Because of their extraordinary surface areas and tailorable porosity, metal-organic frameworks (MOFs) have the potential to be excellent sensors of gas-phase analytes. MOFs with open metal sites are particularly attractive for detecting Lewis basic atmospheric analytes, such as water. Here, we demonstrate that thin films of the MOF HKUST-1 can be used to quantitatively determine the relative humidity (RH) of air using a colorimetric approach. HKUST-1 thin films are spin-coated onto rigid or flexible substrates and are shown to quantitatively determine the RH within the range of 0.1-5% RH by either visual observation or a straightforward optical reflectivity measurement. At high humidity (>10% RH), a polymer/MOF bilayer is used to slow the transport of H2O to the MOF film, enabling quantitative determination of RH using time as the distinguishing metric. Finally, the sensor is combined with an inexpensive light-emitting diode light source and Si photodiode detector to demonstrate a quantitative humidity detector for low humidity environments.