Article — Pressure Unit Conversion
Pressure Unit Conversion: SI, Gauge vs Absolute, and Depth
Pressure unit conversion routes through the pascal (SI base, 1 Pa = 1 N/m²) and produces values in ten other units: kPa, MPa, bar, mbar, atm, psi, mmHg, inHg, torr, and kg/cm². The conversion handles absolute pressure; gauge pressure requires subtracting 101,325 Pa (sea-level atmosphere) to compare with equipment readings.
The same physical pressure can wear ten different unit labels depending on the industry. Medicine uses mmHg. Meteorology uses hPa. Industrial hydraulics uses bar or MPa. U.S. engineering uses psi. Older European specs use kg/cm². Pressure unit conversion brings them all into one system through the pascal.
The physics behind pressure unit conversion
Pressure is force per unit area. In SI units that is newtons per square meter, named the pascal. The pascal is small for human-scale pressures — atmospheric pressure at sea level is 101,325 Pa, around 14.7 psi or 1.013 bar. The pascal works well as a definitional base but rarely as a working unit.
All ten units that pressure unit conversion handles are linear multiples of the pascal. The conversion factor for each is fixed by international standard. No measurement uncertainty enters the unit conversion itself; only the original pressure measurement carries error bars. This makes pressure unit conversion an arithmetic exercise once the right factor is selected.
Atmospheric pressure was discovered before the unit pascal existed. In 1644 Evangelista Torricelli filled a glass tube with mercury, inverted it in a dish of mercury, and watched the column settle at about 760 mm. He correctly inferred that the atmosphere was pressing down on the dish and holding the column up. The 760 mmHg figure became the foundation of every later pressure unit, and is the reason "mm of mercury" survives 380 years later.
Gauge vs absolute in pressure unit conversion
Absolute pressure measures from a perfect vacuum. Gauge pressure measures from local atmosphere. A tire gauge reads zero when held in open air because the tire interior matches atmosphere — but the absolute pressure in either case is still about 1 atm. Adding 14.696 psi (or 101,325 Pa) to a gauge reading gives the absolute equivalent at sea level.
Pressure unit conversion in this calculator handles absolute values. Equipment gauges report gauge values. To compare the two, subtract atmospheric pressure from the absolute number, or add atmospheric pressure to the gauge number. The 101,325 Pa offset matters most for low pressures (under 5 atm) and becomes negligible at high pressures (above 50 atm).
1 Pa = 0.001 kPa = 10^-5 bar1 atm = 101,325 Pa = 14.696 psiP_abs = P_gauge + 101,325 PaWater depth = 9806 Pa per metreWhy pascal is the natural base
The pascal is one newton per square meter. It is the simplest pressure expression in SI base units (kg/m·s²). Every other pressure unit reduces to a pascal multiple: 1 bar = 100,000 Pa, 1 atm = 101,325 Pa, 1 psi = 6894.76 Pa, 1 mmHg = 133.322 Pa.
This makes the pascal the natural pivot for pressure unit conversion. Convert any source unit to pascals; convert pascals to any target unit. Two steps, two factors. The pascal is rarely the final desired unit (it is small for everyday work), but it is always the intermediate.
Pressure unit conversion in fluid statics
Fluid pressure at depth follows the hydrostatic formula: P = ρ g h, where ρ is density, g is gravitational acceleration (9.806 m/s²), and h is depth. For freshwater (ρ = 1000 kg/m³), each meter of depth adds 9806 Pa = 9.806 kPa = 0.0981 bar = 1.422 psi.
This is why the rule of thumb "1 bar per 10 m" works for water depth. Pressure unit conversion lets the same depth pressure appear in any unit: a 30 m dive is about 3 bar of water above atmosphere, which is 2.96 atm, 43.5 psi, or 300 kPa. Total absolute pressure at 30 m: 4 bar = 3.95 atm = 58 psi.
Altitude and atmospheric pressure
Atmospheric pressure drops with altitude. The barometric formula gives an approximate exponential decay: at 5500 m (Everest base camp) pressure falls to about 50 kPa, half of sea level. At 18 km the pressure is around 7000 Pa. The atmosphere becomes essentially a vacuum above 80 km.
This affects gauge pressure conversions. A tire inflated to 32 psi gauge at sea level reads roughly 32 psi gauge at altitude — because the gauge subtracts whatever the local atmosphere is. But the absolute tire pressure has dropped: at Mexico City (78 kPa atmospheric) a 32-psig tire is 32 + 11.3 = 43.3 psia, lower than the 32 + 14.7 = 46.7 psia at sea level. Pressure unit conversion in this calculator uses sea-level reference; altitude-corrected conversions require local atmospheric data.
Pressure unit conversion in vacuum work
Vacuum is measured by how much below atmosphere the pressure has dropped. A "rough vacuum" reaches 10–100 Pa absolute (about 0.1–1 torr). A "fine vacuum" reaches 10^-3 to 10^-6 Pa (10^-5 to 10^-8 torr). Ultra-high vacuum systems for surface-science research achieve 10^-10 Pa or lower — over 17 orders of magnitude below atmosphere.
Pressure unit conversion in vacuum systems uses torr or Pa most often. 1 torr = 133.322 Pa. The pascal scale and torr scale agree well: at typical lab vacuums (10^-4 torr = 0.0133 Pa) the numbers track each other. Mass spectrometry, electron microscopy, and semiconductor wafer processing all rely on precise vacuum pressure unit conversion.
A vacuum reading is negative gauge pressure. A vacuum chamber at 10 kPa absolute is at −91 kPa gauge (about −13.2 psig) at sea level. The minimum possible gauge pressure is −101,325 Pa, the value of 1 atm subtracted from a perfect vacuum. Pressure unit conversion handles negative inputs but the sign convention varies — some industries report vacuum as a positive "inches of vacuum" number, others as negative gauge.
Common pressure unit conversion mistakes
The first mistake is forgetting the gauge-versus-absolute distinction. A "10 bar tank" usually means 10 bar gauge (11 bar absolute). A pressure unit conversion to atm gives the absolute number; subtracting 1 atm gives the gauge equivalent.
The second mistake is mixing units within a calculation. Mixing kPa and bar, or psi and atm, leads to errors of 14× or 6× — large enough to cause safety issues in pressure-vessel design. Convert all pressures to one unit before any computation.
The third mistake is using sea-level atmospheric pressure at altitude. The 101,325 Pa figure is correct only at sea level. For high-altitude installations, replace it with local atmospheric pressure (around 83 kPa in Denver, 78 kPa in Mexico City, 65 kPa in La Paz).
- 1 Pa = 1 N/m² (SI base)
- 1 atm = 101,325 Pa exactly
- 1 bar = 100,000 Pa
- 1 psi = 6894.757 Pa
- 1 mmHg = 133.322 Pa
- Freshwater: 9806 Pa per metre of depth
- Seawater: ~10,055 Pa per metre
- Sea-level atmosphere: 101,325 Pa = 760 mmHg
For depth conversions, the 1 bar per 10 m rule works for both freshwater and seawater within 2.5%. For precise dive planning, use 0.0981 bar per metre for freshwater and 0.1006 bar per metre for seawater. Pressure unit conversion in dive tables typically expresses depth as "feet of seawater" (fsw) or "metres of seawater" (msw), which embeds the conversion factor implicitly.