The terms "sperm concentration" and "sperm count" are not the same measure, although the terms are often used interchangeably.
Sperm concentration is the number of sperm/mL in a semen sample. The number of sperm/mL in a sample is typically determined by counting sperm in a counting chamber. According to WHO 5th edition, the lower reference limit for sperm concentration is 15 x 106 spermatozoa/mL.
Sperm count is the total number of sperm in the entire ejaculation. To determine the sperm count, sperm concentration is multiplied by the total volume of the sample submitted. The lower reference limit for sperm count is 39 x 106 spermatozoa per ejaculate.