Pounds per square inch and the bar
A psi is one pound-force on one square inch, 6,894.757 Pa. A bar is defined as exactly 100,000 Pa (100 kPa), chosen to be close to sea-level atmospheric pressure, which is 101.325 kPa or 1.01325 bar or 14.696 psi. So a bar is 14.5038 psi and a psi is 0.068948 bar. Car tyres run about 2.2–2.5 bar (32–36 psi); a scuba cylinder is filled to 200–232 bar (2,900–3,365 psi); a domestic boiler sits near 1–1.5 bar.
bar = psi × 0.0689476 | psi = bar ÷ 0.0689476
Rough conversion in your head
Divide psi by 15 for bar: 32 psi → 2.1 bar (true 2.21). Multiply bar by 15 the other way, then trim: 2.5 bar → 37.5, so about 36 psi (true 36.3). Gauge pressure (psig) reads zero at atmospheric; absolute pressure (psia) adds the 14.7 psi of the air. Tyre gauges are gauge pressure, which is what this page assumes.