Normal sewing thread is size 50 in most countries, which corresponds to Tex 30 or Denier 300. This size is ideal for general-purpose sewing with medium-weight fabrics. Thread size may appear differently across measurement systems, but size 50 remains the standard for domestic machines.
In this guide, we’ll demystify the confusing world of thread measurements, explain the different sizing systems, and help you confidently choose the right thread for any project. Whether you’re a beginner or experienced sewer, knowing the standard thread sizes will improve your sewing results and save you frustration.
Understanding Sewing Thread Size
Thread size is one of the most confusing aspects of sewing, yet understanding it is essential for successful projects. Throughout the sewing world, there’s no single standard for measuring sewing thread thickness, which makes things particularly challenging for beginners and experienced sewists alike.
What does thread size mean?
Thread size refers to the thickness or diameter of a sewing thread. Unlike fabric, which you can easily see and feel, thread thickness is measured through several different systems that can seem contradictory at first glance.
The most common measurement systems include:
- Weight (wt): The most popular system in the US, where the number represents how many kilometers of thread it takes to weigh 1 kilogram. Counterintuitively, the higher the weight number, the finer the thread. A 30 wt. thread is heavier than a 50 wt. thread because it takes only 30 kilometers to weigh one kilogram.
- Tex: A direct measurement system that measures the weight in grams of 1,000 meters of thread. If 1,000 meters weighs 25 grams, it’s labeled Tex 25. The higher the Tex number, the heavier the thread.
- Denier: Similar to Tex but measures the weight in grams of 9,000 meters of thread. A thread weighing 120 grams per 9,000 meters would be a 120-denier thread.
- Commercial Sizes (V): Used primarily for heavy-duty threads, these are set sizes (30, 46, 69, 92, 138, etc.) derived by dividing the thread’s denier by 10.
To add to this complexity, many manufacturers borrow measurements from other standards without fully understanding them. For instance, the US weight system originated from the Japanese Gunze Count standard, where threads labeled #50 in Japan became known as 50 wt. in America. Furthermore, this system doesn’t account for the number of plies (strands twisted together), so a 50/2 thread (two plies) and a 50/3 thread (three plies) might both be labeled as 50 wt. despite the latter being 50% heavier.
Why thread thickness matters in sewing
The thickness of your thread influences virtually every aspect of your sewing project:
Stitch Quality and Appearance: Thicker threads create more visible stitches, making them ideal for decorative work, while thinner threads blend in better for invisible seams. If you want your stitches to stand out (as in quilting or topstitching), you might choose a thicker 30 wt. thread.
Machine Performance: Using thread that’s too thick for your machine or needle can cause jams, broken threads, or skipped stitches. Most home sewing machines work best with threads up to Tex 70.
Tension Adjustments: Different thread thicknesses require different tension settings. When you switch from a 50 wt. to a heavier 30 wt. thread, the increased diameter pushes tension disks further apart, requiring adjustment.
Needle Compatibility: A general rule is to use a needle whose eye is 40% larger than the thread diameter. For example, a #90/14 needle works well with 40 wt. thread, while a #100/16 needle is better for 30 wt. or 12 wt. thread.
Fabric Matching: Lightweight fabrics need finer threads to avoid overwhelming the fabric’s structure, while heavy fabrics require robust threads for durability.
The Most Common Thread Sizing Systems
Making sense of thread sizing systems requires understanding the four major measurement methods used worldwide. Each system calculates sewing thread thickness differently, which is why the same thread might be labeled with several different numbers.
Tex: weight of 1000 meters in grams
The Tex system stands as the most consistent method for measuring thread thickness and is endorsed by the International Organization for Standardization. Essentially, Tex measures the weight in grams of 1,000 meters of thread. For instance, if 1,000 meters of thread weighs 25 grams, it’s classified as Tex 25.
What makes Tex straightforward is its direct relationship to thickness—the higher the number, the heavier the thread. This creates a logical progression:
- Light-weight threads: Tex 8-25
- Medium-weight threads: Tex 25-40
- Heavy-weight threads: Tex 40 and above
Many textile professionals prefer Tex since it provides a universal measurement that allows for consistency across different thread types.
Denier: weight of 9000 meters in grams
Denier offers another fixed-length measurement system, primarily used for synthetic fibers like nylon and polyester. Under this system, thread thickness is determined by weighing 9,000 meters of thread in grams. A thread weighing 120 grams per 9,000 meters would be labeled as 120-denier.
