How to fix sewing machine tension problems

Tension is the thing beginners fear and intermediate sewists quietly never fully understand. The dial sits there with numbers from 0 to 9, and nobody explains what they mean, so people poke at it randomly and hope. Let’s fix that. Once you understand what tension actually is, diagnosing a bad stitch takes about ten seconds.

What tension actually means

A machine stitch is made of two threads: the top thread coming down through the needle, and the bobbin thread coming up from below. A good stitch happens when these two threads meet and lock in the middle of the fabric, where you can’t see the join. “Tension” is just how hard each thread is being held back as it feeds. If one thread is held tighter than the other, it pulls the locking point towards its own side, and that’s a visible fault.

The numbered dial on the front of your machine controls the top thread tension. Higher number = tighter top thread. Most fabrics are happy somewhere around 4. That’s your home base; start there.

How to read a bad stitch

This is the whole skill. Sew a test seam on a doubled scrap, using different colours top and bobbin so you can see which thread is misbehaving, and look at both sides:

• Loops on the underside (bottom) of the fabric: the top thread is too loose (or, more often, not seated in the tension discs at all). The bobbin thread is dragging the loose top thread through to the back. Increase top tension, or rethread the top with the foot up.
• Loops on the top of the fabric: the top tension is too tight, or the bobbin thread is too loose, so the bobbin thread is being pulled up. Decrease top tension first.
• Top thread lies flat but you can see bobbin thread dots on top: top tension slightly too high. Nudge it down a number.
• Bobbin thread lies flat but top thread shows through underneath: top tension slightly too low. Nudge it up a number.
• Puckered seam: usually overall tension too tight, or stitch length too short for the fabric.

The rule of thumb: loops appear on the opposite side to the problem. Loops on the bottom = top thread issue. Loops on top = bobbin (or over-tight top) issue.

Before you touch the dial, check these first

Nine times out of ten, “bad tension” isn’t the tension setting at all. It’s one of these, and changing the dial only masks the real problem:

1. Rethread the top thread completely, with the presser foot raised so the thread seats in the tension discs. This alone fixes most sudden tension faults.
2. Check the bobbin is in the right way and the thread is pulled through its tension slot. A bobbin dropped in spinning the wrong way mimics a tension problem exactly.
3. Change the needle. A blunt or bent needle makes ugly stitches no dial can fix.
4. Use the same thread top and bobbin where possible. Wildly different thread weights upset the balance.
5. Clean the bobbin area. Lint packed under the bobbin case throws the bobbin tension off. A quick brush-out works wonders.

Adjusting the top tension properly

Once you’ve ruled out the basics, adjust in small steps:

• Change the dial by one number at a time and sew a fresh test seam after each change. Big jumps just confuse you.
• Write down where you started (usually 4) so you can always return to base.
• Remember that different fabrics genuinely want different tensions. Fine silk likes it looser; thick denim with topstitching thread may want it tighter. There is no single “correct” number for all sewing.

Should you ever touch the bobbin tension?

For most sewing: no. The bobbin tension is set at the factory and you can usually balance everything with the top dial alone. Leave the little screw on the bobbin case alone unless you really know what you’re doing: it’s easy to lose the screw and hard to reset.

The exception is if you have a second, dedicated bobbin case for special threads (heavy topstitching, or for free-motion quilting). Many quilters keep a spare case they’ve deliberately adjusted, so the factory one stays untouched. If you only have one, don’t fiddle with it.

A quick diagnostic flow

1. Stitch looks wrong → sew a two-colour test on a scrap.
2. Loops underneath? → rethread the top (foot up), then raise top tension if still loose.
3. Loops on top? → lower top tension; re-seat the bobbin.
4. Still wrong? → new needle, clean the bobbin area, match your threads.
5. Now adjust the dial one number at a time until both threads lock in the middle.

Read the stitch, not the dial

Tension stops being scary the moment you realise the dial isn’t magic; it’s just a clamp on the top thread, and a bad stitch tells you exactly which way to move it. Sew a two-colour test, read which side the loops are on, rethread before you reach for the dial, and adjust one number at a time. Most “my tension is ruined” days end with the discovery that the thread simply wasn’t seated properly to begin with.