Polyester thread comes in eleven sizes or thicknesses. The "right" thread size depends the material, seam strength needs, the desired "look", and machine capabilities. Sometimes, the best size is a compromise that balances competing needs.

Polyester Thread Size - Polyester thread comes in eleven sizes or thicknesses that range from hair-thin with a 1.5 pound tensile strength to 1 millimeter thick with a 98 pound strength. The "right" thread size depends on the material you are sewing, seam strength requirements, the "look" that you want, and your sewing machine's capabilities. Usually, the "right" size is the one that best balances competing needs. Here is a shortcut - go to our  Polyester Thread Use and Size Guide page for size recommendations for many typical polyester thread sewing applications.
Thread size affects almost everything. To make the right choice you need to understand how thread size affects your work and what happens when thread size increases:
Thread diameter increases - Technically, thread sizes refer to the weight of a given length of thread - not its thickness. For example, with a Tex 70 thread 1,000 meters of thread weighs 70 grams. The thread industry uses this standard because thread is too spongy to be measured with calipers. Also, the tightness of a thread's twist and its bonding affect its thickness. Having said this, U.S. Military Specifications give nominal thread diameters that show the relative differences between sizes.
Thread and seam strength increase - The material you are sewing costs far more than the thread you are using. It most cases (parachutes excepted) it is better for the seam to break that to have the material tear. For example, Size 69 / Tex 70 polyester has an 11 pound tensile strength which most people cannot break by pulling. This is compatible with the strength of most lightweight garments and gear.
Thread size matters most when it comes to covers and enclosures. In these cases seam stress increases as the area enclosed and fabric weight increases. This means that thread size needs to increase to handle the stress. The tents on the right illustrate this point. The camping tent would use a sizes between 46 and 92; the shelter tent would use thread in the size 207 to 554 range. tents tents
Stitch visibility increases - This can be either good or bad depending on how you want your work to look. When you use a thread Size 138 / Tex 135 or higher the stitches become noticeable; at Size 277 / Tex 270 they become a focal point as shown on the right. Also, thread colors that contrast with the material and stitch size increase visibility. Finally, uniform stitching is critical because any mistakes really stand out. knife
Needle size increases - A thicker thread requires a larger needle. Our Needle and Thread Size Chart and the Polyester Thread Specifications give the range of needle sizes used with each thread size. We give ranges because the physical thickness of thread varies by size. Generally, it is best to use the smallest needle size that has a large enough eye for the thread.
Material thickness increases - Like thread, fabric and leather thickness cannot be measured with calipers. Instead, it is measured by how much a yard of material weighs in ounces. Here is a table that shows how the range of polyester thread sizes used increases as material thickness increases. Note that leather uses thicker thread sizes because it is denser than fabric.
Fabric Ounces Per Yard Thread Size Range Leather Ounces Per Yard Thread Size Range
2 to 6 Ounces 15 to 33 1 to 8 Ounces 46 to 207
6 to 8 Ounces 33 to 47 8 to 12 Ounces 138 to 277
8 to 10 Ounces 46 to 69 12 to 16 Ounces 138 to 346
10 to 12 Ounces 69 to 92 Multiple Layers 207 to 346
12 to 14 Ounces 92 to 207    
Sewing machine requirements increase - Sewing machines work by punching holes in material and forcing thread through the hole to make the stitch. A thicker thread requires a larger hole and a larger hole requires a more powerful machine. Home machines (flat shank needles) usually can handle polyester thread up to Size 69 / Tex 70, commercial machines can handle thread up to Size 207 / Tex 210, and heavy duty (walking foot) machines are needed for sizes 277 / Tex 270 and up.
Yards per pound decreases - Polyester thread is sold by spool weight -- not the length of thread on the spool. For example, a Size 46 / Tex 45 polyester yields about 10,000 yards per pound; Size 207 / Tex 210 yields about 2,100 yards per pound.
Polyester Thread Size Descriptions - Here are brief descriptions of the eleven polyester thread sizes that we stock. See our Polyester Thread Use and Size Guide  page for size recommendations about specific thread uses, and our  Polyester Thread Specifications page for technical details.
Size 15 / Tex 16 / Govt. 00 Ultra thin thread that is mainly used for light sewing and to embroider leather and fabric.
Size 33 / Tex 35 / Govt. AA Lightweight thread that is used for sewing garments and very thin leather such as purses and wallets.
Size 46 / Tex 45 / Govt. B Lightweight thread used for sewing lightweight outdoor gear (e.g. tents), thin leather, and light outdoor garments.
Size 69 / Tex 70 / Govt. E Most popular size, best color selection. Great for lightweight and middleweight sewing. Use for outdoor furniture, leather, gear, garments, upholstery, drapes. Largest size for most home sewing machines.
Size 92 / Tex 90 / Govt. F Middleweight thread that is used for tents, furniture upholstery, auto upholstery, dog beds and collars, sails, and sand bags.
Size 138 / Tex 135 / Govt. FF Middleweight thread that is sometimes used for heavier weight sewing because stitches are noticeable. Typical uses include auto upholstery and convertible tops, awnings, party tents, pool covers, tarpaulins, and sails.
Size 207 / Tex 210 / Govt. 3-Cord Heavyweight thread. Typical uses include auto upholstery, furniture upholstery, boat covers, awnings, tarpaulins, and saddles and horse tack.
Size 277 / Tex 277 / Govt. 4-Cord Heavyweight thread used for hand and machine sewing. Typical uses include horse saddles and tack, knife sheaths, holsters, tarpaulins, banners, boat and pool covers. Its stitches are a focal point.
Size 346 / Tex 350 / Govt. 5-Cord Heavyweight thread used for hand and machine sewing. Typical uses include canvas tarpaulins, saddles, horse tack, shoe repair, conveyer belts, holsters, and sheaths. Its stitches are a focal point.
Size 415 / Tex 410 / Govt. 6-Cord Heavyweight thread used for hand and machine sewing. It is also a survival pack staple. Typical uses include canvas tarpaulins, saddles, horse tack, shoe repair, conveyor belts, holsters, and sheaths. Its stitches are a focal point.
Size 554 / Tex 600 / Govt. 8-Cord Our heaviest thread with a 1 mm diameter. It is used for very heavy duty hand and machine sewing and for tying, binding, and hanging. Stitches are a focal point.

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