Last updated: August 21, 2026
You have run out of bobbins in the middle of a project, the manual is somewhere in a box, and the store shelf offers Class 15, Class 15J, Style A, Style L and a bag labeled “fits most machines”. Nothing on the machine itself says which one it takes. This is the moment most sewers buy the wrong bag, and then quietly blame their tension for a year.
There are four reliable ways to identify your bobbin, and only one of them involves the manual. This page runs through them in order, from the fastest to the one that works when you have nothing but the machine in front of you.
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Quick Answer
How do you find out which bobbin your machine uses?
- Check the manual first. Machine makers name the bobbin class or the part number in the accessories or specifications section.
- No manual? Search the model number plus the word bobbin on the manufacturer site, since most makers publish parts lists and downloadable manuals.
- Measure the original bobbin across the flat and across the width, and compare it to the class dimensions.
- Take the old bobbin to the store and compare it side by side. Diameter matching is not enough, the width has to match too.
Bottom line: identify the class once, write it on a label stuck to the machine, and never guess in a store again.
Start With What the Manufacturer Says
The manual is the fastest and the most reliable answer, and almost every home machine sold in the last twenty years has a downloadable one. Look under accessories, included parts, or specifications. The maker will either name a class, such as Class 15, or give a part number that a parts retailer can translate.
If the printed manual is gone, the model number on the machine is enough. It is usually on a sticker on the back, the base or inside the free arm cover, and the maker’s support site will have the manual and often a parts diagram behind it.
When the Model Number Is Rubbed Off
Older machines lose their stickers. On Singer and Brother machines the model family is often molded into the body or printed on the front panel, and those families follow patterns that narrow it down quickly. Our pages on Brother model numbers and Singer model numbers explain how those naming systems work.
Measure the Bobbin You Already Have
If you still have one original bobbin, it answers the question by itself. Measure the outside diameter across the flat discs, then the total width from one disc face to the other. Those two numbers place it in a class, and the width is the number that separates the two most confusable classes.
Class 15 is published at roughly 20.3 mm diameter and about 11.7 mm wide. Class L shares the 20.3 mm diameter but is about 8.9 mm wide. Class M is the large one at about 24.9 mm across. Those figures come from the bobbin references published by sewing parts retailers, and they are the numbers to compare your bobbin against.
The Four Ways to Identify It, Ranked
Each method below works on its own. Use the first one available to you, and treat the coin comparison as a rough sorting step rather than a final answer, because it separates diameters and tells you nothing about width.
| Method | What you need | How certain | Watch out for |
|---|---|---|---|
| Manual or maker site | Model number | Definitive | Model variants with different parts |
| Measure the original | A ruler or calipers | High | Measure width, not just diameter |
| Compare in store | The old bobbin in hand | Good | Same diameter, different width |
| Coin comparison | A nickel or a quarter | Rough sorting only | Never use it as the final check |
The coin trick comes from the way parts retailers describe the classes: Sewing Parts Online compares the Class 15 to the size of an American nickel and the Class M to a quarter. It is a useful first sort and no substitute for the width measurement.
Drop In or Front Loading Changes What to Look For
A drop in bobbin sits horizontally under a clear cover next to the needle plate, and the machine usually takes a Class 15 style plastic bobbin. A front loading machine has a removable bobbin case that you pull out from behind a door on the front, and the case itself is a strong clue, because cases are class specific.
The Case Is Half the Fit
On a front loading machine, the right bobbin is the one that sits flush in the case with the thread coming out under the tension spring without binding. If the bobbin protrudes or rocks in the case, it is the wrong class, no matter how close the diameter looks. Hold the loaded case and let the bobbin turn: it should move smoothly with light resistance.
If you are already fighting an uneven stitch and are not sure whether the bobbin is the cause, work through the checks in bobbin tension before buying anything. Most of those checks cost nothing.
Bags Labeled “Fits Most Machines”
Generic multipacks are the single biggest source of bobbin trouble, because most of them are a Class 15 clone with loose tolerances. Sometimes they work. Sometimes they are a fraction too thick, sit slightly proud in the case and start catching, or they are molded thin enough to flex when the thread pulls.
The cost of the mistake is not the bag, it is the evening spent adjusting tension on a machine that was fine. If you buy generic, buy a small quantity, test them on a scrap before a project, and keep them separate from the ones that came with the machine. Full class dimensions and the difference between plastic, metal and prewound are in bobbins explained.
Frequently Asked Questions
What bobbin does my Brother sewing machine use? Most current Brother home machines take a Class 15 style bobbin, but the manual for your specific model is the authority, because Brother lists a specific part number per machine. Look it up by the model number on the back of the machine.
What bobbin does my Singer use? Modern Singer home machines commonly use Class 15, while older models can take Class 66 with its curved side profile. Since the two share a footprint and differ in the side wall, this is one case where matching by eye goes wrong easily.
Can I use any plastic bobbin that fits? Fitting and working are different. A bobbin that is slightly narrow will rattle and give uneven tension, and one that is slightly thick will bind. Both look like machine faults from the top of the fabric.
How do I measure a bobbin without calipers? Lay it on a metal ruler with millimeter markings and read the diameter, then stand it on edge to read the width. You are distinguishing about 11.7 mm from about 8.9 mm, which is visible on a ruler if you look carefully.
Are prewound bobbins the right class? They are sold by class, so you buy the same class you already use. Check that your machine reads them if it has a bobbin sensor, and be aware that sided prewounds vary slightly between suppliers.
Where should I keep spare bobbins? In a case that keeps them from unwinding, ideally beside the matching spool. Loose bobbins in a drawer tangle together, and a tangled bobbin gets wound over rather than emptied, which is how mixed thread ends up in one seam.
The Bottom Line
Look it up in the manual, or measure the original across the diameter and the width, and stop trusting bags that promise to fit most machines. Class 15 and Class L share a diameter and differ in width, which is the mistake almost everyone makes once. Write the class on a label and stick it to the machine. Read next: bobbin classes explained, winding a bobbin, and our needle guide for the other part that quietly decides your stitch quality.