Pressure unit and gauge–absolute conversion tool

What the pressure conversion tool does

Pressure errors in compressed-air work are rarely arithmetic errors. They come from mixing units, mixing gauge and absolute values, or forgetting that atmospheric pressure is part of the conversion. A nameplate in psig, a dryer data sheet in bar(a), a gauge in kgf/cm² and a vacuum pump rated in mmHg can all appear in the same project.

The calculator above answers three questions at once:

• What is this pressure in another unit (bar, kPa, MPa, psi, kgf/cm² and common vacuum units)?
• Is the value gauge, absolute, vacuum or differential?
• Which atmospheric pressure was used when gauge and absolute values were converted?

The result carries its reference tag, such as bar(g) or bar(a), so it can be passed safely to the FAD and reference condition conversion tool, the receiver transient storage calculator or the pipe-sizing tools.

Gauge, absolute, vacuum and differential pressure

A pressure value without its reference is incomplete. “7 bar” on a site report almost always means 7 bar(g), but a process data sheet may mean 7 bar(a), which is about 1 bar less in gauge terms. A differential pressure across a filter is neither gauge nor absolute; label it as mbar or bar differential and keep it separate.

The equations behind the tool

• Absolute pressure = gauge pressure + atmospheric pressure
• Gauge pressure = absolute pressure − atmospheric pressure
• Vacuum depression = atmospheric pressure − absolute pressure

All three terms must be in the same unit before you add or subtract. For a plain unit conversion, such as bar(g) to psig, only a scale factor is needed and atmospheric pressure does not enter. The common factors are:

Why local atmospheric pressure matters in India

Atmospheric pressure falls with altitude. Most coastal sites, such as Visakhapatnam, Kakinada, Chennai or Mumbai, sit close to the standard 1.013 bar. Hyderabad and Secunderabad, on the Deccan plateau, are typically around 0.95 bar. Hill and plateau sites elsewhere in India can be lower still. Weather changes atmospheric pressure slightly, but for compressor work the altitude effect is the one that counts.

A common practical approximation for atmospheric pressure at altitude h in metres is p = 1.01325 × (1 − 2.2558 × 10⁻⁵ × h)^5.256 bar. At 540 m this gives about 0.950 bar. If you have a calibrated barometer reading on site, use it instead.

The difference looks small, but it affects:

• Compression ratio. 7 bar(g) at sea level is 8.01 bar(a), a ratio of about 7.9. In Hyderabad it is 7.95 bar(a) from an inlet of 0.95 bar(a), a ratio of about 8.4. The compressor works harder and discharge temperature rises slightly.
• Free air delivery. Delivered air quoted at reference conditions must be corrected for local inlet pressure and temperature.
• Receiver and storage calculations. Stored air depends on absolute pressures, not gauge readings.
• Diesel portable compressors. On high-pressure borewell and drilling packages, engine output and airend ratio both change with altitude, which is why makers publish altitude ratings.

Worked examples

Example 1: nameplate in psig, site gauge in bar

A portable compressor is rated 150 psig. In bar(g): 150 ÷ 14.504 = 10.34 bar(g). No atmospheric correction is needed because both values are gauge.

Example 2: gauge to absolute in Hyderabad

The header runs at 7.0 bar(g). With local atmospheric pressure of 0.95 bar, the absolute pressure is 7.95 bar(a), or 115.3 psia.

Example 3: absolute value from a data sheet

A dryer is rated at an inlet of 8 bar(a). At sea level that is 6.99 bar(g); in Hyderabad it is 7.05 bar(g). Check which condition the maker used before applying correction factors from the dryer correction factor worksheet.

Example 4: vacuum reading

A vacuum gauge reads −0.60 bar(g) on a conveying line in Hyderabad. Absolute pressure = 0.95 − 0.60 = 0.35 bar(a). The same −0.60 bar(g) at sea level would be 0.41 bar(a). Vacuum equipment is rated in absolute terms, so this difference changes what the pump must achieve.

Example 5: old gauge in kgf/cm²

An old receiver gauge shows 8 kgf/cm². In bar: 8 ÷ 1.0197 = 7.85 bar(g). Treating kgf/cm² as equal to bar overstates pressure by about 2%, which matters when checking against a safety-valve set pressure.

