Since gases fill the entirety of the container which they occupy, the volumetric thermal expansion coefficient at constant pressure, , is the only one of interest.
For an ideal gas, a formula can be readily obtained by differentiation of the ideal gas law, . This yields
where  is the pressure,  is the molar volume (, with  the total number of moles of gas),  is the absolute temperature and  is equal to the gas constant.
For an isobaric thermal expansion, , so that  and the isobaric thermal expansion coefficient is:
which is a strong function of temperature; doubling the temperature will halve the thermal expansion coefficient.