To understand the term Thread count first you should understand the term thread density. The thread density means number of yarns per unit length of the fabric in two principal directions. The two principal direction means length/longitudinal and width/latitudinal of the fabric. In weaving terms length direction is called the warp direction because the yarns running in this direction are called warp yarns and the width direction is called the weft direction because the yarns running in this direction are called weft yarns. The figure below shows the warp and weft directions.
What is thread count?
So the thread density means number of warp yarns and number of weft yarns per unit length. Usually one inch is taken as unit of length. Warp yarns are sometimes also called as ends and weft yarns are called as picks or fillings. Therefore, warp yarns/inch or ends/inch is warp density of the fabric. Similarly weft yarns/inch or picks/inch or fillings/inch is weft density in the fabric.
Now let’s discuss what is thread count of the fabric? Thread count of the fabric or “T” or TPI of the fabric is actually the sum of warp density/inch and weft density/inch. Therefore, the thread count or T or TPI of the fabric can be calculated and expressed as shown in figure below.
So the thread count refers to the total number of threads in every square inch of a woven fabric especially for bed sheeting fabrics and work wear. It is a measure for fabric quality and durability and indicates how tightly the fabric has been woven. To go technical, the total number of warp threads (length) and weft threads (width) per square inch of fabric. ie thread density in a certain area of your fabric. The thread count is the number of threads counted thus added together.
The alternative term used for thread count indicates its meaning well – threads per inch (TPI). So a 400-count fabric will have 200 warp threads and 200 weft threads in a square inch. In countries like Australia, France and Italy the thread count is calculated every 10 square cm which measures 3.16cm x 3.16cm. The count in threads per 10 cm square will tend to be higher than threads per square inch. You have to be aware of this difference if you shop internationally.
Normally a high thread count is a good indicator as to the fabric being of good quality especially for bed sheet fabrics and work wear. But this also depends a lot on the quality of the yarns and the way it is woven and even where it is made. Some low thread count fabric may be of finer quality than a densely woven fabric woven with low-quality yarn.