logP

logP is the standard numerical scorecard for how greasy a molecule is. Imagine shaking a compound in a bottle containing two layers, oily octanol on top and water below, then waiting for it to settle into both. logP is the logarithm of the ratio of concentrations: how much sits in the oil versus the water.

Because it is a logarithm, every whole-number step is a tenfold change. A logP of 0 means the molecule splits evenly between the two layers; a logP of 3 means it is a thousand times more concentrated in oil; a negative logP means it prefers water. Crucially, logP refers to the neutral, un-ionized form of the molecule only.

This makes logP a clean, intrinsic measure of lipophilicity, but also a partly artificial one for ionizable drugs, since at body pH much of the molecule may be charged. For those compounds, the pH-aware cousin logD is usually more predictive of real behavior. logP can be measured experimentally or estimated by software (often written clogP for the calculated value).