We describe a simplified technique for the acquisition of off‐axis beam‐quality data (i.e., half‐value layers in water) for medical linear accelerators. A measurement protocol is presented in which an ordinary beam‐scanning water phantom is used to accurately position an ionization chamber at appropriate measuring positions. Attenuation coefficients are calculated by performing a regression on the measured data. The technique uses the smallest fields for which lateral electron equilibrium can be established.