Propane and Butane Vapor Pressure Chart
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The pressure a closed vessel settles at when liquid and vapor are both present, from -100 to 140°F (-73 to 60°C), pure and blended.
DocumentREF-VP-01
SeriesEngineering Reference
Revision2026.1
Issued24 September 2026
Use this sheet to predict tank and recovery pressure at a given temperature, to check a relief valve setting against a hot room, and to know when a cold system is pulling vacuum. Values below 0 psig are shown as vacuum in inches of mercury.
Saturation pressure, psig
| °F (°C) | Propane | n-Butane | Isobutane | 75/25 B/P | 50/50 B/P |
|---|---|---|---|---|---|
| -100 (-73) | -24.0 inHg | -29.4 inHg | -28.8 inHg | -27.9 inHg | -26.5 inHg |
| -80 (-62) | -18.7 inHg | -28.6 inHg | -27.5 inHg | -25.9 inHg | -23.4 inHg |
| -60 (-51) | -10.2 inHg | -27.3 inHg | -25.3 inHg | -22.7 inHg | -18.2 inHg |
| -40 (-40) | 1.4 | -25.0 inHg | -21.4 inHg | -17.4 inHg | -10.3 inHg |
| -20 (-29) | 10.7 | -21.1 inHg | -15.4 inHg | -9.6 inHg | 0.7 |
| 0 (-18) | 23.7 | -15.2 inHg | -6.4 inHg | 0.8 | 8.8 |
| 20 (-7) | 41.1 | -6.4 inHg | 3.2 | 8.5 | 19.8 |
| 40 (4) | 63.9 | 3.0 | 11.9 | 18.8 | 34.3 |
| 60 (16) | 93.0 | 11.3 | 23.5 | 32.3 | 52.9 |
| 70 (21) | 110.2 | 16.5 | 30.6 | 40.4 | 64.0 |
| 80 (27) | 129.3 | 22.5 | 38.7 | 49.5 | 76.3 |
| 100 (38) | 173.9 | 36.9 | 58.0 | 71.0 | 105.2 |
| 120 (49) | 227.8 | 55.1 | 82.0 | 97.5 | 140.2 |
| 130 (54) | 258.7 | 65.9 | 95.9 | 112.8 | 160.2 |
| 140 (60) | 292.3 | 77.9 | 111.4 | 129.5 | 182.1 |
Blends are butane / propane by liquid volume at 60°F (16°C). Calculated bubble point (GERG mixture model), within about 1.5 psi of published blend data. 70°F row in bold.
Fixed points
| Solvent | Boiling point at 1 atm | Critical temperature | Critical pressure | Pressure at 70°F (21°C) |
|---|---|---|---|---|
| Propane | -43.8°F (-42.1°C) | 206.1°F (96.7°C) | 616.6 psia | 110 psig |
| n-Butane | 31.1°F (-0.5°C) | 305.6°F (152.0°C) | 550.6 psia | 16.5 psig |
| Isobutane | 10.9°F (-11.8°C) | 274.4°F (134.7°C) | 526.3 psia | 30.6 psig |
Using this chart
- Sealed tank pressure. A tank with any liquid in it sits at the saturation pressure for its liquid temperature, regardless of fill level. A 100 lb tank of propane at 100°F (38°C) reads about 174 psig whether it holds 10 lb or 80 lb.
- Relief valves and ratings. Propane reaches 258 psig at 130°F (54°C), above a 250 psi vessel or clamp rating. Keep propane and propane rich blends out of hot rooms and direct sun, and keep jacket water below the temperature that matches the lowest rated component. See Cryogenic Relief Valve Selection.
- Recovery. Recovery works by holding the collection side warmer (higher pressure) and the solvent tank colder (lower pressure). The pressure difference between the two temperatures on this chart is the driving force.
- Vacuum at the cold end. At -40°F (-40°C) n-butane is at 25 inHg vacuum and propane is only about 1.4 psig. Air leaks into a cold butane system, not out, so leak check under pressure before chilling.
- Blends fall between the two pure solvents, closer to propane than a straight average. Blend values are estimates. A gauge on a tank at a known temperature is the final word.
Sources. NIST Chemistry WebBook, Thermophysical Properties of Fluid Systems (propane C74986, n-butane C106978, isobutane C75285). Reference equations of state: Lemmon et al. 2009 (propane), Bücker and Wagner 2006 (butanes). Blend check: Engineering ToolBox, Propane Butane Mix. psig = psia - 14.696.
Questions about your system? Call the shop at 541-330-6409, Monday to Thursday, 8 to 4 Pacific, or email info@nboler.com.