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The impact of fluorescence microscopy has been limited by the difficulties of expressing measurements of fluorescent proteins in numbers of molecules. Absolute numbers enable the integration of results from different laboratories, empower mathematical modelling, and are the bedrock for a quantitative, predictive biology. Here we propose an estimator to infer numbers of molecules from fluctuations in the photo bleaching of proteins tagged with Green Fluorescent Protein. Performing experiments in budding yeast, we show that our estimates of numbers agree, within an order of magnitude,with published biochemical measurements, for all six proteins tested. The experiments we require are straightforward and use only a wide-eld fluorescence microscope. As such, our approach has the potential to become standard for those practising quantitative fluorescence microscopy.