Pressure Unit Converter

Pressure units are a mess: a tire shop quotes psi, a weather app shows hPa, a hospital monitor uses mmHg, and an industrial drawing labels a pipe in MPa. This converter handles 13 of them - the SI family (pascal Pa, hectopascal hPa, kilopascal kPa, megapascal MPa), engineering and meteorological units (bar, millibar mbar, standard atmosphere atm, technical atmosphere kgf/cm²), medical and vacuum units (Torr, millimetre of mercury mmHg), imperial units (psi, ksi, inHg), and the water column unit (mH₂O). Enter a value, pick the source and target units, and get the cross-unit answer with the conversion factor shown. All calculations run in your browser; nothing is uploaded.

Pressure is force per unit area. Its SI derived unit is the pascal (Pa) = N/m² = kg·m⁻¹·s⁻². Since the 2019 SI redefinition, the newton comes from m·kg·s⁻² with the kilogram fixed by the Planck constant h = 6.626 070 15 × 10⁻³⁴ J·s, the metre by the vacuum speed of light c, and the second by the caesium-133 hyperfine transition - so the pascal is exact and self-consistent inside SI. Every other unit in the converter is stored as a fixed scalar multiple of Pa, and conversion is a single floating-point operation value × (factor_from / factor_to). The core constants are: 1 atm = 101 325 Pa exactly (10th CGPM, 1954); 1 bar = 100 000 Pa exactly; 1 Torr = 101 325 / 760 Pa ≈ 133.322 368 Pa; 1 mmHg ≈ 133.322 387 415 Pa (mercury density 13.5951 g/cm³ at g_n = 9.806 65 m/s², differs from Torr by < 2 × 10⁻⁷ and is treated as equivalent in engineering); 1 psi = lbf/in² ≈ 6 894.757 Pa using the international avoirdupois pound (0.453 592 37 kg) and inch (0.025 4 m); 1 inHg (at 0 °C convention) ≈ 3 386.388 64 Pa; 1 mH₂O (with ρ = 1000 kg/m³ at g_n) = 9 806.65 Pa; 1 kgf/cm² = 98 066.5 Pa (the technical atmosphere 'at'). All of these are essentially table lookups; the units themselves do not introduce additional error. The distinction between absolute, gauge, and vacuum pressure is a choice of zero point, not of unit - absolute uses true vacuum as zero (psia, Pa abs), gauge uses local atmospheric pressure as zero (psig), and vacuum measures negative gauge pressure. This converter only changes the unit; it does not change the reference point. To turn a gauge reading into an absolute pressure, add the local atmosphere (≈ 101.325 kPa at sea level, falling to about 89.9 kPa at 1000 m elevation). Numerical accuracy is bounded by IEEE 754 double precision (about 15-17 significant decimal digits, machine epsilon 2⁻⁵² ≈ 2.22 × 10⁻¹⁶). The widest span in this converter is ksi → Pa (≈ 6.89 × 10⁶), far below the double-precision overflow limit, so chained multiplications never overflow or underflow. Display, however, should still be rounded to 6-9 significant figures so the output does not imply more precision than the original measurement.

• Pascal definition: 1 Pa = 1 N/m² = 1 kg·m⁻¹·s⁻²; the 2019 SI fixes h, c, and Δν_Cs, making Pa exact inside the SI.
• Standard atmosphere 1 atm = 101 325 Pa exactly (1954 CGPM), the anchor for historical units atm, Torr, mmHg, and inHg.
• 1 bar = 100 000 Pa exactly; 1 hPa = 100 Pa = 1 mbar; meteorological hPa is numerically identical to the legacy mbar.
• 1 Torr = 101 325/760 Pa ≈ 133.322 Pa; 1 mmHg ≈ 133.322 387 Pa (mercury density 13.5951 g/cm³ at g_n = 9.806 65 m/s²) - the two differ by less than 2×10⁻⁷ Pa.
• 1 psi ≈ 6 894.757 Pa; 1 inHg(0 °C) ≈ 3 386.39 Pa; 1 mH₂O = 9 806.65 Pa; 1 kgf/cm² = 98 066.5 Pa (technical atmosphere, about 2% smaller than 1 bar).
• Absolute, gauge, and vacuum pressures differ in their reference point, not their unit; this tool only converts units and preserves the chosen zero.
• IEEE 754 double precision gives ~15-17 significant decimal digits; the maximum ksi → Pa span (~6.9 × 10⁶) is far below the overflow limit, but displayed results should be rounded to 6-9 significant figures to avoid false precision.