The aim of this paper is to describe a user-friendly spreadsheet culling model that was constructed to support economical, optimal breeding and replacement decisions on dairy farms. The model was based on the marginal net revenue technique. Inputs for the model can be entered for specific farm conditions, and the output is easily accessible. In the model, the retention pay-off (RPO) value of individual dairy cows was calculated. The RPO value of a cow is equal to the total additional profits that a producer can expect from trying to keep the cow until her optimal age, taking into account the changes of involuntary removal compared with her immediate replacement. To calculate the RPO values, the future production, revenues, and costs of dairy cows at different levels of milk production with different numbers of days open (DO) were determined. Furthermore, the ranges of carcass value, calf revenues, and the range of involuntary disposal rates of cows within and across lactations were taken into account. To illustrate the model, parameters in the model were chosen to represent a typical Holstein dairy herd in Pennsylvania. The results of this model are very comparable with earlier, more complex models that are more difficult to use on the farm. In addition to using the RPO values to evaluate the decision to breed or replace a cow, the costs per additional DO were estimated. Early conception was most profitable with the costs per additional DO varying from $0 to more than $3/d. The model can be used as a decision-supporting tool for producers, extension personnel, veterinarians, and consultants. In addition, researchers, economists, and government organizations can use the model to determine the costs of culling dairy cows in a disease control program. The model and manual are available at http://cahpwww.vet.upenn.edu/software/econcow.html.