Since the initial detection in dairy herds in March 2024, Highly Pathogenic Avian Influenza H5N1 has spread extensively in the United States with over 1000 confirmed cases in dairy cattle across 17 states. Data on within-herd transmission of H5N1 are limited with fundamental knowledge gaps. We used a simplified disease transmission model and an Approximate Bayesian Computation (ABC) algorithm to estimate H5N1 within-herd transmission model parameters using disease morbidity, laboratory testing data from an outbreak herd, and experimental inoculation studies. The estimated adequate contact rate was used to simulate disease spread and predict the time to exceed various threshold fractions of cattle with clinical signs. The estimated adequate contact rate in the baseline model scenario was 0.78 (95% C.I.,0.64–0.97) per day and the estimated basic reproduction number was 8 (95% C.I.,7–11). Based on simulation model predictions for a known infected herd with 3,433 lactating cows, it took more than 2 weeks from disease introduction to attain a clinical signs prevalence threshold of 5 percent. The estimated parameters are essential for informing surveillance design, outbreak management approaches, risk analyses, and regional transmission models. Incorporating data from future experimental studies and outbreak herds may enhance model precision and help characterize the variability of transmission patterns.