In the design of microwave linear amplifiers , s parameters are used extensively. It is assumed that the amplifier will be bilateral. i.e. the s parameter, s12 will be non – zero. However, if the effect of s12 on gain is relatively low, then we can accept this and assume that s12 is zero. The reverse transmission from the output to the input of the amplifier is negligible. Under this condition, the design of the amplifier can be done under a unilateral assumption. This type of design is much simpler than a full bilateral amplifier design. A way to quantify the conditions under which we can, indeed, use the unilateral assumption is to use the unilateral figure of merit U. This predicts the amount of error in the gain in dB, if the unilateral assumption is used. If the error in gain is small enough, then the designer can revert to a unilateral design. This saves time and effort. This factor is simply calculated from the s parameters of the device. A simple javascript has been written by the design team at Signal Processing Group Inc. It allows engineers to calculate U quickly. The script can be accessed from the website by visiting complementary items in the menu and selecting the calculator item in the list.