Pressure Converter: Tips & How-Tos

Best Practices

Always check tyre pressure when cold. Driving heats the air inside tyres, increasing pressure by 4–6 PSI. Pressure checked after driving will read artificially high. Check first thing in the morning or after the car has sat for at least three hours.
The tyre sidewall shows maximum pressure, not recommended. The maximum PSI moulded into the tyre is the tyre's structural limit, not your target. The manufacturer-recommended pressure is on a sticker inside the driver's door jamb or in the owner's manual.
Pressure changes roughly 1 PSI per 10°F of temperature. As seasons change, tyres lose or gain pressure with ambient temperature. Check pressure when temperatures shift significantly — a tyre that was at 32 PSI in summer may be at 28 PSI in winter.
Front and rear recommended pressures often differ. Many vehicles specify different pressures for front and rear tyres, particularly SUVs and vehicles with rear-wheel drive. Check the door sticker for both axle specifications.
Inflate to the higher end for heavy loads or motorway driving. Most manufacturer recommendations include a higher-pressure setting for loaded vehicles or sustained highway driving. Use the door-sticker table, not just the basic recommendation.

Know whether your reading is gauge or absolute pressure. Gauge pressure is measured relative to atmospheric pressure (so a flat tyre reads 0 PSI gauge, not 14.7 PSI). Absolute pressure includes atmosphere. Tyre gauges read gauge pressure; scientific instruments often specify.
Never exceed the rated pressure of a container. Pressure vessels, hoses, and fittings are rated to a maximum working pressure. Exceeding it risks catastrophic failure. Check ratings before connecting any pressurised system.
Bar and atm are close but not equal. 1 atmosphere (atm) = 101,325 Pa; 1 bar = 100,000 Pa. They differ by about 1.3%. For engineering calculations, use the precise value rather than treating them as interchangeable.
Use kPa for scientific and medical reporting. The pascal (and kilopascal) is the SI unit of pressure and should be used in scientific papers, medical records, and international documents to avoid ambiguity.
mmHg is still the standard for blood pressure. Despite the adoption of SI units in most of medicine, blood pressure measurement remains in millimetres of mercury (mmHg) by convention worldwide. The converter can cross-reference this with kPa if needed.

Checking whether your European car's tyre pressure specification meets US gauge readings.

Verifying that the pressure shown on your gauge matches the manufacturer's specification.

Find the recommended tyre pressure on the door-jamb sticker (usually in PSI or bar).
Use a tyre gauge to check the current pressure in your preferred unit.
If the units don't match (sticker in bar, gauge in PSI), open the Pressure Converter.
Enter the sticker value and convert to the gauge's unit.
Inflate or deflate to match the converted target value.

Expressing blood pressure readings in SI units for a medical record.

Knowing which type of pressure measurement your instrument is reporting.

Gauge pressure = absolute pressure minus atmospheric pressure.
A flat tyre reads 0 PSI on a tyre gauge (gauge pressure) but actually contains air at ~14.7 PSI (atmospheric pressure, absolute).
Most everyday gauges (tyre, bicycle, compressor) read gauge pressure.
To convert gauge to absolute: add 14.696 PSI (or 101.325 kPa) to the gauge reading.
Scientific instruments and laboratory equipment typically specify which type they measure.

Understanding the pressure number shown on a weather app or barometer.

Weather apps typically show pressure in hPa (hectopascals) or mbar (millibars) — these two are identical units.
Standard sea-level pressure is 1013.25 hPa.
Low pressure (below ~1000 hPa) typically indicates stormy weather.
Enter the weather value in the Pressure Converter and select 'kPa' as the To unit to see the SI equivalent.
1013 hPa = 101.3 kPa = 1.013 bar ≈ 14.7 PSI.

Understanding whether a compressor's rated output is sufficient for a pneumatic tool.

Find the required operating pressure for your tool (usually in PSI or bar).
Find the compressor's rated working pressure (often in PSI or bar).
If the units differ, open the Pressure Converter and convert both to the same unit.
The compressor's rated pressure must exceed the tool's required pressure.
Also check the compressor's CFM (airflow) rating against the tool's requirement — pressure alone is not sufficient.