How to Adjust the Thread Tension on Your Sewing Machine

Your seam looks tidy on top, but there are little loops on the underside? It's a classic at the sewing table, and it's usually quicker to fix than you might think. When you adjust the thread tension on your sewing machine, it's worth not reaching for the tension dial straight away. Often the real culprit is a wrongly threaded top thread, an unsuitable needle or fluff around the hook.

Thread tension determines where the top and bobbin threads meet in the fabric. Ideally, this interlock sits right between the two layers of fabric. The seam then looks even on both sides, is hard-wearing and doesn't add unnecessary bulk to garments. Especially with jersey, fine viscose fabrics or several layers of canvas, the right setting makes a noticeable difference.

What thread tension does on your sewing machine

Every domestic sewing machine works with two threads: the top thread from the reel and the bobbin thread from the bobbin. The top thread tension is usually set with a dial or on the display. On many machines, a middle setting, often around 4, is a good starting point.

The bobbin thread runs through the bobbin case or the built-in hook. Its tension is controlled by a small spring. Unlike the top thread tension, it should rarely need changing in everyday sewing. If you adjust both tensions at the same time, you make troubleshooting unnecessarily complicated.

It's also worth remembering that a number on the tension dial isn't a mark of quality. A setting of 4 suits many projects, but not necessarily every thread, needle and fabric combination. A thin cotton fabric behaves differently from bulky fleece or stretchy jersey.

Adjusting thread tension: check first, then turn the dial

Before you change the tension, it's best to sew a short test seam on the same fabric as your project. Use the same number of layers, the same thread and ideally the same needle too. For a lined pouch or a softshell jacket, a single piece of fabric isn't enough as a test, because several layers can affect how the thread runs.

Then take a look at the top and underside of the seam. If the threads sit neatly in the fabric with no visible loops, the setting is right. Only once you can see a clear problem should you start correcting it step by step, always in small increments and with a fresh test seam each time.

Loops on the underside of the fabric

If loops of top thread form on the underside, the top thread tension is often too loose. Start by turning the tension dial up by half a step to one step and test again.

Before you do that, though, it's best to rethread the top thread completely. The most common reason for loops underneath isn't actually the tension setting, but the top thread not sitting properly between the tension discs. Raise the presser foot to do this: the tension discs then open, and the thread can settle into place correctly. Only then lower the presser foot again and start sewing.

Loops on the top of the fabric

If there are loops on the top or the bobbin thread is clearly visible, the top thread tension is often too tight. Reduce it carefully by half a step to one step.

Here too, it pays to check the basics. Make sure the bobbin has been inserted correctly and the thread runs neatly through the guide provided. On some machines, the bobbin has to be inserted clockwise or anticlockwise. The little marking on the machine or in the manual will tell you reliably.

The seam puckers

If the fabric puckers or ripples along the seam, the thread tension may be too tight. This happens especially easily with fine cotton fabrics, lightweight viscose or thin jersey. Reduce the top thread tension slightly and check the stitch length at the same time.

A very short stitch length can also make fabric pucker, even if the tension is right. For many woven fabrics, around 2.5 millimetres is a good starting value. For jersey, a narrow zigzag stitch or a stretch stitch may be the better choice, because the seam needs to stretch as you wear the garment.

Causes that are often overlooked

If your seam suddenly becomes uneven, thread tension isn't necessarily to blame. Especially if the machine was sewing reliably before, it's worth quickly checking the following points:

• Needle: A bent, blunt or unsuitable needle can skip stitches and put unwanted strain on the thread. For jersey, a jersey or stretch needle is usually the right choice, for denim a jeans needle, and for fine woven fabric a universal needle in a suitable size.
• Thread: Very old, fluffy or unevenly wound thread can run jerkily through the thread guides. For most sewing projects, good-quality all-purpose thread is a reliable choice.
• Dirt and fluff: Lint under the needle plate, in the hook or between the tension discs changes the way the thread runs. Brushing out your machine regularly after sewing fluffy fabrics such as fleece or teddy plush saves a lot of troubleshooting later on.
• Wrong bobbin: Not every bobbin fits every machine, even if it looks similar at first glance. Wherever possible, use the bobbins designed for your model.

With thick projects such as bags, fabric storage baskets or several layers of canvas, the presser foot can also tilt. The fabric then isn't fed evenly, and the seam can quickly look wobbly. A hump jumper or a folded piece of fabric behind the presser foot helps to level out the height at the start of the seam.

Only change the bobbin tension for a good reason

There is often a tiny screw on the bobbin case. It regulates the bobbin tension, and it's not for spur-of-the-moment tweaks between two seams. Only turn it if you're sure that the top thread, threading path, needle and thread have already been checked and the problem persists.

If an adjustment really is necessary, mark the starting position and move the screw only very slightly. An eighth of a turn may be enough. Make a note of the change and sew a test seam. If you own several machines or often work with different threads, it's a good idea to use a separate bobbin case for speciality thread, provided your machine model allows it.

On machines with a horizontal drop-in bobbin, the bobbin tension is often harder to get to. Here, a noticeable and persistent fault is better left to a sewing machine specialist. The same applies if the thread keeps breaking despite correct threading, the machine makes unusual noises or the hook area no longer moves smoothly.

Choosing the right tension for your fabric and project

There's no single setting that suits every fabric. For a woven cotton for cushions, patchwork or aprons, a medium top thread tension is usually right. Thin fabrics, on the other hand, often need a little less pull so the seam doesn't pucker.

Jersey and ribbing have their own demands: the fabric stretches, and the seam should too. Work with a suitable jersey or stretch needle, a stretch stitch or a narrow zigzag stitch. The thread tension shouldn't be so tight that the fabric gets stretched as you sew. Afterwards, test the seam by gently pulling the piece of fabric apart. If the thread snaps, you usually need a stretchier stitch setting, not simply less tension.

With thick fabrics such as softshell, canvas or bag fabrics, the tension doesn't automatically need to be set higher. What matters is whether the threads still meet in the middle of the fabric. A heavier needle, a slightly longer stitch and even feeding are often more helpful than a tightly pulled top thread.

The same goes for topstitching with thicker thread: test first. Wherever possible, use matching thread weights for the top and bobbin threads, or adjust your settings deliberately. A thick decorative thread on top and fine thread underneath can work, but it calls for a careful test seam.

A small test seam saves big frustrations

Before starting a new project, have two or three fabric scraps ready. Sew forwards, backwards and, if your project calls for it, over several layers. Check both sides of the seam in good light and pull it apart gently. That way you'll know before cutting out whether thread, needle, stitch and tension are working well together.

For more demanding projects, it's well worth keeping your successful test pieces. When you later sew the same raincoat fabric, jersey or home décor fabric again, you'll have a handy reference for needle size and settings. And if questions remain despite all your checks, personal advice, for example from Die Stoffkönigin, will often get you further faster than puzzling over the tension dial for ages.

Your sewing machine doesn't need to be perfectly preset for every fabric. Give it clean threading, the right needle and a little time for a test seam, and your fabric, thread and ideas will once again turn into seams you can rely on.