About

Pressure miscalculation propagates catastrophically through engineering systems. A 1 % error in reactor vessel pressure ratings can exceed yield strength margins, risking rupture. This tool converts the standard atmosphere (atm), defined as exactly 101325 Pa by the 10th CGPM (1954), against 20+ pressure units using NIST-traceable conversion factors. All conversions are exact or carry full IEEE 754 double-precision floating-point fidelity.

The tool assumes static gauge-independent absolute pressure. It does not account for gauge vs. absolute distinctions. If converting gauge readings, add local atmospheric pressure (typically 1 atm at sea level) before input. Note: mmHg conversions use the conventional value (1 mmHg = 133.322 Pa), not the temperature-dependent mercury density definition.

Formulas

All pressure conversions use a two-step normalization through Pascals (Pa), the SI coherent unit of pressure.

P_out = P_in × F_fromF_to

Where P_in is the input magnitude, F_from is the number of Pascals per one unit of the source unit, F_to is the number of Pascals per one unit of the target unit, and P_out is the converted result. The defining relation anchors all factors:

1 atm ≡ 101325 Pa = 760 Torr = 14.696 psi = 1.01325 bar

For example, converting 2.5 atm to psi: P_out = 2.5 × 1013256894.757 = 36.7399 psi.

Reference Data

Unit	Symbol	1 atm Equivalent	Domain
Pascal	Pa	101325	SI Base
Kilopascal	kPa	101.325	Engineering
Megapascal	MPa	0.101325	Materials Science
Gigapascal	GPa	0.000101325	Geology
Hectopascal	hPa	1013.25	Meteorology
Bar	bar	1.01325	Industrial
Millibar	mbar	1013.25	Meteorology
Pound per sq inch	psi	14.69595	US Engineering
Torr	Torr	760	Vacuum Science
mmHg (0 °C)	mmHg	760	Medicine
cmHg	cmHg	76	Legacy Lab
inHg (32 °F)	inHg	29.9213	Aviation (US)
cmH₂O (4 °C)	cmH₂O	1033.23	Respiratory
inH₂O (60 °F)	inH₂O	407.189	HVAC
mmH₂O	mmH₂O	10332.3	Hydraulics
kgf/cm²	kgf/cm²	1.03323	Legacy metric
kgf/m²	kgf/m²	10332.3	Civil Eng.
Technical atm	at	1.03323	DIN Standards
Dyne/cm²	dyn/cm²	1013250	CGS System
Pound per sq ft	psf	2116.22	Structural
Short ton/ft²	tsf	1.05811	Soil Mechanics
Foot of seawater	fsw	33.0	Diving (US Navy)
Meter of seawater	msw	10.0	Diving (metric)

Frequently Asked Questions

The standard atmosphere (atm) equals exactly 101325 Pa, defined by international agreement at the 10th CGPM. The technical atmosphere (at) equals 1 kgf/cm² = 98066.5 Pa. The technical atmosphere is approximately 3.2 % smaller. Confusing them in boiler pressure ratings or autoclave specifications can push equipment beyond safe operating limits.

By modern convention, 1 Torr is defined as exactly 101325760 Pa = 133.3224 Pa. The conventional mmHg uses the same value. The actual pressure exerted by a column of mercury varies with temperature (density changes ~0.018 %/°C), but the unit definition is fixed at 0 °C and standard gravity. For clinical blood pressure, the fixed conventional value is always used.

Gauge pressure reads zero at local atmospheric pressure. To convert to absolute: P_abs = P_gauge + P_atm. At sea level, P_atm ≈ 1 atm. At altitude, use barometric pressure. For example, a tire gauge reading 32 psi corresponds to approximately 32 + 14.696 = 46.696 psi absolute. Enter that absolute value into this tool.

All conversion factors are stored as IEEE 754 double-precision floats (approximately 15 - 17 significant decimal digits). The display rounds to 6 significant figures by default to avoid false precision. For the exact definitions (e.g., 1 atm = 101325 Pa), conversions are exact. For derived factors (e.g., psi), the conversion factor 6894.757293168 Pa is used, traceable to NIST definitions of pound-force and inch.

In the US and Canada, altimeter settings use inches of mercury (inHg). Standard sea-level pressure is 29.9213 inHg. In most other ICAO member states, hectopascals (hPa) are used, with standard pressure 1013.25 hPa. A 1 hPa error corresponds to roughly 27 ft (8.2 m) altitude error, which is critical during instrument approaches.

Vacuum pressure is typically expressed in Torr or mbar. Rough vacuum is 760 - 1 Torr. High vacuum is 10⁻³ - 10⁻⁹ Torr. Ultra-high vacuum goes below 10⁻⁹ Torr. When converting very small pressures, verify that your input value has sufficient significant figures. This tool handles scientific notation inputs for such cases.