Just as with Tex, larger denier numbers indicate heavier threads. Notably, many embroidery threads are labeled as “120/2,” meaning they consist of two strands of 120-denier thread twisted together, resulting in a 240 denier total.
Commercial sizes: V numbers for heavy-duty threads
Commercial sizes, denoted by “V” numbers, are specifically designed for heavy-duty applications such as upholstery, canvas, or marine sewing. These sizes follow a standardized numbering system: 30, 46, 69, 92, 138, 207, 277, 346, 415, and 554.
Rather than a direct measurement, commercial sizes are calculated by dividing the thread’s denier by 10. For example, a thread with a denier of 900 would be classified as V-90. This practical system helps sewists quickly identify robust threads suitable for demanding projects.
According to industry standards, commercial sizes from V-30 to V-70 are generally compatible with domestic sewing machines, whereas heavier options are typically reserved for industrial equipment.
Weight (wt): indirect sizing system
Unlike the other methods, the weight system works inversely—smaller numbers indicate heavier threads. Additionally, it measures length rather than weight, creating potential confusion for beginners.
Under this system, a thread labeled “40 wt.” means that 40 kilometers of that thread weighs 1 kilogram. Subsequently, a 30 wt. thread is heavier than a 40 wt. thread because it takes fewer kilometers (30 vs. 40) to reach the same weight.
This counterintuitive relationship explains why many sewists find the weight system puzzling at first. To complicate matters further, some manufacturers began labeling Japanese standard threads (#30, #40, #50) as weight measurements (30 wt., 40 wt., 50 wt.) regardless of the actual plies, creating inconsistencies across brands.
For quick reference, a common conversion between systems is: 40 wt. = 240 denier = Tex 25.
As you explore various threads, you’ll notice these different sizing systems on labels. Understanding how they relate to actual sewing thread size empowers you to make informed choices regardless of how a manufacturer labels their product.
How to Read and Compare Thread Sizes
Deciphering sewing thread sizes can feel like cracking a secret code. Thread labels contain valuable information beyond just color and brand name, yet many sewists struggle to interpret them properly.
Understanding thread labels and markings
Thread labels typically contain several key pieces of information about the thread’s characteristics:
- Thickness/Weight: Often shown with a # sign followed by a number (like #40) or with “wt” (40wt)
- Ply: Indicates how many strands are twisted together (e.g., 2-ply, 3-ply)
- Composition: Sometimes displayed as two numbers separated by a slash (50/2 or 30/3)
The first number in a composition label (like 50/2) typically represents the thread’s thickness, while the second number indicates the number of plies. Therefore, 50/2 means a 2-ply thread with a size of 50, whereas 30/3 is a 3-ply thread with a size of 30.
Moreover, you might encounter terms like “mercerized,” “glazed,” or “gassed.” Mercerized cotton has been treated to increase luster, glazed thread has a wax coating (not suitable for machine sewing), and gassed thread has been passed over a flame to reduce fuzz.
Thread size vs. thread weight
Although often used interchangeably, “thread size” and “thread weight” aren’t identical concepts:
- Thread size refers to the overall thickness or diameter of thread
- Thread weight specifically refers to the indirect measurement system where the number represents how many kilometers of thread equal 1 kilogram
Indeed, this is where confusion commonly arises. In the weight system, a 30wt thread is actually heavier than a 50wt thread, since it takes fewer kilometers (30 vs. 50) to reach 1 kilogram.
How to convert between Tex, Denier, and Weight
Converting between different thread thickness systems follows these formulas:
- Weight to Denier: 9000 ÷ weight
- Weight to Tex: 1000 ÷ weight
- Denier to Weight: 9000 ÷ denier
- Denier to Tex: denier × 0.111
- Tex to Denier: tex × 9
- Tex to Weight: 1000 ÷ tex
For instance, a standard 40wt thread equals 225 denier or Tex 25. This equivalence helps when comparing threads labeled in different systems.
When examining sewing thread sizes in stores, trust your eyes and fingers as much as the label. Manufacturers sometimes borrow measurements from different systems without fully understanding the conversions, resulting in inconsistencies.
To determine the actual Tex of any thread yourself, measure 10 meters, weigh it in grams, and multiply by 100. This gives you the weight in grams of 1,000 meters—the true Tex value.
What Size Is Normal Sewing Thread?
When shopping for sewing supplies, you’ll often notice most thread spools fall within a specific range of sizes. Let’s explore what sizes are considered “normal” in the sewing world.
