Hydrotest pressure calculator: hydrostatic and pneumatic test pressure from MAWP

Work out the minimum hydrostatic or pneumatic test pressure for a pressure vessel from its MAWP and the stress ratio you enter. For a hydrostatic test, see how much higher the pressure is at the low point once the test liquid's static head is added.

Free hydrotest and pneumatic test pressure calculator in psi, kPa or bar, with a show-the-steps panel. You enter the allowable stresses or their ratio from your own code book; no stress tables are included. No account needed.

Test, MAWP and stress ratio

Results update as you type. All pressures are gauge pressures. You enter the allowable stresses or their ratio from your code edition.

Test type

MAWP (psi gauge)Pressure unit

Use the MAWP on the nameplate or data report, referred to the top of the vessel. 150 psi is an example.

Stress ratio input

Allowable stress at test temperatureAllowable stress at design temperature (same unit)

Use the lowest ratio among the vessel's pressure parts, from your own code edition. The default stresses are examples.

Static head at the low point

Vertical height, top to low point (ft)Height unitTest liquid specific gravity

Use vertical height, not vessel length. For a horizontal drum this is about the outside diameter; for a tower it is the full height of the liquid column.

Minimum hydrostatic test pressure at the top

216.667 psi

Static head of test liquid: 5.20233 psi
Pressure at the low point: 221.869 psi

Show the steps

Stress ratio: 20,000 ÷ 18,000 = 1.11111
Minimum test pressure at the top: 1.3 × 150 × 1.11111 = 216.667 psi
Static head of the test liquid: SG 1 × 1,000 kg/m³ × 9.80665 m/s² × 3.6576 m = 5.20233 psi
Pressure at the low point: 216.667 + 5.20233 = 221.869 psi

Formulas used

MAWP is the maximum allowable working pressure. LSR is the lowest stress ratio: allowable stress at test temperature divided by allowable stress at design temperature, taken across the vessel's pressure-boundary materials. UG-100(b) leaves bolting out of the LSR for a pneumatic test, except for bolts that the test would load close to their yield strength. For the hydrostatic case, check the UG-99(b) code text in your edition for how it treats bolting. The formulas are:

Hydrostatic test, minimum: P_test = 1.3 × MAWP × LSR. ASME Section VIII, Division 1, UG-99(b).
Pneumatic test, minimum: P_test = 1.1 × MAWP × LSR. ASME Section VIII, Division 1, UG-100.
Static head of the test liquid: Δp = ρ × g × h, with ρ = SG × 1,000 kg/m³ and g = 9.80665 m/s². In US units, about 0.433 psi per foot of water.
Pressure at the low point = test pressure at the top + Δp.

Both test forms are printed by Engineers Edge; the paragraph numbers are from the code. The static head formula is from NASA Glenn Research Center, and standard gravity from NIST. Open your own copy of UG-99 and UG-100 for the full rules.

Worked example: hydrostatic test with static head

These are fictional study inputs, not values from any stress table or any real vessel. MAWP is 150 psi. The allowable stress is entered as 20,000 psi at test temperature and 18,000 psi at design temperature. The vessel stands 12 ft from its top to its low point and is filled with water, SG 1.

Stress ratio: 20,000 ÷ 18,000 = 1.11111.
Minimum hydrostatic test pressure at the top: 1.3 × 150 × 1.11111 = 216.667 psi.
Static head: 12 ft = 3.6576 m, so 1,000 × 9.80665 × 3.6576 = 35,868.8 Pa = 5.20233 psi. The 0.433 shortcut gives 5.196 psi.
Pressure at the low point: 216.667 + 5.20233 = 221.869 psi.

A pneumatic test of the same vessel uses 1.1 instead of 1.3: 1.1 × 150 × 1.11111 = 183.333 psi. The weight of a gas column is negligible, so the pressure is the same at the top and the low point.

Worked example in SI units

A fictional vessel with MAWP 10 bar, an LSR of 1 and 5 m of water from top to low point. The hydrostatic test pressure is 1.3 × 10 × 1 = 13 bar at the top. Static head is 1,000 × 9.80665 × 5 = 49,033.25 Pa = 0.490333 bar, so the low point sees 13.4903 bar. A pneumatic test would be 1.1 × 10 × 1 = 11 bar.

Top of the vessel or the low point?

The multiplier sets a minimum, and UG-99(b) applies it at every point in the vessel. The top sees the lowest pressure, so with the test gauge at the top, a reading at or above the minimum meets it everywhere. Every lower point sees more than the gauge reading by the static head of the liquid above it. On a tall tower that difference is large, so check the low-point pressure against what the lowest parts and any attached components can take.

Hydrotest pressure calculator FAQ

Why does the hydrotest pressure change with the stress ratio?

The hydrostatic minimum is 1.3 × MAWP × LSR, and LSR is the allowable stress at test temperature divided by the allowable stress at design temperature. In the worked example, 20,000 ÷ 18,000 = 1.11111 makes 1.3 × 150 × 1.11111 = 216.667 psi. With an LSR of 1 and a 10 bar MAWP, the minimum is 13 bar.

Is the pressure at the low point higher than the gauge reading?

For a hydrostatic test, yes. The low point sees the test pressure at the top plus the static head of the liquid above it. For 12 ft of water that is 5.20233 psi, so 216.667 + 5.20233 = 221.869 psi. The top sees the lowest pressure, so check the low-point pressure against what the lowest parts and attached components can take.

How is a pneumatic test pressure different?

The minimum uses 1.1 instead of 1.3: 1.1 × 150 × 1.11111 = 183.333 psi for the same vessel. The weight of a gas column is negligible, so the pressure is the same at the top and the low point. The calculator does not plan the pressurizing steps or the safety measures your procedure requires.

Can I use the result as the test pressure for my vessel?

No. The calculator does not look up allowable stresses, pick the governing part for the LSR, or check that any part can take the test pressure. The governing code edition, the nameplate and manufacturer's data report, your approved test procedure and the responsible inspector or engineer control.

What this calculator does not do

It does not look up allowable stresses, pick the governing part for the LSR, check that any part can take the test pressure, set the examination pressure, or cover other divisions, piping or other code editions. For a pneumatic test it does not plan the pressurizing steps or the safety measures your procedure requires.

The governing code edition, the vessel's nameplate and manufacturer's data report, your approved test procedure and the responsible inspector or engineer control. This tool does not set a test pressure for a real vessel or authorize a test.

Related tools: work out the MAWP itself with the MAWP and thickness calculator, and the head between any two elevations with the static head calculator. Section I.A.6 of the API 510 Body of Knowledge lists test pressure compensated for temperature; see the API 510 exam page.