Units · NIST SP 811

Plant unit converter: psi to bar, gpm, °F to °C

Match the data sheet units to the gauge on your plant.

Type a pressure, flow or temperature in any listed unit and read it in every other unit as you type.

Last updated 2026-09-27. Math runs in your browser, no data leaves your computer.

Each card converts as you type, so there is no button to press. The factors are the NIST Special Publication 811 values, and the answers carry more digits than a gauge can read so that you round once, at the end.

Pressure

Convert gauge to gauge and absolute to absolute, because the tool does not add atmospheric pressure between psig and psia.

    Flow

    These are volume flows, so a pump curve in US gallons per minute converts directly, but the imperial gallon is a larger unit and will not.

      Temperature

      This converts a temperature reading, not a temperature difference, because a difference of 1 °C equals 1.8 °F with no offset.

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        Where the factors come from

        Mixed units are how a correct part becomes a wrong part. A hose, valve or pump gets specified from a data sheet in one system and checked against a plant standard in the other, and a rough mental conversion lets an under rated item through goods receiving and onto the line.

        Every factor on this page comes from NIST Special Publication 811, the NIST guide to the SI. One psi is 6.894757 kPa, one bar is exactly 100 kPa, and one MPa is 1,000 kPa. One US gallon is 3.785412 liters, so one US gpm is 3.785412 L/min, and one m3/h is 1,000 liters over 60 minutes.

        Temperatures follow the NIST formulas: Celsius is Fahrenheit minus 32, divided by 1.8, and kelvin is Celsius plus 273.15. The tool refuses readings below absolute zero, since those can only come from a typing error or the wrong unit.

        Worked example: a hose rated in bar on a line rated in psi

        The booking. The plant standard calls for transfer hose rated to at least 150 psi, and a supplier quotes an assembly with a working pressure of 10 bar. The buyer remembers roughly 15 psi to the bar, reads 10 bar as about 150 psi, and approves the quote, which looks fine on paper.

        The failure. One bar is 14.50 psi, not 15, so the hose is rated to 145.04 psi and sits below the 150 psi the standard requires. Goods receiving rejects the delivery against the purchase order, and the plant either waits for replacements or signs a deviation that the next audit will ask about.

        The fix. Type 150 psi into the pressure card and it returns 10.34 bar, so the buyer asks for the next pressure class the supplier offers at or above that figure. The hose that arrives meets the standard on paper and on the test rig, and it goes straight into service.

        Sizing a tank by liquid level and need the result in gallons or liters? The tank volume calculator handles vertical and horizontal cylinders and gives gallons per inch.

        Correcting a liquid density from the lab temperature to the loading temperature? The density at temperature calculator applies the thermal expansion correction for common chemicals.

        Frequently asked

        How do I convert psi to bar?

        Multiply psi by 6.894757 to get kPa, then divide by 100 to get bar, using the factor in NIST Special Publication 811. One bar is 14.50 psi, so 10 bar is 145.04 psi and 150 psi is 10.34 bar. The same factor works for gauge and absolute readings, as long as you keep both sides on the same basis.

        How do I convert US gpm to m3/h?

        One US gallon is 3.785412 liters, so 1 gpm is 3.785412 L/min or 0.2271 m3/h. Going the other way, 1 m3/h is 4.403 US gpm. The imperial gallon is a different, larger unit, so check which gallon a pump curve uses before you convert it.

        What is the difference between psig and psia?

        psig reads pressure above the local atmosphere, while psia reads it above a perfect vacuum, so psia is psig plus the atmospheric pressure at the time and place of the reading. This converter changes units only and does not add or remove atmospheric pressure, so convert gauge to gauge and absolute to absolute.

        How do I convert Fahrenheit to Celsius and kelvin?

        Subtract 32 from the Fahrenheit reading and divide by 1.8 to get Celsius, then add 273.15 to get kelvin, as set out in NIST Special Publication 811. A tank at 122 degrees Fahrenheit is at 50 degrees Celsius, or 323.15 K.

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