Bar, pascal, PSI — how much air is in your tires

The weather report gives pressure in hectopascals. The gauge on a bike pump carries two scales — bar and PSI — and the sticker in your car's door jamb speaks in bar. The gas regulator on a camping trip runs on millibars, and the cuff at the doctor's on millimeters of mercury. It is all one and the same physical phenomenon, merely dressed in different units and different reference points. No wonder it is easy to get lost — and a mistake of one order of magnitude can end in a burst tire or a flat wheel. Let's take it apart.

One phenomenon, a dozen units

Pressure is simply a force spread over an area:

p = F / A — the same force on a smaller area gives a higher pressure.

The SI unit is the pascal (Pa) — the pressure a one-newton force exerts on one square meter. It is a very small unit: a single pascal is roughly the push of a sheet of paper lying on a table. That's why we almost never use bare pascals in practice, only their multiples and derived units, each tied to its own domain.

Meteorology counts in hectopascals (hPa) — and since 1 hPa is exactly 100 Pa, it is numerically equal to the older millibar (mbar). "1013 hPa" on a synoptic chart and "1013 mbar" are the same value. Engineering and the car world prefer the bar, conveniently close to atmospheric pressure. Chemistry and physics fall back on the atmosphere (atm), while the Anglosphere — and the spec sheets of American tire makers — use PSI (pounds per square inch). All these units are bound by rigid arithmetic:

A few relations are worth keeping in your head: 1 bar = 100 kPa = 1000 mbar, 1 atm = 1013.25 hPa = 760 mmHg, and 1 bar ≈ 0.987 atm ≈ 14.50 PSI. And one more, the easiest to trip over in the workshop: 1 bar = 0.1 MPa, so 50 bar is 5 MPa, not 50 MPa. The NebulaMath pressure calculator converts all of these both ways — but it helps to understand where the numbers come from.

Absolute or gauge? A zero that isn't zero

Here lies the trap that catches the most people. The atmospheric pressure in a weather report is an absolute value — its reference point is a perfect vacuum, the true zero of pressure. The pressure in a tire is a relative (gauge) value: the gauge shows how much the pressure inside exceeds the surrounding atmosphere. They are joined by a simple equation:

p_abs = p_gauge + p_atm

That's why a completely flat tire reads 0 bar on the gauge — even though there is then about 1 bar of absolute pressure inside, exactly as much as outside. The gauge doesn't measure "how much air there is," only "how much more than around it."

The consequences of this difference can be surprising. A tire inflated to 15 PSI gauge, placed in a vacuum chamber, will read 30 PSI — because the external pressure that used to "subtract" from the reading has vanished. The same thing, only gentler, happens in the mountains. A car climbing to 3,000 m above sea level meets thin air at about 0.7 bar absolute. The absolute pressure inside the tire doesn't change (it's a sealed batch of gas), but the gauge reading rises: from 2.5 bar at sea level to about 2.8 bar in the mountains. The tire physically expands, its contact patch with the road shrinks — and the pressure has to be corrected downward to keep grip.

The "absolute vs. gauge" distinction is no academic curiosity. A level sensor in an open tank must be gauge, or else the daily weather swings of ±30 mbar would masquerade as a level change of ±30 cm over a motionless liquid. Vacuum food packaging, by contrast, needs an absolute sensor — the residual pressure of about 65 mbar decides the product's shelf life and must not "drift" with the weather.

The thermodynamics of a tire: why cold lets air escape without a hole

Drivers are baffled every year that frost "deflates" their tires. Nothing escapes — it is pure physics of a gas trapped in a fixed volume. It is described by the ideal-gas law (Clapeyron's equation), pV = nRT, and at constant tire volume and unchanged mass of air it reduces to Gay-Lussac's law: the ratio of absolute pressure to absolute temperature is constant.

The lower the temperature, the slower the air molecules move and the more weakly they bombard the tire walls — the pressure drops. Let's do the math. A tire inflated in a heated garage (+20 °C, i.e. 293.15 K) to 2.2 bar gauge holds 3.2 bar absolute. After driving out into −20 °C frost (253.15 K):

p₂ = 3.2 bar · (253.15 / 293.15) ≈ 2.76 bar absolute

Subtract the atmosphere and about 1.76 bar gauge remains — a drop of over 0.4 bar from the recommended 2.2. The rule of thumb says every 10 °C shifts the pressure by 0.1–0.2 bar: at 0 °C the drop is about 0.2 bar, at −20 °C it reaches 0.4 bar. That's why in winter it pays to preemptively bump the base pressure by 0.2 bar.

