In chemistry, the percent transmittance indicates the amount of light that passes through a sample. Absorbance is the quantity of light that the sample absorbs. Higher absorbance generally means lower transmittance. You can measure absorbance by using a spectrophotometer to shine light through a sample; the device displays the absorbance value as a number. Absorbance and the percent transmittance are related by the following equation: Absorbance = -log (percent transmittance/100).

Measure or find out the absorbance value for your sample. For example, the spectrophotometer indicates your sample's absorbance is 0.532.

Multiply the absorbance by -1 to calculate the negative value. In this example, the negative absorbance is 0.532 x (-1) = -0.532.

Raise 10 to the power of the negative absorbance. In this example, 10^(-0.532) = 0.2938.

Multiply the result from the last step by 100 to calculate the percent transmittance. In this example, the percent transmittance is 0.2938 x 100 = 29.38 percent.