Pressure Converters

Pressure: From Tyres to Blood Pressure

You're at a servo and your tyre gauge shows "45". Is that good? Do you add air or let some out? The answer depends whether your gauge is showing PSI, kPa, or bar, and this distinction has caused flat tyres across Australia because tyre sidewalls list pressure in kPa, but some gauges show PSI, and they look similar but aren't.

Start with tyre pressure because it's the most common Australian mistake. Your car's tyre sidewall says something like "230 kPa max pressure". That's kilopascals, Australia's standard. Many cheap digital gauges sold at Bunnings are calibrated in PSI because they're imported. 230 kPa is roughly 33 PSI. If you overinflate to 45 PSI thinking you're adding a small amount, you've actually added 310 kPa and your tyre is now dangerously overinflated. Overinflated tyres wear faster, reduce traction in wet weather, and reduce ride comfort. Check your car's door jamb (not the tyre sidewall) for the manufacturer-recommended pressure, which is usually in kPa for Australian vehicles.

Bar and millibar (mbar) appear in weather and scuba diving. When the Bureau of Meteorology reports air pressure as "1015 hPa" (hectopascals), that's the same as 1.015 bar. Weather websites often show this as millibars. High pressure means clear weather. Low pressure (below 1000 hPa) usually means rain is coming. The difference matters for storm prediction.

Atmospheric pressure (the weight of air above you) is about 101.325 kPa at sea level. The higher you go, the lower the pressure. This is why cooking times change at altitude, why mountaineers need oxygen, and why sealed bags puff up when you fly. At Denver (1600m elevation), atmospheric pressure is only 83.5 kPa, so water boils at 95 degrees Celsius instead of 100. If you're caravanning in the high country and your oven isn't cooking right, pressure difference is probably why.

Blood pressure is measured in millimetres of mercury (mmHg), a unit that sounds archaic because it is. A doctor says "140 over 90" meaning 140 mmHg systolic (pressure when your heart beats) and 90 mmHg diastolic (pressure between beats). Modern machines also report this in kPa, but clinics still use mmHg because that's what the medical literature uses. Normal is roughly 120/80 mmHg or 16/11 kPa. The reason we still use mercury is historical and kind of stubborn.

Scuba diving uses bar and atmospheres (atm). Diving depth calculations rely on pressure because pressure increases by 1 atmosphere (1 bar) for every 10 metres underwater. At 20 metres, you're at 3 atmospheres total pressure (1 from the air above plus 2 from the water). This is critical for decompression tables and air consumption calculations. If you've done open water certification in Australia, your instructor used bar for tank pressure.

For everyday use, just remember: tyres use kPa in Australia, weather uses hPa or mbar, blood pressure uses mmHg, and diving uses bar. Converting between them is why this tool exists. A quick check before inflating your tyres is worth 20 dollars in fuel efficiency.