Specify the external design pressure. CodeCalc computes the required thickness of the given geometry for the external pressure you specified. If you are designing for a full vacuum, type a value of 14.7 psig (or rounded off to 15 psig) or 1.0133 bars.
CodeCalc calculates the required thickness for both external and internal pressure. Then, the software chooses the greatest required thickness, tr, and proceeds with the calculations.
If external pressure governs, the software automatically reduces the required area of reinforcement by 50 percent.