A thread label can contain a size, a fiber name, a construction description, a spool length, and a color code. Confusing any two of those fields makes comparison difficult. The safest approach is to write them separately before deciding whether one spool can replace another in your leather sewing machine.
Tex is a linear-density system: it describes the mass in grams of one thousand meters of thread. A higher Tex number therefore means more mass per unit length. It does not directly state the thread's diameter, breaking strength, needle requirement, or suitability for your particular machine.
Why #69 and Tex 70 appear together
Commercial thread designations do not always match Tex numerically. Service Thread explains that nylon and polyester products sold under a size 69 designation can both fall in the nominal Tex 70 category despite differences in their component filaments. This is a useful shopping relationship, not a reason to treat every number printed on a spool as interchangeable.
Our catalog includes products explicitly labeled #69/T70. That combined labeling is more useful than guessing a conversion from an isolated number. When a brand supplies its own table, follow that table for its product. Save the product sheet with your machine notes so a future reorder does not depend on memory.
Bonded describes something different
The word bonded is not a size. Neither are nylon and polyester. A listing that says bonded nylon heavy duty but omits a numeric size has not given you enough information to make a dependable substitution. Ask for the stated Tex or the manufacturer's equivalent designation and intended sewing application.
Likewise, a spool length such as one thousand meters describes quantity, not thread thickness. Large spools can hold fine thread, and small spools can hold relatively coarse thread. Do not estimate size from the apparent diameter of a cone in a product photograph. Our thread purchasing guide covers other listing details that deserve confirmation.
Read a label as separate fields
Write a short purchasing note with the product name, fiber, nominal size, finish, color, and quantity. Add the manufacturer's needle recommendation and any machine-specific limitation. If one field is missing, mark it unknown rather than filling it with a familiar value from another brand.
For example, a clearly described bonded nylon #69/T70 spool can be compared with another documented product in that category. A black nylon spool without a size cannot join that comparison just because both are marketed for leather. The missing specification matters more than a lower price per spool.
Do not convert size into a machine guarantee
A thread that belongs to a common leatherworking size category is not automatically suitable for all leather sewing machines. Check the machine's published thread range, the allowed needle sizes, and any differences between upper and bobbin capacity. A household machine described as heavy duty may have very different limits from an industrial compound-feed machine.
The industrial machine guide helps explain that distinction. If your intended thread falls outside the machine's documented range, selecting a larger needle alone does not establish compatibility. Treat the thread, needle, hook system, and material as a working combination.
Use supplier guidance to choose a needle
Start with the thread supplier's suggested needle range, then intersect it with the machine's allowed system and sizes. Test within that overlap. The goal is smooth thread passage and consistent stitch formation without unnecessarily large holes. Our leather needle size guide describes a controlled comparison.
Avoid publishing or relying on one rigid needle chart for every thread carrying a similar number. Product construction and material resistance can affect the useful pairing. A chart is a starting reference; the machine manual and a representative sewn sample decide whether the combination works for your project.
Why size alone does not predict appearance
The same nominal thread size can look different against two leather colors or with different point shapes and stitch spacing. A pale thread on a dark smooth panel appears more prominent than a close color match on a textured surface. Inspect an actual seam before buying a large quantity for a visible design.
Keep topstitch appearance separate from seam function. Choosing a heavier thread for a bold border may require a different supported machine setup, while hidden assembly seams may need another combination. Our stitch length guide discusses spacing as another deliberate design decision rather than a fixed number tied to thread size.
Compare quantity without losing the specification
Only compare price by length after establishing that the products meet the same requirements. A cheaper spool of unknown size is not necessarily a saving, especially if it creates setup work or cannot be used. Confirm whether the advertised quantity refers to one spool or an entire color set.
For a multicolor pack, calculate how much of the needed color is actually included. Eight colors are useful for varied small projects, but most of that inventory may sit unused during a run of identical black bags. Buy around the planned work, not the largest total figure printed in the listing.
Keep a substitution record
When switching products, retain a short length of the old thread and a successful seam sample. Record the new product's full label and sew a comparison using the actual construction. Note any changes to the needle or tension so the result can be repeated.
If the new thread fails, return to those records rather than concluding that the entire size category is wrong. A consistent specification makes troubleshooting and restocking easier. It also prevents a familiar term such as #69 from hiding differences that matter to your machine and finished leatherwork.