A leak rate is a throughput: the quantity of gas crossing the leak per unit time, expressed as a pressure-volume product per second. Every common unit is the same physical quantity dressed differently — mbar·L/s in European vacuum practice, Pa·m³/s in SI, atm·cc/s in North American and aerospace specifications, Torr·L/s in older vacuum work, and sccm or scfh where the specification came from a flow engineer rather than a leak-test engineer. Mixing them is the most common error on a leak-test report, and the factors are not round numbers.

The SI anchor is 1 mbar·L/s = 0.1 Pa·m³/s, which follows directly from 1 mbar = 100 Pa and 1 L = 10⁻³ m³. The atmosphere-based units carry the standard atmosphere with them: 1 atm·cc/s = 1013.25 mbar × 10⁻³ L/s = 1.01325 mbar·L/s, so a leak quoted in atm·cc/s is about 1.3% larger than the same number in mbar·L/s. Standard cubic centimetres per minute are the same pV product spread over a minute: 1 sccm = 1.01325/60 = 0.0168875 mbar·L/s, and 1 scfh = 28316.85 std cm³/h works out at 7.97 mbar·L/s.

Two cautions. First, these conversions are exact only for the same gas at the same pressure difference — a leak rate is not transferable between helium and air without the molecular-flow or viscous-flow correction. Second, the pV throughput conversions above are independent of the reference temperature, but converting to a mass flow (g/s, or grams of refrigerant per year) does need the standard temperature stated, because that is where the number of moles enters.

Worked example

Leak rate value	1
Units of the value entered	mbar_l_s
mbar·L/s	1
Pa·m³/s	0.1
atm·cc/s	0.9869
Torr·L/s	0.7501
sccm	59.22
scfh	0.1255

1 mbar·L/s = 0.1 Pa·m³/s exactly. In atm·cc/s it is 1/1.01325 = 0.98692. In Torr·L/s, 1/1.333224 = 0.75006. In sccm, 1/0.0168875 = 59.215. In scfh, 1/7.970012 = 0.12547.