Standardized measurement protocols are required to reduce ammonia (NH3) emissions. In vitro measurement of NH3 emissions consists in trapping the emission from an emitting source in an acidic solution under controlled conditions. The objective of this study was to assess the in vitro NH3 measurement method from pig slurry with acid wet traps, as regards to the following: (i) the variation between replicates of NH3 emissions measured in vitro, (ii) the relationships between partial and accumulated emissions, and (iii) the reduction of measurement frequency. For this study, a total of 60 pig slurry samples from different animal types (sows and growing animals) were collected from commercial farms. The coefficient of variation among replicates of accumulated NH3 emission during 15 days was 6.73 %. Emissions tended to decrease with time, and an average reduction of NH3 emissions about 16 % was found in the period 96–240 h with respect to the 0–96-h period. However, samples continued emitting considerable amounts of NH3 after 360 h. Linear regression models allowed predicting emissions accumulated for 15 days using only the first 8 days (R2 > 0.90). Reducing NH3 measurement frequency (from 24 to 48 h) did not significantly affect measured emissions (P > 0.05). The results of this study confirm that replication of measurements is required and a coefficient of variation of 10 % may be established as quality control requirement. The study also suggests that reducing the duration and frequency of measurements is a tangible option to simplify this methodology.