Introduction The use of ultrasound to measure back fat thickness (BET) of cows is established in modern dairy herd management. The measurement of BFT enables an estimation of the cow's energy balance. An optimal range of conditions for German Black Pied cows and crossbreeds with Holstein Friesian has been published but there are only few published studies of BET of dual-purpose Simmental cows. The aim of this study was to determine an optimal body condition range for Simmental cows to be used in dairy herd management. Materials and Methods The back fat thickness of Simmental cows was evaluated over a period of more than three years on eleven Austrian dairy farms. Herd sizes ranged from 27 to 99 cows and average milk yield was 5,715 to 10,051 kg per year. A total of 5,026 BFT-values were analysed. The mean BET each day was calculated and categorized into classes from -60 days in milk (DIM) to 360 DIM. A BET reference curve for Simmental cows was computed by a third-order regression model. Regression equations were established for lactating and dry-off cows. Results The regression model revealed a mean BET for cows at drying off of 16 to 18 mm. After parturition, the average nadir of BFT was reached on day 110 with a value of 12 mm. The BET values were a good fit, as indicated by coefficients of determination of 0.62 and 0.95 for cows at drying off and lactating cows. Conclusions Results of BET regression revealed a reference of 16 to 18 mm BFT for Simmental cows at drying off. After parturition, the body condition reached its nadir at about 110 DIM with a mean BFT of 12 mm. This is the first description of a BFT reference curve for Simmental cows. The reference curve should help farmers and veterinarians monitor the body condition of cows and should contribute to modern herd health management.