Inspired in the relative error between two quantities, we define a functional ΔD that operates on two non‐negative scalar fields, which are integrable in a given open connected set D. We prove that ΔD defines a metric, but not an ultrametric nor a translation invariant metric. We show how to apply ΔD to evaluate the performance of analytical approximations of PDEs. The principal advantage of using ΔD instead of other distances given in the literature is that the value given by ΔD has a very easy interpretation: a value close to 0 means relatively near, but a value close to 1 means relatively infinitely far. Copyright © 2013 John Wiley & Sons, Ltd.