The bobbin holds the thread that forms the bottom stitch
The bobbin is a small spool that sits inside your sewing machine, directly below the needle plate. It holds thread that works together with the thread from the top spool to create a stitch. When you sew, the needle pushes thread down through the fabric, and the bobbin thread comes up from underneath to lock that thread in place. Without a bobbin, you would only have thread on one side of your fabric—no stitch would form at all.
Think of it this way: the top thread and bobbin thread are partners. The needle carries the top thread down, the bobbin thread wraps around it, and they lock together inside the fabric layers. That locked loop is what holds your seam together. The bobbin does half the work of every stitch you make.
Key Takeaways
- The bobbin is a small spool that holds thread underneath the needle plate and works with the top thread to form a complete stitch.
- Most bobbins are plastic or metal and come in standard sizes, though some machines use proprietary bobbins that only fit that brand.
- You wind thread onto the bobbin using your machine's built-in winding mechanism before you start sewing.
- If your bobbin is empty, wound unevenly, or threaded incorrectly, your stitches will skip, bunch, or look loose on the underside of your fabric.
- Checking and replacing your bobbin is one of the first troubleshooting steps when your machine stops sewing properly.
How the bobbin and needle thread work together
Your sewing machine creates a stitch through a precise mechanical dance. The needle moves down and pushes the top thread through the fabric. As the needle rises back up, a small hook or looper (depending on your machine type) catches the loop of top thread and wraps the bobbin thread around it. This creates an interlocked stitch that holds the two threads together inside the fabric.
The bobbin thread is just as important as the top thread—if either one breaks, runs out, or is threaded wrong, your machine will not sew properly. Many beginners focus only on the top thread and forget the bobbin exists, then wonder why their stitches look perfect on top but messy underneath. That mess is usually a bobbin problem.
Types of bobbins and how they fit your machine
Bobbins come in a few standard sizes, but not all machines use the same bobbin. Most modern home machines use either a Class 15 bobbin (the most common plastic type) or a Class A metal bobbin. Some older machines and specialty machines use different sizes. Check your machine's manual or look at the bobbin case itself—it usually has the size printed on it.
Many sewing machine brands also make proprietary bobbins that only fit their machines. Bernina, Janome, and some Singer models use bobbins designed specifically for them. Buying the wrong bobbin for your machine will not fit into the bobbin case, and forcing it can damage your machine. If you are unsure which bobbin you need, take your machine's model number to a fabric store or call the manufacturer.
How to wind and insert a bobbin
Before you can sew, you need thread on your bobbin. Most machines have a bobbin winder built into the machine itself—usually a spindle on top or to the side. You place an empty bobbin on the winder, thread it from your top spool, and press a button or foot pedal to wind. The machine winds the thread onto the bobbin automatically until it is full, then stops.
Once your bobbin is wound, you insert it into the bobbin case. On most machines, the bobbin case is a small metal or plastic housing that sits under the needle plate. You drop the wound bobbin into the case, thread the thread through a small slot (called the tension spring), and close the case. The thread should sit snugly in that slot—if it is loose or not threaded through, your stitches will not lock properly.
Some machines have a drop-in bobbin system where you straightforward drop the bobbin into a hole under the needle plate without a separate case. Others have a removable bobbin case that you pull out to insert the bobbin. Your manual will show you which system your machine uses and the exact steps for your model.
Signs your bobbin needs attention
If your machine starts skipping stitches, making loops on the underside of your fabric, or producing loose stitches, the bobbin is often the culprit. Check these things in order: Is the bobbin empty or nearly empty? Is the thread wound evenly on the bobbin, or is it bunched on one side? Is the bobbin threaded correctly through the tension spring in the bobbin case?
Uneven winding happens when you wind too fast or do not guide the thread smoothly onto the bobbin. Loose bobbin tension (the small screw on the bobbin case) can also cause problems, but do not adjust it unless you have checked everything else first. Many machines have a tiny screw on the bobbin case that controls tension—turning it slightly clockwise tightens the tension. If you turn it too far, you can break the bobbin thread or damage the case.
When to replace your bobbin
Bobbins wear out over time. Plastic bobbins can crack or warp after years of use, especially if they have been wound very tightly or stored in heat. Metal bobbins last longer but can dent or rust. If your bobbin is cracked, dented, or visibly damaged, replace it. A damaged bobbin can catch the thread and cause breaks or skipped stitches.
You should also have a few extra bobbins on hand so you can wind one while you are sewing with another. This saves you from stopping mid-project to wind a new bobbin. Most fabric stores sell replacement bobbins in packs of three to five for a few dollars. Keep them in a small container so they do not get lost.
Bobbin problems and quick fixes
If your machine is not sewing, start with the bobbin. First, remove it and check that it has thread on it. If it is empty, wind a new one. If it has thread, unwind a few inches and look for knots, tangles, or breaks in the thread. If the thread is damaged, wind a fresh bobbin.
Next, rethread the bobbin case. Remove the bobbin, pull the thread out of the tension spring, and reinsert it, making sure it sits in the slot with a slight click. Sometimes the thread slips out of the tension spring during sewing, and straightforward rethreading fixes the problem. If you have done all this and your machine still skips stitches, the bobbin case tension may need adjustment, but that is a step to take only after you have ruled out these simpler causes. If you are not comfortable adjusting tension, a sewing machine repair technician can do it for you.
Frequently Asked Questions
Can I use any bobbin in my sewing machine?
No. Most machines use a standard Class 15 or Class A bobbin, but some brands make proprietary bobbins that only fit their machines. Using the wrong bobbin can damage your machine or cause it not to fit in the bobbin case. Check your manual or the bobbin case itself to find out which type your machine needs.
How tight should the bobbin thread be?
The bobbin thread should be snug in the tension spring but not so tight that it is hard to pull. If you hold the bobbin case and gently tug the thread, it should move with slight resistance. If it pulls out easily, the tension is too loose. If you cannot move it at all, it may be too tight. Most machines come with the tension set correctly from the factory.
Why does my bobbin keep running out of thread?
You may be winding bobbins too loosely, so they hold less thread than they should. Wind slowly and guide the thread evenly across the bobbin so it fills completely. You can also wind multiple bobbins at once and keep them ready. If your machine winds bobbins automatically, make sure you let it finish completely before removing the bobbin—stopping early leaves it underfilled.
What happens if I forget to put thread in the bobbin case?
Your machine will still move the needle up and down, but no stitch will form. You will see only top thread on your fabric with no locking stitch underneath. The needle may also break because there is no bobbin thread to catch and lock it. Always check that the bobbin is wound and threaded before you start sewing.
Can I wind a bobbin by hand?
You can, but it is slow and the winding will likely be uneven, which causes tension problems. Most machines have a built-in winder that does the job in seconds and winds evenly. If your machine's winder is broken, a repair technician can fix it, or you can ask at a fabric store if they offer bobbin-winding services.