Förster resonance energy transfer (FRET) is a versatile method for analyzing protein-protein interactions within living cells. This protocol describes a nondestructive live-cell FRET assay for robust quantification of relative binding affinities for protein-protein interactions. Unlike other approaches, our method correlates the measured FRET efficiencies to relative concentration of interacting proteins to determine binding isotherms while including collisional FRET corrections. We detail how to assemble and calibrate the equipment using experimental and theoretical procedures. A step-by-step protocol is given for sample preparation, data acquisition and analysis. The method uses relatively inexpensive and widely available equipment and can be performed with minimal training. Implementation of the imaging setup requires up to 1 week, and sample preparation takes ∼1-3 d. An individual FRET experiment, including control measurements, can be completed within 4-6 h, with data analysis requiring an additional 1-3 h.