When scuba diving what is the pressure at thirty feet?

The method is the same, but the formula differs depending on whether you want it in meters or feet.

The easy method in feet is to add .3 atmospheres (ata) every 10 feet (or every 3 meters) and then add a surface atmosphere, so 30 feet would be:

[(.3 x 3=.9) +1] = 1.9 ata

The simple formula for feet is: (Depth + 33)/33

For meters it is: (Depth +10)/10

One atm is defined as 101.325 kPa or 14.7 psig. A somewhat newer unit has been introduced to replace atm, particularly when working in metric units. The "bar" is defined as exactly 100 kPa or 14.5 psig. One bar is equivalent to 32.81 feet of sea water.