Current practice is to ＇correct＇ the actual/gross efficiency curves of hydrocyclones using the Kelsall correction for bypass. This gives corrected efficiency curves for which the efficiency is zero as the size approaches zero and unity when the actual efficiency is unity. The corrected efficiency curves so obtained are used for comparative purposes by plant engineers and designers alike. Following a brief discussion on the present understanding and basis for correcting/＇normalising＇: the efficiency curves, we bring to light that the so-called bypass mechanism suggested by Kelsall is beyond the reach of experimental methods. Consequently, no physical/process explanation can be attributed to it. As such, it is regarded as a simple mathematical transformation only. A generalised mathematical method for normalisation of efficiency curves is then proposed. The important feature of our method is the inclusion of an actual efficiency dependent normalising function It is shown that the Kelsall transformation is the simplest of the available options, when a constant normalising function is assumed. Finally, we illustrate our method with examples, which give normalised efficiency curves closer to the actual efficiency than with Kelsall correction.