某种氨基酸的亲水指数（hydropathy index）是一个描述其支链的亲水性或疏水性程度大小的值。“亲水指数”于1982年被Jack Kyte与Russell Doolittle提出。亲水指数越大，这种氨基酸的疏水性就越强。

亲水指数是根据蒸汽至水相转移的Δ_trG_m以及在形成蛋白质时氨基酸在表面和内部的分布测算的。疏水性最强的两种氨基酸分别是异亮氨酸（亲水指数为4.5）与缬氨酸（亲水指数为4.2）；亲水性最强的两种氨基酸分别是精氨酸（亲水指数为-4.5）和赖氨酸（亲水指数为-3.9）。

Hydrophobicity scales are values that define relative hydrophobicity of amino acid residues. The more positive the value, the more hydrophobic are the amino acids located in that region of the protein. These scales are commonly used to predict the transmembrane alpha-helices of membrane proteins. When consecutively measuring amino acids of a protein, changes in value indicate attraction of specific protein regions towards the hydrophobic region inside lipid bilayer.