How to read and use the results

1. Check that the reference tag on the output is the one your next calculation expects. Gas-law and FAD calculations need absolute values.
2. Note the atmospheric pressure used and keep it with the result, so another engineer can repeat the conversion.
3. Round only at the end. Rounding 14.504 to 14 psi per bar introduces an error of more than 3%.
4. For high-pressure work, such as 25 bar (362.6 psig) borewell air or 30 bar PET air, the gauge–absolute difference is a small fraction of the value, but unit errors are still serious.

Common conversion mistakes on site

• Adding 1 bar to every gauge reading regardless of altitude, then using the result in a sensitive calculation.
• Reading psi on a dual-scale gauge as bar, or the reverse. On a 0–16 bar gauge the psi scale reaches about 232 psi.
• Comparing a compressor rated in bar(g) with a data sheet quoting MPa(a) without converting both units and reference.
• Treating a filter differential of 350 mbar as 0.35 bar(g).
• Assuming a negative gauge reading below −1 bar is possible. It is not: −0.95 bar(g) is close to a perfect vacuum in Hyderabad.

Pressure ranges you will meet in compressor work

Knowing the usual ranges helps you spot a conversion error at a glance. If a converted figure falls far outside the range expected for that equipment, check the unit and reference before using it.

For example, a borewell compressor reported as “25” is almost certainly 25 bar(g), not 25 psig, while a breaker quoted at “100” is in psig. A dryer or filter differential of “0.35” is in bar, and “350” in mbar. Reading the number against its duty is a simple and reliable cross-check.

Limits of the tool

The converter handles units and pressure references. It does not correct for gauge accuracy, temperature effects on instruments or the elevation difference between a gauge and its tapping point on liquid-filled lines. It does not decide whether a vessel, hose or fitting is safe at a given pressure. Those decisions belong to the equipment ratings, the relevant codes and the responsible engineer.

Barg to bar: what the “g” means

“barg” and “bar(g)” mean bar gauge: pressure measured above the local atmosphere, which is what almost every compressor gauge, pressure switch and controller shows. “bara” or “bar(a)” means bar absolute: pressure measured above a perfect vacuum. Plain “bar” with no suffix is ambiguous. On site and on most nameplates it usually means bar(g). In thermodynamic data, gas tables and many process data sheets it usually means bar(a). So converting barg to bar means one of two things. If “bar” means gauge, nothing changes. If it means absolute, add the local atmospheric pressure: bar(a) = bar(g) + p_atm. Converting bar absolute to bar gauge is the reverse: bar(g) = bar(a) − p_atm. When a document uses plain “bar”, ask which is meant before you use the number.

Ready reckoner: bar(g) to bar(a), psig, kPa, MPa and kgf/cm²

The table converts common gauge pressures at two reference atmospheres: standard sea level (1.01325 bar(a)) and Hyderabad and Secunderabad (0.950 bar(a)). The Hyderabad figure comes from the standard barometric formula p = 1.01325 × (1 − 2.25577 × 10⁻⁵ × h)^5.25588 bar(a), with h = 540 m. That gives 0.9500 bar(a). Weather moves the real value by roughly ±0.01 bar. All values are computed from 1 bar = 14.5038 psi = 100 kPa = 0.1 MPa and 1 kgf/cm² = 0.980665 bar. Sea-level absolute values are rounded to 3 decimals, Hyderabad absolute values to 3 decimals, psi to 1 decimal and kgf/cm² to 2 decimals.

Read the first row carefully. A gauge reading of 0 bar(g) is not zero pressure. It is the local atmosphere, about 1.013 bar(a) at the coast and about 0.95 bar(a) in Hyderabad. Likewise, 1 bar gauge pressure is 2.013 bar(a) at sea level and 1.950 bar(a) in Hyderabad. The gauge-only columns (psig, kPa(g), MPa(g), kgf/cm²) are pure unit conversions and do not depend on altitude.

kPa gauge to kPa absolute

Data sheets, transmitters and vacuum equipment often use kPa. The rule is the same as for bar, but the atmospheric term is in kPa: kPa(a) = kPa(g) + p_atm. Standard atmosphere is 101.325 kPa(a). Hyderabad is about 95.0 kPa(a) and Bengaluru about 90.8 kPa(a).

Example A: transmitter in kPa(g), calculation needs kPa(a)

A header transmitter in Hyderabad reads 600 kPa(g). Absolute pressure = 600 + 95.0 = 695.0 kPa(a), which is 6.950 bar(a). Using the sea-level figure instead (701.3 kPa(a)) would overstate it by 6.3 kPa, or about 0.9%.

