Although standards for single-zone air leakage tests are widely used, there are no existing standards for several multi-zone cases including: 1) testing air leakage between adjacent zones or 2) testing leakage to the outside from a single unit in a multi-zone building. While a range of test procedures have been used to determine inter-zone leakage using fan-pressurization, the accuracy of the methods can vary significantly. Using field measurements and simulations, we compared the uncertainty in the leakage between two adjacent zones for different measurement and calculation methods. The most accurate method for determining leakage between two adjacent zones using a single blower door has 25% uncertainty and the most accurate two-door test has 16%. In multi-family housing buildings, air leakage from a single zone to the outside is often measured by pressurizing adjacent units to the same test pressure, i.e., a guarded test approach. We investigated how two common sources of uncertainty affect zone-to-outside leakage test results using the guarded zone test method: 1) pressure fluctuations in the different units, and 2) interconnected zones that are not pressurized during the test. While the uncertainty in leakage to outside due to pressure fluctuations and calibration error in guarded test results is relatively small (4-14%), leakage to interstitial zones may have a much more substantial impact (on the order of 30-100% of the leakage directly to the outdoors).