In more detail
In PV = nRT, P is absolute pressure in pascals, V volume in cubic metres, n the amount in moles, T temperature in kelvin and R = 8.314 J/(mol·K). The model treats molecules as points that do not attract each other, which works well for air and most gases near room conditions and less well at high pressure or close to condensation. For a fixed amount of gas it becomes the combined gas law, P₁V₁ / T₁ = P₂V₂ / T₂, with Boyle's and Charles's laws as special cases. One mole at 273.15 K and 101,325 Pa occupies 22.41 litres.
Why it matters
The two classic mistakes are using degrees Celsius instead of kelvin (add 273.15) and using gauge pressure instead of absolute pressure: a tyre gauge reads pressure above the atmosphere, so about 14.7 psi (101.3 kPa) must be added before calculating. It explains why tyres lose pressure in winter and how much gas a cylinder holds.
Example
Tyre at 32 psi gauge, 20 °C → absolute 32 + 14.7 = 46.7 psi
Cooled to 0 °C: 46.7 × 273.15 / 293.15 = 43.5 psi absolute → 28.8 psi on the gauge
2 mol in 10 L at 25 °C: P = 2 × 8.314 × 298.15 / 0.010 = 495,791 Pa ≈ 4.96 bar