Example B: blower data sheet in kPa(a)

A process blower is rated for a discharge of 250 kPa(a) at sea-level test conditions. In gauge terms that is 250 − 101.325 = 148.7 kPa(g), or 1.487 bar(g). Set the site pressure switch in gauge units, not at “250”.

Example C: vacuum in Bengaluru

A vacuum gauge reads −40 kPa(g) at a site about 920 m above sea level. Absolute pressure = 90.8 − 40 = 50.8 kPa(a). The same −40 kPa(g) reading at the coast would be 101.3 − 40 = 61.3 kPa(a). The vacuum pump’s capacity curve is plotted against absolute pressure, so use 50.8 kPa(a) when reading it.

Atmospheric pressure at Indian site altitudes

The table applies the same barometric formula to typical site elevations. Elevations are approximate, and actual sites within a city vary. For a real calculation, use the site’s surveyed elevation or a local barometer reading. The last three columns show what a 7 bar(g) setting means at each site, and the most negative gauge reading physically possible there, which equals full vacuum.

The pressure ratio column explains why the same compressor works harder at altitude. At Leh a 7 bar(g) machine compresses through a ratio of 11.64 instead of 7.91. Discharge temperature, power per unit of free air and the load on each stage all rise. Machines for high sites need the maker’s altitude rating, not a sea-level selection.

Compression ratio: why it must use absolute pressure

Pressure ratio is r = p₂(a) ÷ p₁(a). Gas laws, ideal discharge temperature and compressor power all use this ratio, and all of them fail if gauge values are used. A gauge suction pressure is usually 0, which would make the ratio infinite.

Worked example: single-stage, 7 bar(g), Hyderabad

Suction is at atmosphere, 0.950 bar(a), and air enters at 35 °C (308.15 K). Delivery is 7 bar(g) = 7.950 bar(a).

• Correct ratio: 7.950 ÷ 0.950 = 8.37.
• Common mistake: using 7 bar(g) ÷ 1 bar, giving a ratio of 7.00, which understates the true ratio by 16%.
• Ideal adiabatic discharge temperature, T₂ = T₁ × r^(γ−1)/γ with γ = 1.4, so the exponent is 0.2857:
• Correct: 308.15 × 8.37^0.2857 = 565.4 K = 292 °C.
• Wrong ratio: 308.15 × 7.00^0.2857 = 537.3 K = 264 °C.

The gauge-based figure is 28 °C too low. That error matters for valve and oil temperature checks on dry reciprocating stages. These are ideal values. Oil-injected screw compressors run much cooler because the oil absorbs the heat.

Worked example: booster from 7 to 30 bar(g), Hyderabad

Here the error runs the other way. Gauge ratio = 30 ÷ 7 = 4.29. Absolute ratio = 30.95 ÷ 7.95 = 3.89. Using gauge values overstates the ratio by about 10% and would point to the wrong booster stage count or power. The direction of the error depends on the pressures involved, so the only safe rule is to convert both pressures to absolute first.

How Reddy Compressors helps

We sell, rent and service compressors of many brands, including ELGi, Atlas Copco, Ingersoll Rand, KAESER and Chicago Pneumatic, as an independent multi-brand specialist rather than an authorised dealer. When we size a compressor, dryer or receiver for a site, we work in consistent units and references and correct for local conditions, including the lower atmospheric pressure on the Deccan plateau and summer ambient temperatures of 40–45 °C, and above 45 °C on open sites. Our engineers also check gauges and transmitters during service visits, because many pressure “problems” turn out to be instrument errors.

We are based at Tarnaka, Secunderabad, with service teams covering Telangana and Andhra Pradesh, and we supply equipment, rentals and spares across India.

What to send us

• The pressure values you are working with, their units and whether they are gauge, absolute or differential
• Site location and approximate altitude
• The compressor, dryer or vessel nameplate data, or a clear photograph
• What the converted figure will be used for: selection, FAD check, receiver sizing or troubleshooting

Frequently asked questions

What is the difference between bar(g) and bar(a)?

