The "Highway Capacity Manual" (1997) does not provide a method for the prediction of right turns on a red signal (RTOR). A model for prediction of the RTOR volumes at isolated and coordinated signals is presented. The model has been derived by using clear assumptions and equations consistent with these assumptions. The method was implemented in the form of a spreadsheet and was evaluated without any previous calibration. CORSIM was used to generate benchmark data. The evaluation results show that the model produces unbiased results if the impedance from the queue on the approach with the right turns is the prevailing factor in the RTOR volume. A limited bias appears if the impedance from the flows on the cross street is critical for the RTOR volume. Adequate setting of the model parameters (critical gap and follow-up time) should reduce this bias. A way of estimating delays of right-turning vehicles is briefly addressed.