwise you’ll end up with a giant wedgie or a much more high cut pair of togs than you were expecting!), while others work beautifully with stable knits, like French Terry for light jumpers.

Knowing your stretch percentage helps you:

• Choose the right pattern size

• Avoid neckbands that don’t fit

• Prevent baggy fits or too tight ones

• Sew with more confidence

• Make more informed choices when ordering your fabric

For a 2 way stretch fabric like cotton interlock or cotton jersey, which only stretches horizontally from selvedge to selvedge, you will be able to measure horizontal stretch only. For a 4 way stretch fabric, which generally has a lycra content, you can measure the stretch factor both horizontally and vertically as it stretches both ways (but still pay attention to which way the pattern states to cut the fabric, as cutting it across the grain for more stretch can affect the print direction, as well as have unintended fit consequences.)

How to Calculate Stretch Percentage with a ruler (Step-by-Step)

You only need:

• Your fabric

• A ruler or measuring tape

• A flat surface

  
  

Step 1: Measure 10cm (or 4 inches)

Lay your fabric flat (without pulling).Measure exactly 10cm across the direction of stretch without including the selvedge edge. 

Step 2: Gently Stretch the Fabric

Hold the fabric at each end of the 10cm section and stretch it until you feel resistance, but don’t overstretch. You want to make sure it can also spring back to its original length (This shows you haven’t overstretched it or the fabric has good recovery.)

Step 3: Measure the Stretched Length

Let’s say your 10cm stretches to 15cm.

That means it stretched an extra 5cm.

Step 4: Calculate the Percentage

Extra stretch divided by original length × 100 = Stretch Percentage

In this example:

5 / 10 × 100 = 50% stretch

So your fabric has 50% stretch.

What Do Stretch Percentages Mean?

Here’s a simple guide:

• 20–30% stretch → Stable knits (great for structured projects)

• 40–60% stretch → Great for garments like tshirts, light jerseys, dresses etc.

• 70%+ stretch → High stretch. Good for activewear, swimwear etc. Can be used for garments designed for the 40-60% category, but you’ll need to size down, or expect a looser, more ‘soft’ fit due to the fact that the fabric can stretch more. 

  
  

When you’re shopping for knit fabrics, knowing this difference makes preorder decisions much easier. If you’re browsing bases in our preorder, we’ve done the work for you for our specific fabrics! (Or if they're still blank because I had to press publish before I was actually ready, we will have done the work at some point soon!)

Cotton Lycra 220:

Cotton Lycra 270:

French Terry:

Rib Knit:

Modal:

Swim:

Bamboo Lycra: 

Cotton Interlock: 

If you have fabric at home already, you can use either the above method, or our easy ruler download that requires no maths to compare stretch across your fabrics!

Free Printable Stretch Percentage Ruler

To make things even easier, we’ve created a printable ruler with built-in stretch percentage markings.

It allows you to:

• Mark 10cm instantly

• See percentage indicators visually without needing to do any maths

• Keep it near your sewing machine (you could even stick it to your cutting table!)

• Quickly test new fabrics as they arrive

  
  

👉 Download the free printable stretch ruler here.

Print at 100% scale for accurate results.

Common Mistakes to Avoid when Measuring

Even experienced sewists sometimes:

• Pull too aggressively

• Ignore recovery (how well it snaps back)

Stretch percentage matters, but recovery matters too. A good fabric should have good recovery, meaning it returns to its original size without losing shape. If your fabric doesn’t return to the same 10cm after being stretched, the recovery isn’t good enough for the amount you stretched it. 

Sewing with 3am Threads Knits

If you’re unsure whether a base will work for your project, you can:

• Check our base info page for stretch percentages

• Use this stretch guide

• Or reach out, we’re always happy to help

Holly and Jen

Related Resources

You might also find these helpful:

• Cotton Lycra vs Rib vs French Terry: What’s the Difference?