The effect of leakage in Helmholtz resonators has been experimentally and numerically investigated. Transmission loss of a Helmholtz resonator having a gap between the cavity and main duct is measured using an impedance tube setup. The effect of leakage on the transmission loss is examined using different amounts of gap openings. Experimental results are then compared with the predictions from the boundary-element method. The study shows that the leakage increases the resonance frequency substantially and widens the transmission loss. Hence, the leakage needs to be taken into account for accurate predictions of Helmholtz resonators.