The effect works both ways. After a dozen kilometers of driving, friction and flexing heat the air in the wheels, raising the pressure by 0.2–0.4 bar; a sun-baked parked car can gain as much as 15%. Hence the iron rule: measure pressure on cold tires, ideally after at least 30 minutes at rest. In motorsport, to shed the influence of humidity and temperature swings, tires are filled with pure nitrogen — more thermally stable than ordinary air.

The gauge and its two scales: where mistakes come from

Most analog gauges — at stations, in workshops, on pumps — use a Bourdon tube: a flattened metal tube bent into an arc that tries to straighten under pressure. Its free end moves a few millimeters, and a system of levers and gears turns that motion into the rotation of a needle. Under vibration (say, at a compressor) the gauge is filled with glycerine, which damps the needle and prolongs the mechanism's life. Precision instruments are calibrated at least once every 12 months.

The dial's dual scale — bar and PSI — is a legacy of two markets: Europe counts in bar, the Anglosphere in PSI. Convenient, but also the source of three classic errors:

• Parallax error — reading the needle "at an angle" falsifies its position. Look at the dial straight on.
• Scale mix-up — the user sees a "3" and inflates to that value without checking which scale it belongs to. 3 bar is a good pressure for a gravel bike, but 3 PSI (0.2 bar) is a nearly flat tire. The reverse can be worse: inflating a car tire to 33 bar instead of 33 PSI (2.3 bar) would rupture it with a bang.
• Order-of-magnitude error — copying a number without converting, e.g. taking 50 bar to be 50 MPa. Correctly, 1 bar = 0.1 MPa, so 50 bar = 5 MPa. The slip inflates the reading tenfold.

How much air in a car's tires

For a typical passenger car the values sit in the 2.1–2.3 bar range under normal load, but they depend on tire size, vehicle mass, and how the load is split between the axles. Smaller wheels (13–14") run lower, larger ones (16–18") and SUVs higher, up to 2.7–2.8 bar. The maker's sticker in the door jamb or on the fuel flap always decides, not a "generic" table.

The crucial point is that a loss of about 30% (close to 1 bar) cannot be judged by eye. A too-soft tire flexes excessively, rolling resistance and fuel use climb, the tread edges wear, and cornering and emergency braking get riskier. An over-inflated tire bulges in the middle, loses contact patch and grip on wet roads. That's why new cars carry TPMS systems monitoring pressure — in two wildly different technologies:

The direct system gives a real readout of the values and detects a puncture instantly — invaluable on the motorway. The indirect one is cheaper and maintenance-free, but reacts with a delay and needs a simple reset in the menu after every pressure change.

Bikes: less air, higher pressure

A bicycle wheel works on a far smaller volume of air than a car's, yet carries a comparable load per unit area — so it needs higher pressures. Too low a value risks a "pinch flat" (colloquially a "snake bite"): hitting an edge, the tire compresses down to the rim and pinches the tube, cutting it in two spots. In mountain biking and gravel the problem is solved by tubeless systems — a liquid sealant instead of a tube lets you safely lower the pressure for better damping and grip.

Beware modern hookless rims (without the inner hooks that hold the tire's bead). Under the 2023 ETRTO standards, the maximum pressure for tires on such rims is a strict 5.0 bar (72.5 PSI) — exceeding it risks the tire blowing off the rim. The valve type matters too: Schrader (car-type) is measured directly; Presta needs its top nut unscrewed first; and the classic Dunlop doesn't allow a static gauge reading at all — the value can only be read dynamically, with a pump that has a built-in gauge.

The right pressure depends on the rider's weight and the discipline. Here are the typical ranges:

Four rules to close with

Correct pressure is one of the simplest and most rewarding pieces of maintenance — it bears directly on safety, handling, and fuel use. A few rules are worth keeping. First, weight distribution: on a bike the load falls roughly 40% front, 60% rear, so the rear tire should be pumped 0.2–0.5 bar harder; with panniers or a child seat, higher still. Second, weight correction: every ±5 kg of rider weight is about ±0.1–0.2 bar within the maker's range. Third, surface: smooth asphalt likes the top of the range, boggy or wet terrain wants a 0.2–0.3 bar drop for traction. And fourth, regularity: air slowly diffuses through rubber, so check car tires once a month and bike tires (high pressure, low volume) before every longer ride. Always cold, always on the right scale.

Further reading

• OpenStax, University Physics, Vol. 1 (§14.2 "Measuring Pressure") — the definition of pressure and gauge units.
• WIKA, Gauge and absolute pressure measurement — reference points and sensor selection.
• WIKA Blog, The Bourdon tube pressure gauge — how it works — how the dial reading is produced.
• ETRTO / Continental Tires, Tire/Rim combinations — hookless standards and bicycle-rim pressure limits.
• Polish Tyre Industry Association (PZPO), A safe car — tire pressure — practical advice and the effects of under-inflation.