Sewing Machine 101: What is Tension on a Sewing Machine?
Cover Image: Sewing Machine 101: What is Tension on a Sewing Machine?

Have you ever excitedly undertaken a new sewing project only to realize that your thread is bunching underneath your fabric? Can’t figure out why your stitches are puckering? You’ve probably heard the term “tension” before, and may even recognize that it is the key to producing strong, even stitches, but even advanced sewers have difficulty answering the question: what is tension on a sewing machine?

To cut to the chase, it is one of the most important factors affecting stitch quality and understanding how to adjust tension is crucial in order to achieve stable, attractive seams. Knowing the parts can make a difference.

What is Tension?

Close-up shot of needle and thread of sewing machine

All sewing machines utilize what is known as a “lock stitch,” meaning the top and bottom thread interlock with one another to form a single stitch in a piece of fabric. Sewing machines are factory pre-set to impart the same amount of tautness on both so that the needle thread and bobbin thread pull on the fabric equally. However, it is occasionally necessary to adjust these settings because external factors can alter the amount of stress in each thread.

When Do I Need to Adjust Tension?

There are a few common circumstances in which you will need to adjust the tension on your sewing machine:

  1. You are using a different type of thread on the top and bottom of your piece. For example, you may be working on a quilt with a decorative top thread that is not as strong as standard 50 wt. thread.
  2. You are using batting or stabilizers, which place additional stress on the top thread
  3. You are working with a densely woven fabric like denim, which creates friction and adds the right amount of strain to the top thread

closeup of woman with a sewing machine and blue jeans hem

If you have threaded the machine properly and are using the correct needle size but notice that the threads are not lying flat, are looping against the fabric, or can be easily pulled out of the seam, then you will also need to adjust the tension.

How Do I Know Which Thread to Adjust?

Perhaps the simplest way to determine which thread is responsible for the problem is by making a test swatch. If you are working with dense fabric or cotton batting, start by threading the machine with different colors of thread in the needle and bobbin. If you are already using different threads, you can simply thread the machine as you normally would. Sew a few inches into your test fabric and then take a look at both sides of the piece. If the bobbin thread is visible at the top of the piece, then there is too much tension in the needle thread. If the needle thread is visible on the bottom side of the fabric, then there is not enough tension in the top thread.

The Adjustment Process

Closeup women sewing on a machine

There are two main ways to adjust the tension of your machine. The first is by adjusting the needle tension. The second is by learning how to adjust the tension on the sewing machine bobbin. You should always begin by adjusting the needle tension, regardless of whether you must make it tighter or looser, simply because it is the easiest to do. Using trial and error, turn the tension dial on your machine up or down one unit and continue making test swatches until the stitches are balanced and lie smoothly along both sides of the fabric.

When you find the proper tension setting for the particular fabric or thread combination you are using, make a note of it! If you keep track of the tension settings for all of your projects, the next time you use something similar, you won’t need to waste any time making test swatches.

How to Adjust the Tension on Sewing Machine Bobbin

Woman opens the sewing machine and cleans it inside

While most tension problems can be quickly solved by adjusting the top thread, you may find the only way to achieve perfectly even stitches is by changing the tension of your bobbin. Remember that because there are two threads forming the lock stitch, you can also increase or decrease the tension of the lower thread to match that of the needle.

Most machines contain a small screw on the inside surface of the bobbin case. Using the screwdriver from your machine tool kit, you can turn the screw a quarter-turn counter-clockwise to loosen the bobbin tension, or a quarter-turn clockwise to tighten it.

One important thing to remember is that anytime you make adjustments to the bobbin tensity, you have to be sure to reset it to its original setting when you are finished with your product. Alternatively, you can invest in extra bobbin cases for very low cost, allowing you to simply change out the bobbin case when working on special projects.

The Bottom Line

Adjusting the tension on your sewing machine is a straight-forward and intuitive process that will help you to maintain a stable, balanced, attractive seams regardless of the type of fabric or thread you plan on using. You can even purchase inexpensive bobbin cases to make the process of adjusting tension even simpler.

a close up shot of a person's hand holding the bobbin case

If you have any questions, feel free to leave a comment. And remember, getting perfect stitches is an easy 4 step process:

  1. Perform a test swatch before beginning a new project, and determine whether the top thread or bottom thread is too tight
  2. Turn the tension dial one unit up or down and continuing performing test swatches until your stitches lie flat
  3. Adjust your bobbin tension if the tension dial doesn’t solve your problem
  4. Get sewing!