This calculator is intended to help you determine the reference efficiency of any loudspeaker. There is no standard for stating the efficiency of a loudspeaker, so some manufacturers will give a rating at 1kHz for a subwoofer. This makes the speaker seem like it is a lot more efficient, but it is unfair when comparing to another subwoofer with an efficiency rated at 100 Hz (and not many subwoofers are used at 1kHz anyway). This formula will give you a reference number that will allow you to compare subwoofers on an equal basis.
This will give you the efficiency as a decimal. To convert it to a percentage, multiply by 100. Common percentages range from 0.3 to about 1.6.