Bar to PSI

Reading a Bar Placard on a Forecourt Gauge Marked in PSI

The sticker inside the driver's door pillar says 2.2 bar. The air line at the petrol station counts in psi, the pencil gauge in the glovebox counts in psi, and the tyre fitter down the road quotes psi as well. One number, three instruments, and nobody wants to guess — a tyre set 0.2 bar light wears its shoulders away and drinks fuel doing it.

Conversion factor: 1 bar = 14.5037738 psi, so multiply the placard figure by 14.5. A common family-hatchback setting of 2.2 bar is 31.9 psi, which is why a forecourt gauge clicking in whole psi should be taken to 32 rather than left at 30.

Before You Reach for the Air Line

The placard beats the sidewall

The label on the door pillar, fuel flap or glovebox lid is the carmaker's figure for that car on those tyre sizes. The moulded number on the tyre wall is a ceiling for the casing, not a target.

Gauges disagree by a little

Forecourt machines drift, and a stick gauge can read a psi or two out. Two readings from two gauges that land 1 psi apart are normal; 5 psi apart means one of them is wrong.

Weather moves the number

Air contracts as it cools, so the same tyre reads roughly 1 psi lower for every 10 °F drop — about 0.1 bar per 10 °C. Autumn is when the dash warning lights start appearing.

Loading changes the target

Most placards carry a second, higher column for a full car or a towing setup, usually on the rear axle. Holiday luggage plus four adults is exactly the case it was printed for.

Setting Four Tyres from One Bar Figure

Do the arithmetic once in the car park, write the psi numbers on your hand or your phone, and the rest is just walking round the car.

Put the placard value in the bar box

2.2, 2,4 or 2.5 — a comma is accepted just as readily as a full stop, and stray spaces are ignored. The psi equivalent counts up beside it before you finish typing.

Do the front and the rear separately

Many placards ask for a different pressure at each axle, and a loaded car adds a third figure. Change the bar value and read the new psi target rather than adding a rough margin by eye.

Take the number with you

The copy button lifts the bare figure — digits only, nothing else — so it pastes cleanly into a note on your phone. Ctrl + C inside either field does the same job on a desktop.

Work the other way at the tyre shop

When the fitter says "I put 36 in them", the swap button (↔) turns the page into psi → bar — a multiplier of 0.068947573 — and 36 psi comes back as 2.48 bar, so you can check it against a placard printed in bar.

Check them cold: the placard figure assumes tyres that have been standing for a few hours or driven under a couple of miles. Come off a motorway and the same tyre can read 4–6 psi higher, and bleeding that "extra" air away leaves the car under-inflated once everything cools.

Placard Pressures by Passenger-Car Class

The bands carmakers usually land in, converted to the psi your gauge shows. These are ranges to sanity-check a reading against — the label on your own car always wins, because it accounts for its weight, tyre size and speed rating.

Vehicle class	Condition	Placard (bar)	Gauge (psi)
City car / kei class	Driver plus one, narrow tyres	2.0 bar	29.0 psi
Supermini	Normal load	2.1 bar	30.5 psi
Family hatchback	Normal load	2.2 bar	31.9 psi
Mid-size saloon	Normal load	2.3 bar	33.4 psi
Compact crossover / SUV	Normal load	2.4 bar	34.8 psi
Battery-electric hatch or saloon	Heavier kerb weight	2.6 bar	37.7 psi
Estate or MPV	Fully laden, rear axle	2.8 bar	40.6 psi
Space-saver spare	Temporary use only	4.2 bar	60.9 psi

Everything a passenger car asks for sits between roughly 29 and 41 psi, which is why the space-saver at 60.9 psi looks so startling on the gauge — a narrow temporary wheel needs far more pressure to carry the same corner of the car.

Handy Details When You Are Stood at the Air Line

Both boxes move together

Neither side is locked, so you can nudge the bar value up in tenths and watch the psi target follow, or type a psi figure and see what the placard would have called it.

One tap to read psi back in bar

The direction arrows reverse the pair, which is what you want when the workshop quotes psi and the handbook in the glovebox is printed in bar.

kPa and kg/cm² sit in the same list

Searchable menus on both sides cover 26 pressure units, so an imported car with a kPa placard or an older sticker in kg/cm² is handled without leaving the page.

A comma works like a decimal point

Handbooks printed on the continent write 2,4 bar; typing it exactly that way is fine, and the psi answer appears with the decimals you need to round sensibly.

Tyre Pressure Questions at the Air Line

What does "check them cold" actually mean in practice?

Tyres that have sat for at least three hours, or been driven no more than a mile or two at low speed. Rolling warms the air inside and pushes the reading up by several psi; a hot tyre showing 34 psi may only hold 30 psi once it has cooled. If the nearest air line is a long drive away, set the pressures a couple of psi above target and re-check them cold at home.

The sidewall is stamped 3.5 bar — why does the door label say 2.2?

They answer different questions. The 3.5 bar moulded into the tyre (50.8 psi) is the maximum cold pressure the casing is rated to hold at its maximum load — a limit for the component. The 2.2 bar on the placard is the pressure the vehicle maker chose for ride, braking, wear and load on that particular car. Inflate to the sidewall figure and the tyre rides on its centre band with a harsh, skittish contact patch.

How far does the reading drift between a cold morning and a hot afternoon?

Roughly 1 psi for every 10 °F, or about 0.1 bar per 10 °C. A car set to 2.2 bar (31.9 psi) on a 25 °C afternoon will show closer to 2.0 bar on a 5 °C morning without losing any air at all. That seasonal swing is the usual reason a warning lamp appears after the first cold night of autumn.

Why does the placard print a second, higher pressure for a fully loaded car?

Extra weight squashes the tyre flatter, which flexes the sidewall harder and builds heat at motorway speed. Adding pressure restores the shape of the contact patch and the casing's load capacity. On many family cars the laden column asks for something like 2.8 bar (40.6 psi) at the rear against 2.3 bar (33.4 psi) unladen — and it should be wound back down once the car is empty again.

My space-saver spare is labelled 4.2 bar — why so much more than the road wheels?

That narrow temporary wheel still carries a full corner of the car on a fraction of the footprint, and only pressure can make up the difference — 4.2 bar is 60.9 psi. Spares also sit for years and leak slowly, so check it whenever the road tyres are done. The speed and distance limits printed next to that figure are part of the same bargain.