Compute it from the known composition of gasses.

Let “all” the air out of a rigid container (reducing it to regular atmospheric pressure). Weigh it. Now pump it up to 2 atmospheres of pressure. Weigh it again. Divide the added weight by the internal volume.

Look it up in a reference book.

Are any of these on a useful track?

you would be able to desire to take a stable tension vessel (such as a metallic propane tank), fill it with room-temperature air at established sea-point tension (14.7 pounds in step with sq. inch, or one hundred and one.3 kiloPascals), placed it on a delicate scale, and be certain its weight. then you incredibly pressurize it with one extra 14.7 psi (one hundred and one.3 kPa) of air above the ambient tension, and weigh it returned. the load would desire to improve fairly. the form in weight is only by the extra effective mass of air, in accordance to this formula: Mass = Weight / Gravity finally, you establish the quantity of the tank via liberating the gas, filling it with water, and measuring what quantity of water in good shape interior. then you incredibly calculate the density of air at 14.7 psi (sea point tension) employing this formula: Density = Mass / quantity undergo in concepts that air at bigger altitudes is decrease in density, till you attain deep area, at which element there is distinctly much no density in any respect.