Sewing thread is one of the basic and very important ingredients of a garment. Thread consumption for lock-stitch (301) is calculated. Using basic geometry (shape) of lock-stitch, a formula has been developed. The formula is mathematically derived and takes into account different variable; i.e. stitch length, SPI, material thickness and interlacing. At the end, formula’s results for 11 samples are compared with practical results. The formula has overestimated the results about 5 % on average. This difference is due to some of the assumptions taken into account.