Divide the actual vapor density -- the current amount of moisture in the air -- by the saturation vapor density, or the dew point. If the actual vapor density is 15 g per square meter and the saturation vapor density is 58 g per square meter, the answer is .272727 (15 / 58 = .272727).
Multiply the answer by 100 (.272727 x 100 = 27.2727).
Round the number to the nearest tenth to determine the relative humidity. The relative humidity of the air when the actual vapor density is 15 grams per square meter and the saturation vapor density is 58 g per square meter is 27.27 percent.