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In order to optimize the design of microcalorimeters and bolometers, it is necessary to be able to predict the resolution of any given configuration. For detectors using ion-implanted silicon thermistors, we have sufficient engineering data on their non-ideal characteristics to predict the noise contributions from all currently known sources, given the basic physical parameters of the thermometer. We have constructed an analytical detector model that includes the effect of a sensor hot electron system that is thermally connected to the lattice structure of the detector, and an absorber that is thermally connected, but physically separated from the structure of the detector. The noise analysis incorporates terms for thermistor Johnson and 1/f noise, amplifier noise, load resistor Johnson noise, and thermodynamic fluctuations between the electron and phonon systems in the thermometer as well as between the absorber, the thermistor, and the heat sink. For the calculation, block diagram algebra [1], traditionally used in electrical engineering, has been demonstrated to be a very powerful tool for the complex calculations necessary in our model. The model has been checked by comparing its predictions to data obtained from existing detectors developed for the XRS spectrometer on Astro-E. We also compared the analytical results with a similar model that uses numerical methods to solve the detector differential equations. Rather than review the algebra of block diagrams in this paper, we will describe the effects that are included in the model and the accuracy in the prediction that can be made with it.