This calculator gives you two options:
If you input altitude, it will calculate atmospheric pressure or
if you input atmospheric pressure it will calculate altitude.

Example 1:
What is the atmospheric pressure at the peak of Mount Everest (altitude 29,029 feet)?
Enter 29029 (no commas) in the "Quantity" box.
Select "Feet" from the "Type of Unit" drop down menu.
Click "Enter"
and your answer is 3.5493e+4 pascals, 354.93 millibars and 6 other units.

Example 2:
If the atmospheric pressure is 14.25 pounds per square inch, what is the altitude?
Enter 14.25 in the "Quantity" box.
Select "Pounds per Square Inch" from the "Type of Unit" drop down menu.
Click "Enter"
and your answer is 852.74 feet, 259.91 meters and 2 other units.