Calcium is a ubiquitous intracellular messenger that has important functions in normal neuronal function. The pathology of Alzheimer’s disease has been shown to alter calcium homeostasis in neurons and astrocytes. Several calcium dye indicators are available to measure intracellular calcium within cells, including Oregon Green BAPTA-1 (OGB-1). Using fluorescence lifetime imaging microscopy, we adapted this single wavelength calcium dye into a ratiometric dye to allow quantitative imaging of cellular calcium. We used this approach for in vitro calibrations, single-cell microscopy, high-throughput imaging in automated plate readers, and in single cells in the intact living brain. While OGB is a commonly used fluorescent dye for imaging calcium qualitatively, there are distinct advantages to using a ratiometric approach, which allows quantitative determinations of calcium that are independent of dye concentration. Taking advantage of the distinct lifetime contrast of the calcium-free and calcium-bound forms of OGB, we used time-domain lifetime measurements to generate calibration curves for OGB lifetime ratios as a function of calcium concentration. In summary, we demonstrate approaches using commercially available tools to measure calcium concentrations in live cells at multiple scales using lifetime contrast. These approaches are broadly applicable to other fluorescent readouts that exhibit lifetime contrast and serve as powerful alternatives to spectral or intensity readouts in multiplexing experiments.