In this study, a digital realisation of the IEC (International Electrotechnical Committee) flickermeter using root mean square (RMS) of the voltage waveform as its input, instead of the voltage waveform, is presented. The aim of this research work is to compute the flicker severity according to the IEC flickermeter standard, IEC 61000-4-15, when only the RMS values of the voltage waveform are available. Hence, a new method is proposed to compute the light flicker directly from half-cycle-RMS values of the voltage, which are computed based on the IEC power quality standard, IEC 61000-4-30. Beginning from the signal model for a voltage with flicker provided in IEC 61000-4-15, the RMS of the voltage waveform is derived in terms of the flicker frequency components and the corresponding amplitudes. It has been shown on simulation data that short-term flicker severity can be computed by the proposed method with an average error rate of 0.021%. The method has also been applied on field data collected at a transformer substation supplying an electric arc furnace plant, and has been shown to successfully generate flicker severity from RMS voltage waveforms, when compared with the IEC flickermeter results.