Standard sizes for home sewing machines
Home sewing machines have limitations when it comes to thread thickness. Most domestic machines can handle thread up to Tex 70 (Size 69), which is the heaviest commercial size recommended for standard home sewing machines. Attempting to use thicker threads may cause jams, broken threads, or machine damage.
For optimal performance with home machines:
- Lightweight threads: Tex 8-25 (50-60wt)
- Medium-weight threads: Tex 25-40 (40wt)
- Heavy-duty threads: Up to Tex 70 (30wt)
Common thread sizes for general sewing
Among sewing enthusiasts, certain thread sizes dominate the market due to their versatility. The most widely used thread size is 40wt (equivalent to Tex 25 or 225 denier), which serves as the standard all-purpose thread for most sewing projects.
T-40 (40wt) thread strikes the perfect balance between strength and fineness, making it ideal for general sewing work. Consequently, many manufacturers digitize their embroidery designs specifically for 40wt thread to ensure optimal coverage without causing thread jams.
For lightweight fabrics, 50wt thread offers a finer alternative, whereas 30wt provides more visible stitching for decorative work or heavier fabrics.
Examples of normal thread sizes (e.g., 40wt, Tex 30-40)
Looking at specific examples across different measurement systems, these are considered normal thread sizes:
For General Sewing:
- 40wt = Tex 25 = 225 denier
- 50wt = Tex 20 = 180 denier
- 30wt = Tex 34 = 300 denier
For Specialized Applications:
- Fine detailed work: Tex 15-20 (blind stitching, heirloom projects)
- Quilting: Tex 25-40 (40wt thread)
- Visible topstitching: Tex 30-40 (30wt thread)
- Heavy-duty applications: Tex 70 (Size 69 thread)
As thread gets heavier, it becomes stronger and more visible in your finished project. This explains why decorative stitching typically uses thicker thread than seams meant to blend in. When selecting thread, remember that thread size affects both the appearance of your stitches and how well your machine handles the thread.
Tips for Choosing the Right Thread Size
Selecting the appropriate sewing thread thickness involves more than just grabbing whatever’s available in your sewing box. From my experience, matching the right thread to your project makes the difference between professional-looking results and frustrating failures.
Match thread to fabric type
First and foremost, pair your thread with your fabric’s properties. As a rule, match thread content to fabric content for best results—natural fiber threads with natural fabrics, synthetic threads with synthetic materials. For lightweight fabrics like silks or chiffons, choose finer threads (50-60wt), while heavy materials like canvas or denim require robust threads (30wt or heavier). Remember that thread thickness increases as the weight number decreases, so a 30wt thread is actually thicker than a 50wt thread.
Consider needle size and machine compatibility
The relationship between thread and needle is crucial. Ideally, use a needle with an eye approximately 40% larger than your thread diameter. For instance:
- 12/80 needle for fine T30 thread
- 14/90 needle for medium T45 thread
- 16/100 or 18/110 needle for T70 thread
Be aware that Tex 70 is the heaviest thread recommended for most home sewing machines. Using thread that’s too thick risks damaging your machine and may cause skipped stitches.
Adjust tension for different thread thickness
Whenever you change thread size, adjust your machine’s tension accordingly. Thicker threads typically require looser tension, while finer threads need tighter settings. Machines come preset for 50 or 60wt threads, so significant adjustments are necessary when switching to heavier options. When using decorative threads like metallics, you might need to reduce tension dramatically—sometimes as low as 1.0—to achieve balanced stitches.
Use charts for quick reference
Thread charts eliminate guesswork when pairing thread with needles and fabrics. Many manufacturers provide conversion charts showing the relationships between different sewing thread sizes. These references help you quickly determine which needle size works with specific thread weights and what tension adjustments might be needed. With this in mind, testing your setup on scrap fabric before starting your actual project is always wise.
Conclusion
Understanding sewing thread sizes certainly seems daunting at first glance. After exploring the various measurement systems, we can see why many sewists feel confused when facing labels with T-40, Tex 30, or 50wt designations. Nevertheless, this knowledge proves essential for achieving professional results in your projects.
Thread thickness matters significantly across all aspects of sewing. The standard 40wt thread works perfectly for most general sewing tasks, though different fabrics and applications may require thinner or thicker options. Remember that home sewing machines typically handle threads up to Tex 70, while anything heavier might require industrial equipment.
Additionally, thread selection affects your entire sewing process. Your choice impacts stitch visibility, machine performance, and overall project durability. Therefore, matching your thread to your fabric type, selecting compatible needle sizes, and adjusting tension accordingly creates the foundation for successful sewing.