Sap velocity measurements are useful in fields ranging from plant water relations to hydrology at a range of scales. Heat-pulse based techniques are among the most common methods to measure sap velocity, but most lack the ability to measure velocities across a wide range, including very high, very low and negative velocities (reverse flow). We propose a new method, the double-ratio method (DRM), which is robust across an unprecedented range of sap velocities and can provide real-time estimates of the thermal diffusivity of wood. The DRM employs one temperature sensor proximal and two distal to the heat pulse probe and facilitates several theoretical, heat-based approaches to quantifying sap velocity. We tested the DRM using whole-tree lysimetry in Eucalyptus cypellocarpa and found strong agreement across a wide range of velocities.