Pressure Converter

Pressure Conversions

Conversion Factors

Common Reference Points

Applications

• Weather: Barometric pressure in mbar or inHg
• Tires: PSI or bar
• Science: Pascals (SI unit)
• Diving: atmospheres
• Medical: mmHg (blood pressure)

What Is Pressure?

Pressure is force spread over area: P = F / A. The SI unit is the pascal (Pa) — one newton per square meter. A single pascal is tiny (roughly the weight of a small apple spread over a square meter), so everyday pressures use kilopascals, bar, or psi instead.

Key Conversion Formulas

Weather reports use hectopascals (hPa), which are numerically identical to millibars: 1,013.25 hPa = 1,013.25 mbar = 1 atm.

Step-by-Step: Converting 32 psi to bar

• Step 1 — Convert to pascals (the common base): 32 × 6,894.757 = 220,632 Pa.
• Step 2 — Divide by the target factor: 220,632 ÷ 100,000 = 2.206 bar.
• Step 3 — Round: about 2.21 bar — the number on a European tire-pressure chart.
• Step 4 — Sanity check: 1 bar ≈ 14.5 psi, so 32 psi should be a bit over 2 bar. It is.

Worked Examples

Example 1: Blood pressure to kilopascals

A normal reading of 120 mmHg: 120 × 133.322 = 15,998.7 Pa ≈ 16.0 kPa. Medical literature outside the US increasingly reports both units.

Example 2: Altimeter setting

Standard sea-level pressure of 1,013.25 hPa: 101,325 ÷ 3,386.39 = 29.92 inHg — the "29.92" pilots dial into their altimeters.

Example 3: Hydraulic press

A press rated at 2,500 psi: 2,500 × 6,894.757 = 17,236,893 Pa ≈ 172.4 bar. Safety calculations for hydraulics always work at these magnitudes — getting the unit wrong by a factor of 100 is how equipment fails.

Gauge vs Absolute: The Pressure Distinction That Matters

A tire gauge reading 32 psi measures gauge pressure — pressure above the surrounding atmosphere. The absolute pressure inside the tire is 32 + 14.7 = 46.7 psi. Unit conversion never changes which one a reading is: 32 psi gauge converts to 2.21 bar gauge, and both leave the atmosphere out.

This is the single most common pressure mistake in engineering — and no converter can fix it, because it is about what the reading means, not what unit it is in.

Common Mistakes

• Treating bar and atm as identical: 1 bar = 100,000 Pa but 1 atm = 101,325 Pa — a 1.3% difference that matters in calibration.
• Confusing hPa with Pa: 1 hPa = 100 Pa, not 1 Pa.
• Assuming mmHg and torr are exactly identical: they agree to about 0.00001% — fine for blood pressure, worth knowing in metrology.
• Forgetting gauge vs absolute: see the section above — the classic engineering trap.
• Using water-head pressure with the wrong density: 1 m of water ≈ 9.81 kPa, but seawater and hot water differ.

Pressure, Weather, and Altitude

Atmospheric pressure falls about 12 hPa (0.35 inHg) for every 100 m of altitude near sea level. That is why aircraft altimeters work: set the local sea-level pressure (for example 1,013.25 hPa = 29.92 inHg) and the instrument converts the outside pressure into an altitude reading.

Airliner cabins are pressurized to roughly 75–81 kPa (11–12 psi) — the pressure at about 1,800–2,400 m — which is why ears pop on descent: the cabin pressure is climbing back toward 101 kPa as the aircraft lands.

Pressure Ladder: From Vacuum to Hydraulics

Notice the scale spans four orders of magnitude — which is why the unit must always travel with the number. "200" means a tire in kPa and a deep-sea pressure in psi.

Diving: Pressure Every 10 Meters

Water is heavy, so pressure rises fast underwater: every 10 m of seawater adds about 1 atm (101.325 kPa). A diver at 20 m experiences 3 atm absolute — 1 from the air plus 2 from the water — about 304 kPa, or 44 psi. Gauges on scuba tanks read gauge pressure (tank pressure above ambient), which is why a "3,000 psi" tank reads 3,000 psi at the surface and the same 3,000 psi at depth: the gauge subtracts the ambient water pressure automatically.

Blood Pressure and Medical Units

Blood pressure is reported in millimeters of mercury (mmHg) because the first sphygmomanometers literally measured how high a column of mercury the arterial pressure could push. A reading of 120/80 mmHg is about 16.0/10.7 kPa — the systolic and diastolic pressures in SI units. Mercury is now rare in clinics (most devices are electronic), but the unit survived because generations of treatment thresholds — 120, 130, 140 mmHg — are written in it. Converting a patient's history between mmHg and kPa is a routine step when records move between countries.

Frequently Asked Questions

What is the most common pressure unit? The pascal (and kPa/MPa) is the official SI unit worldwide; psi dominates in the US, and bar remains standard in European engineering.

Is mmHg the same as torr? For all practical purposes, yes — 1 mmHg ≈ 1 torr = 133.322 Pa.

Why do weather maps use hPa instead of Pa? Sea-level pressure is about 101,325 Pa — an awkward number. Hectopascals give the familiar 1,013 figures, and 1 hPa = 1 mbar exactly.

Can pressure be negative? Gauge pressure can read negative (a vacuum below atmosphere); absolute pressure cannot go below zero — a perfect vacuum.