A simplified method of designing R-C active low-pass filters is presented. The filters employ solid-state differential operational amplifiers, and are designed to present high input impedance and low output impedance. R-C active filters can provide stable and distortion-free operation at low frequencies without the inherent limitations of passive R-C and L-C filters. Two active low-pass filters suitable for use in electromagnetic flowmeter output stages are illustrated; one with a pass-band gain of ×1, uniform frequency response to approximately 20 c/s and linear phase shift of 3°/cycle per second; and a second with a pass-band gain of ×2, uniform frequency response to approximately 30 c/s and linear phase shift of 2·2°/cycle per second. The attenuation rate of these filters is 12 dB/octave. A more versatile R-C active low-pass filter has been developed using two similar stages in cascade. The choice of three cut-off frequencies, 25 c/s, 50 c/s and 100 c/s is provided. Pass-band gain is ×1, while attenuation rates of 12 dB/octave or 24 dB/octave are selected by taking the output from either the first or second stage. Dynamic calibration shows uniform frequency response and linear phase shift in the pass-band, while the response to a step function shows 3 per cent overshoot from the 12 dB/octave channel, and 11 per cent overshoot from the 24 dB/octave channel.