What tension does and why it goes wrong
Tension is the amount of pull on the thread as it moves through your machine. When tension is correct, the top and bottom threads lock together inside the fabric in a straight line, and your seams look even on both sides. When tension is off, you get loose loops, puckered fabric, or thread that breaks.
Tension drifts out of adjustment for straightforward reasons: you changed needle size or thread type without adjusting the dial, lint built up in the tension discs, or the machine sat unused for months. The good news is that fixing it usually takes five minutes and no tools.
Most machines have a tension dial on the front (numbered 0 to 9, or sometimes 1 to 10) and a bobbin tension screw underneath. You will almost always fix the problem by turning the top dial. The bobbin screw is a last resort — it is straightforward to overtighten and damage the bobbin case.
Key Takeaways
- Correct tension means the top and bottom threads lock evenly inside the fabric, with no loops or puckering visible on either side.
- Start by cleaning out lint from the tension discs with a small brush or folded paper, because buildup is the most common cause of tension problems.
- Test tension on scrap fabric that matches your project, using the same needle size and thread type you plan to sew with.
- Turn the tension dial up (toward higher numbers) if the bottom thread shows on top, and down (toward lower numbers) if the top thread shows on the bottom.
- Leave the bobbin tension screw alone unless you have already adjusted the top dial and the problem persists.
Clean the tension discs before you adjust anything
Lint and thread bits get trapped between the tension discs — two metal plates that pinch the thread as it feeds through. When they are clogged, the thread cannot move smoothly, and tension readings become unreliable. Cleaning them often fixes the problem without touching the dial.
Raise the presser foot (this opens the discs) and use a small soft brush, an old toothbrush, or a folded piece of paper to gently brush between the discs. Slide the brush or paper in and out a few times. You may see a surprising amount of lint come out. Lower the presser foot again.
If you have compressed air, a short burst works too — but do not use it on the bobbin area, because the pressure can knock the bobbin case out of place. Stick to the top tension discs.
Run a tension test on scrap fabric
Before you adjust the dial, sew a test seam on fabric that matches your project. Use the same needle size, thread type, and stitch length you plan to use. Sew a straight line for three or four inches, then stop and look at both sides of the seam.
Correct tension looks the same on both sides: a straight line of interlocked stitches with no loops, puckering, or visible thread color from the opposite side. If the seam looks different on the front and back, tension is off.
Keep this test piece — you will use it to check your adjustment after you turn the dial.
Adjust the top tension dial based on what you see
The tension dial usually sits on the front of the machine, marked with numbers. The default setting is often 4 or 5, but it varies by machine. Higher numbers mean tighter tension; lower numbers mean looser tension.
If the bottom thread shows on the top of your seam, the top tension is too loose. Turn the dial up one number at a time (toward 5, 6, 7, and so on). Sew another test seam and check again. Keep adjusting until the threads lock evenly.
If the top thread shows on the bottom of your seam, the top tension is too tight. Turn the dial down one number at a time (toward 3, 2, 1, and so on). Test again after each turn. Stop when the seam looks balanced on both sides.
Make small moves — one number at a time — because the dial is sensitive. A change from 4 to 5 can make a big difference.
Check your needle and thread first
Before you assume the tension dial is broken, rule out the most common culprits: a bent or dull needle, the wrong needle size for your fabric, or thread that does not match your needle.
Replace the needle with a fresh one. A bent needle (even slightly) throws off tension because it does not sit straight in the needle clamp. Use the needle size your fabric calls for — a size 70/10 needle for lightweight cotton, a size 90/14 for medium weight, and so on. The package tells you what weight each size handles.
Check that your thread is wound on the spool correctly and feeding smoothly from the spool pin. Old thread can be brittle and break easily, which feels like a tension problem but is not. If you have been using the same spool for months, try a fresh one.
When to adjust the bobbin tension screw
The bobbin tension screw sits on the bobbin case, usually underneath the needle plate or on the side of the machine. It is a small screw with a spring underneath. Most machines come with bobbin tension set correctly at the factory, and you should not need to touch it.
Only adjust the bobbin screw if you have already cleaned the tension discs, replaced the needle, and turned the top dial through its full range without fixing the problem. Even then, make tiny adjustments — a quarter turn at a time — and test after each one. Turning it too far can damage the bobbin case or break thread.
If you do adjust it, turn it clockwise (to the right) to tighten and counterclockwise (to the left) to loosen. Write down the original position first, so you can return to it if something goes wrong. Many people find it easier to take the machine to a technician rather than risk the bobbin screw.
Tension problems that mean you need a professional
If you have cleaned the discs, replaced the needle, adjusted the top dial, and the tension is still wrong, something inside the machine may be bent or misaligned. A bent tension disc, a damaged needle clamp, or a timing issue cannot be fixed at home.
Similarly, if the machine breaks thread constantly even with a new needle and fresh thread, or if it makes grinding noises when you turn the dial, the tension assembly may need professional service. A technician can check the discs, the tension spring, and the feed dogs in ways you cannot do yourself.
Most sewing machine shops offer tension checks and cleaning for a modest fee — often less than fifty dollars — and it is worth the cost if you use the machine regularly.
Frequently Asked Questions
Why does my tension change when I switch from cotton to polyester thread?
Different thread types have different thicknesses and stiffness, so they move through the tension discs differently. Polyester is often thicker than cotton, so you may need to loosen the dial slightly when you switch. Always test on scrap fabric after changing thread type, and adjust the dial as needed.
What number should the tension dial be on?
Most machines work well at 4 or 5, but the right number depends on your fabric, thread, and needle. There is no universal "correct" number — correct means your test seam looks balanced on both sides. Write down the number that works for your usual fabric and thread combination, so you can return to it.
Can I fix tension by adjusting the bobbin instead of the top dial?
Technically yes, but you should not. The top dial is designed to be adjusted regularly; the bobbin screw is not. Adjusting the bobbin screw is harder to control and easier to damage. Always try the top dial first, and only touch the bobbin screw if the top dial does not solve the problem.
Does tension need to be the same for all stitch types?
Not always. Straight stitches usually work at your standard tension, but zigzag and decorative stitches sometimes need slightly looser tension to prevent puckering. Test on scrap fabric and adjust the dial if needed. You can return to your standard number when you switch back to straight stitches.
What if I accidentally turned the bobbin tension screw too far?
If you wrote down the original position, turn it back to that spot. If you did not, try turning it gently clockwise until you feel slight resistance, then back it off a quarter turn. If the machine still does not work, take it to a technician — forcing the screw can break the spring inside the bobbin case.