Bar(g) is gauge pressure, measured above the local atmosphere, and is what an ordinary pressure gauge shows. Bar(a) is absolute pressure, measured from a perfect vacuum. To convert, add the local atmospheric pressure to the gauge value. At sea level that is about 1.013 bar; at Hyderabad’s altitude it is about 0.95 bar. Gas-law, FAD and receiver calculations must use absolute values.

How do I convert psig to bar?

Divide the psig value by 14.504 to get bar(g). For example, 100 psig is 6.9 bar(g), 150 psig is 10.3 bar(g) and 300 psig is 20.7 bar(g). Both values are gauge pressures, so local atmospheric pressure does not enter this conversion. You only add atmospheric pressure when you need an absolute value such as psia or bar(a).

Is kgf/cm² the same as bar?

No, but it is close. One kgf/cm² equals about 0.981 bar, and one bar equals about 1.02 kgf/cm². Many older Indian gauges and receivers are marked in kgf/cm². Treating the two as equal introduces an error of about 2%, which is small for everyday checks but matters when comparing readings with safety-valve settings or test pressures.

Why does altitude affect compressor performance?

At higher altitude the air at the compressor inlet is at a lower absolute pressure, so each cubic metre drawn in contains less air by mass. To reach the same gauge discharge pressure the compressor must work at a higher compression ratio, which raises discharge temperature and changes delivered capacity. Diesel engines also lose power at altitude, which is why portable compressor makers publish altitude ratings.

What does a negative gauge pressure mean?

A negative gauge pressure means the pressure is below local atmospheric pressure, which is a partial vacuum. To find the absolute value, subtract the depression from local atmospheric pressure. For example, −0.6 bar(g) in Hyderabad is about 0.35 bar(a). A gauge can never read lower than minus the local atmospheric pressure, because that would be below a perfect vacuum.

Which pressure should I use in compressed-air calculations?

Use gauge pressure for setpoints, pressure bands and comparing tool requirements, because that is how equipment is normally specified. Use absolute pressure for anything based on gas laws: compression ratio, free air delivery, receiver storage, dryer performance and leak-flow estimates. Whichever you use, label it clearly and keep the atmospheric pressure you assumed with the result.

How do I convert 1 bar gauge pressure to absolute pressure?

Add the local atmospheric pressure. At sea level, 1 bar(g) + 1.013 bar = 2.013 bar(a), which is 201.3 kPa(a) or 29.2 psia. In Hyderabad and Secunderabad, at about 540 m, atmospheric pressure is about 0.950 bar(a). So 1 bar(g) = 1.950 bar(a), which is 195.0 kPa(a) or 28.3 psia. The rough rule of “add 1 bar” works at the coast. In Hyderabad it gives 2.000 bar(a) instead of 1.950, about 2.6% high. At Leh, where the atmosphere is about 0.658 bar(a), it gives 2.000 instead of 1.658, about 21% high.

Is barg the same as bar?

barg is bar gauge, pressure above the local atmosphere. Plain “bar” is ambiguous. On compressor nameplates and site gauges it almost always means bar(g). In gas tables, physics and many process data sheets it means bar(a). Numerically the two differ by the atmospheric pressure, about 1.013 bar at sea level and about 0.95 bar in Hyderabad. When a document says only “bar”, confirm which is meant before using the figure in a calculation.

How do I convert bar absolute to bar gauge?

Subtract local atmospheric pressure: bar(g) = bar(a) − p_atm. For example, 10 bar(a) is 10 − 1.013 = 8.987 bar(g) at sea level and 10 − 0.950 = 9.050 bar(g) in Hyderabad. If the result is negative, the pressure is below atmosphere, which is vacuum. The lowest possible gauge reading equals minus the local atmospheric pressure, for example −0.95 bar(g) in Hyderabad.

How do I convert kPa gauge to kPa absolute?

Add atmospheric pressure in kPa: kPa(a) = kPa(g) + p_atm. Use 101.325 kPa at sea level, about 95.0 kPa in Hyderabad, about 94.8 kPa in Pune and about 90.8 kPa in Bengaluru. For example, 600 kPa(g) in Hyderabad is 695.0 kPa(a). To go back, subtract the same value. Because 100 kPa equals 1 bar, divide kPa by 100 to get bar, keeping the same gauge or absolute tag.

Related pages

• Compressed-air units and symbol dictionary
• FAD and reference condition conversion tool
• Compressed-air engineering formula register
• Pressure budget builder
• Compressed-air knowledge and calculation centre