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The conventional BTPS (body temperature and pressure, saturated with water vapour) correction factor varies with ambient barometric pressure (PB) and many lung function laboratories measure PB daily. The aim was to investigate whether a fixed value for PB could replace daily measurements. PB was measured daily over a 12-month period. The highest and lowest values in Melbourne in the last century were also recorded from data published by the Bureau of Meteorology. Using these PB values, the BTPS factor was determined for a range of spirometer temperatures and compared to the BTPS factors obtained using a fixed ambient pressure of 101.3 kPa. The mean (SD) PB measured over the 12-month period was 102.2 kPa (0.64) with a range of 99.9–103.6 kPa. The level of disagreement between the BTPS factor calculated using a PB of 101.3 kPa instead of the measured value was greater at lower temperatures. Over the extremes of PB during the last century (98.0–104.3 kPa) the use of a standard pressure (101.3 kPa) produced an error in the BTPS factor of ≤ 0.16%. Daily variations in PB do not significantly affect the magnitude of the conventional BTPS correction factor and a fixed value, such as 101.3 kPa at sea level, can be used with little error.