We present a simple method to determine the refractive index of fluids that is suitable for real-time integrated measurements by imaging a collimated beam through a fluid volume and determining its diameter on a CMOS sensor. Our experimental results agree with the prediction of our analytical model, and the resulting refractive index agrees with the measurements obtained with a commercial refractometer with an RMS deviation of just ±0.003. This method requires only inexpensive components: a light source, two lenses, and a camera sensor; it is suitable for real-time monitoring, and it is essentially unlimited in the range of refractive indices.