Note: standard pressure is 29.92126, if you're trying to test for zero.

Like the FAA, this model uses the dry air approximation. Density altitude is also affected by humidity. For a more exact calculator that includes the dewpoint, go here. That calculator will yield the dry-air result if you set the dewpoint at -65°C (-85°F).

On the hottest, most humid day of the year, the dry-air approximation will
generally give a result that is too low by 500 feet at most. If this is
critical to your operation, it may be time to re-think the whole thing. Your
airplane's performance numbers were generated by skilled test pilots with
brand-new engines, on smooth runways. What are **you** flying and where?