Measurements and a model of gas flow through a helical duct of rectangular cross section are reported. Observations of the rate of rise of pressure in a known volume located downstream from the duct yielded molar flow rates of helium, nitrogen, argon and sulfur hexafluoride. The model predicts flow rates from the ducts' entrance pressure, exit pressure, and temperature. It accounts for the gas's nonideal equation of state, gas expansion along the duct's length, the increase of kinetic energy near duct's entrance, and slip. After adjusting only one parameter (the duct's height), the model agrees with the data to within 0.2% in the range of Reynolds number 0.08 < Re < 40. To extend the model to Re = 1,000, the centrifugal effects were accounted for by rescaling Targett et al.'s (1995) numerical calculations. The extended model agrees with the data to within 0.2% in the range 40 < Re < 400 and to within 1% in the range 400 < Re < 1,000.