What tension number should I use for knit fabric?

There is no universal knit number. Begin with the machine manual's default for the selected stitch, then test the same fabric, thread, needle, and seam layers. The result—not a number copied from another machine—determines whether upper tension needs a small change.

Do loops underneath mean upper tension is too loose?

Not necessarily. Singer identifies incorrect upper threading as the usual cause of loops underneath. Remove and rethread the upper thread with the presser foot raised before changing tension.

Should I adjust bobbin tension for knit fabric?

Not as a routine first step. Confirm the correct bobbin, bobbin orientation, upper threading, needle, thread, and upper setting first. Follow the model manual or seek service guidance before changing a factory-set bobbin case.

Why does my knit seam wave even when the stitches look balanced?

The fabric may be stretching during feeding, presser-foot pressure may be unsuitable, or the stitch may be too dense or rigid. Tension cannot correct every feed or stitch-choice problem.

Sewing Machine Tension for Knits: Diagnose Before You Turn the Dial

Do not lower tension automatically for knit fabric. Rethread with the presser foot raised, fit the right fresh needle, test the machine's default on matching scraps, and change upper tension only when the stitch formation points there.

The safest starting tension for knit fabric is the machine’s documented default for the selected stitch—not “one notch lower than woven.” A dial number is model-specific. First prove that the machine is threaded correctly and the needle matches the fabric; only then use the stitch itself to decide whether upper tension needs a small change.

The five-minute reset before any adjustment

Remove the upper thread. Do not try to pull it back into the tension path.
Raise the presser foot and rethread. Singer explains that raising the foot lets the thread seat in the upper tension mechanism.
Confirm the take-up lever is threaded. A missed take-up lever commonly produces loops underneath.
Remove and reinstall the bobbin. Use the exact bobbin type and orientation specified by the manual.
Install a fresh appropriate needle. SCHMETZ identifies Jersey needles for knits and some stretch fabrics, and Stretch needles for elastic materials and highly elastic knitwear.
Select the intended stitch and restore its default. Computerized machines may load a factory starting tension with each stitch; mechanical machines have their own normal range.
Test the real seam stack. Use the same layers, thread, direction, and stabilizer as the project.

If that reset fixes the seam, tension was not the root problem. The thread or bobbin was not seated correctly, or the needle was unsuitable.

Read the symptom before touching the dial

What you see	Most useful first check	When upper tension becomes a reasonable test
Loose loops underneath	Upper threading, take-up lever, presser foot raised during threading	Only after a complete correct rethread still leaves top thread too loose
Bobbin thread pulled onto the top	Upper thread obstruction, bobbin installation, upper setting	After thread paths are clear and the correct bobbin is seated properly
Skipped stitches	Needle condition and Jersey-versus-Stretch choice	Rarely first; solve needle and threading causes before tension
Wavy or stretched seam	Fabric handling, presser-foot pressure, stitch choice	Only if the stitch balance also shows an upper-thread problem
Tunneling between twin-needle rows	Thread bulk, stabilizing, stitch density, model setup	After compatibility and setup checks, test a small upper-tension change
Puckering with a balanced-looking lock	Dense stitch, heavy thread, light fabric, feed	Tension may not be the cause at all

This table keeps “bad seam” from being treated as one diagnosis. Singer’s troubleshooting says loops on the underside normally point to incorrect upper threading; simply turning the dial can hide the clue without fixing the path.

How to make an upper-tension adjustment safely

Change upper tension only after the reset and only when both sides of the test seam support that diagnosis.

Photograph or write down the default.
Make one small change in the direction indicated by the stitch.
Sew a new sample on the same layers.
Compare top, underside, and relaxed fabric shape.
Stretch the seam only as the finished garment will stretch.
Keep the change only if stitch balance improves without new distortion.

If the result does not improve, return to the default before testing another cause. Do not keep walking the dial farther away from normal in search of a feed, needle, or stitch-choice fix.

What loops on the underside actually tell you

Loops underneath are upper thread, not bobbin thread. Before tightening upper tension:

confirm the presser foot was raised during threading;
make sure the thread entered the tension discs;
check the take-up lever;
inspect the spool cap and thread path for a snag;
rethread the needle in the direction specified by the manual.

Singer suggests a simple mechanism check: with the presser foot raised, the upper thread should pull easily; with the foot lowered, resistance should increase. If there is no meaningful change, the upper thread is not correctly engaged and the dial is not the real fix.

What bobbin thread on top can mean

Bobbin thread appearing on the top can indicate upper tension that is too tight, but Singer also points to an obstructed upper thread path or a bobbin that is not seated correctly. Check the simple causes first:

Is this the exact bobbin class specified by the machine?
Is it unwinding in the manual’s direction?
Is the bobbin thread under the correct guide or spring?
Can the upper thread move freely from the spool?
Did the machine start from its normal setting?

Do not loosen a bobbin-case screw as a generic knit adjustment. A removable bobbin case may be designed for service or specialty setup, but many home machines expect the factory setting to remain stable. Use model-specific instructions if the upper system has been ruled out.

When the needle is the better purchase

Needle choice is more specific than “ballpoint for every knit.” SCHMETZ says its Jersey needle uses a medium ball point for knits and some stretch fabrics. Its Stretch needle adds a special eye and scarf for elastic materials and highly elastic knitwear, specifically to help prevent skipped stitches.

For routine jersey, the exact Euro-Notions assorted Jersey ball point needles, ASIN B01GNE3YB8, are a more relevant buy than a random universal-needle search. If a fresh Jersey needle still skips on fabric containing elastic, Lycra, or Spandex, test SCHMETZ’s Stretch family instead. Match size to fabric and thread; do not assume one size suits every knit.

If the current needle is fresh, appropriate, and forming stitches cleanly, buying another pack will not solve fabric that is being pulled through the machine.

When tension is not the problem

The seam is wavy but thread balance looks normal

Support the fabric and stop pulling from either side of the foot. If the machine offers adjustable presser-foot pressure, test a documented reduction on a scrap. If it does not, try a less dense stitch or an approved even-feed solution. Tension cannot undo fabric that was stretched during feeding.

Straight stitches break when the seam stretches

The stitch lacks the movement the finished area needs. Change to a narrow zigzag, stretch stitch, approved twin-needle hem, or serger construction rather than loosening the stitch until it looks unstable. The knit topstitching decision guide separates those finishes.

Skipped stitches continue after rethreading

Replace the needle and reassess Jersey versus Stretch. Confirm the needle is fully inserted in the correct orientation and that the thread/needle size combination is suitable. Persistent skipping across multiple correct setups can indicate timing or service trouble.

The problem starts only at a thick seam crossing

The foot may be tilting and the layers may not be feeding evenly. Level the foot with a model-compatible tool or scrap, slow down, and use the handwheel where the manual recommends it. A global tension change for one bulky intersection can spoil the rest of the seam.

A repeatable test card

Keep a scrap of the project fabric and record:

machine and stitch number;
default upper-tension setting;
needle family and size;
thread type and weight;
presser foot and pressure setting, if adjustable;
stitch length and width;
result after the fabric relaxes and after a gentle stretch test.

That record is more useful than a universal dial number. It lets you reproduce a working setup and shows whether the next problem is genuinely new.

Bottom line

Do not lower tension merely because the fabric is knit. Rethread with the presser foot raised, confirm the bobbin, install the correct fresh needle, and test the machine’s documented default on the real seam layers. Adjust upper tension one small step only when the thread balance points there. Treat rippling, skipped stitches, and seam breakage as separate diagnoses rather than asking one dial to fix all three.