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Hydrostatic Test Pressure Calculator

The hydrotest pressure is a multiple of design pressure, corrected for the difference in allowable stress between the test temperature and the design temperature. This tool gives bar, MPa and psi, plus the gauge range you need on the rig.

Your numbers

bar

Design pressure at the design temperature.

MPa
MPa

B31.3 limits the ratio ST/S to 6.5.

minutes

Set by the procedure. Many shop procedures use 10 to 60 minutes after stabilisation.

Your results

Test pressure
69
In bar gauge.
Stress ratio used
1.15
Allowable at test temperature divided by allowable at design temperature.
Test pressure in psi
1,001
Test pressure in MPa
6.9
Minimum gauge full scale (1.5 x test)
103.5
Maximum gauge full scale (4 x test)
276

Always confirm the test factor, temperature correction and any maximum test pressure limits in the governing code edition and your approved test procedure.

Save results:

Test pressure vs design pressure

Sweeps Design pressure from 5 to 100, holding your other inputs fixed.

Digitise your test records

Optional review of how to link hydrotest results to work orders and serial numbers in your ERP.

Optional. The tool and every export work without this. No spam.

How to use it

Enter the design pressure and pick the factor that your code requires. For B31.3 piping the hydrostatic test is at least 1.5 times design pressure, adjusted by the ratio ST/S where ST is the allowable stress at test temperature and S is the allowable stress at design temperature. Enter both stresses. The ratio is capped at 6.5 for piping. The primary output is the test pressure in bar, and the other outputs convert it for gauges calibrated in psi or MPa.

Where the numbers come from

Piping and vessel codes require the test to prove the pressure boundary at a level above design while keeping membrane stress below yield. The stress ratio corrects for hot design temperatures: if the metal is weaker at design temperature than at ambient, the test must be higher to give the same margin. Pressure gauges used in the test are typically required to have a range between 1.5 and 4 times the test pressure, with 2 times being the common target for readability.

  • Use ambient allowable stress for ST on a normal shop test.
  • Check that test pressure does not exceed flange or valve body ratings.
  • Record gauge calibration numbers on the test report.

Worked example

A line with a design pressure of 40 bar at a design temperature where S is 120 MPa, tested at ambient where ST is 138 MPa, gives a ratio of 1.15. The B31.3 test pressure is 1.5 x 40 x 1.15 = 69 bar, about 1,001 psi. The gauge must read at least 103.5 bar full scale, and 4 times the test pressure (276 bar) is the upper limit. A 160 bar gauge fits well.

Common mistakes

Teams often apply 1.5 to the operating pressure rather than the design pressure, forget the stress ratio, or exceed the rating of the weakest component such as a valve seat or a spectacle blind. Another trap is air left in the system, which stores energy and causes a pressure drop that looks like a leak. In the ERP, link the test record to the work order and the serial number so the pressure, hold time and gauge ID are traceable.

Frequently Asked Questions

Is the hydrotest factor 1.5 or 1.3?

ASME B31.3 uses 1.5 times design pressure for hydrostatic testing, with the stress ratio correction. ASME VIII Division 1 uses 1.3 in current editions, with older editions using 1.5. Always check the edition on your contract.

Why is the stress ratio capped at 6.5?

In B31.3 the ratio of allowable stresses cannot exceed 6.5 so that the test does not overstress the piping at ambient temperature. The calculator applies the cap you enter.

What gauge range should I use?

Test gauges are normally chosen so the test pressure sits near the middle of the scale, with full scale between 1.5 and 4 times the test pressure. Lower than 1.5 risks overpressure damage to the gauge.

Can I use this for a pneumatic test?

The tool can compute a number, but pneumatic tests store far more energy and have separate code limits and safety rules. Confirm the factor with the code and your safety procedure before choosing it.

If your test records are still on paper, we can help you capture them against work orders and serials in the ERP so each hydrotest is traceable and auditable.

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