Simply put, thread count is a measure of the fineness or coarseness of fabric. It is measured by how many individual threads there are in one square inch (or in ten square centimeter (3.16cm x 3.16 cm) for countries using the metric system) of the sheet. Note that thread count is not an indicator of the quality of fabric; rather a description of what the fabric should feel like, and an indication of how tightly the fabric has been woven. This is an industry standard that is used to regulate how sheets are marketed and how they are labeled. While this cannot be an exact science, mainly due to the differences in the weave type and quality of the fiber materials, thread count is still a reasonably reliable way of determining how luxurious a set of sheets is going to feel. 

Technically speaking, thread count is measured by counting all of the threads in one square inch (or in ten square centimeter) of fabric, both those going across (weft) and those running lengthwise (warp). The thread count is the number of threads counted along two sides (up and across) of the square inch (or of ten square centimeter), added together. This means that if a sheet says that it has 400 thread count (sometimes denoted by 400tc), it has 200 threads running lengthwise and 200 threads running across. These thread counts usually span from 100 to 800, while some manufacturers claim to sell sheets of up to 1,200 threads per square inch (or per ten square centimeter).

It is worth emphasizing that quality of fiber is a key contributing factor to the overall quality, feel and durability of fabric. In order to fit more threads into a square inch (or ten square centimeter), the selected threads will have to be finer, and therefore, softer, than those used in sheets with less thread count. With softer and longer staple in the fiber material used in the fabric (such as Egyptian cotton fiber, Pima cotton fiber, Supima cotton fiber), higher thread count sheets are texturally denser, thicker and more durable. As a general rule of thumb, the higher the thread count, the denser and thicker the texture of the fabric.     

When it comes to picking sheets based on thread count, staying above 225 and below 400 is usually recommended. While there is generally a noticeable difference in between low thread count sheets and higher ones (but usually not beyond 400), most people cannot tell the difference between 400 thread count sheets and 800 thread count ones. Also, keep in mind that the higher the thread count, the higher the price.

Some people believe they can save money by purchasing lower thread count sheets, especially if they do not mind the sheets being of a lower quality. However, sheets that have fewer than 150 threads per square inch usually will not stand up well to the common wear and tear of usage. Bear in mind that the fiber material matters just as much as the thread count does. While 200tc cotton sheets might not be soft enough for your tastes, 200tc silk ones may be far softer, simply because the threads themselves are naturally softer.