Entropy production is the hallmark of nonequilibrium physics, quantifying irreversibility, dissipation, and the efficiency of energy transduction processes. Despite many efforts, its measurement at the nanoscale remains challenging. We introduce a variance sum rule for displacement and force variances that permits us to measure the entropy production rate in nonequilibrium steady states. We first illustrate it for directly measurable forces, such as an active Brownian particle in an optical trap. Data for this analysis can be found in the repository (1) described below. We then apply the variance sum rule to flickering experiments in human red blood cells (repositories (2-4)). We find that the entropy production rate is spatially heterogeneous with a finite correlation length (in particular, data in the repository (4)) and its average value agrees with calorimetry measurements. The dataset is composed of 4 repositories: 1) SwitchingTrap.zip, containing data from Optical-tweezer experiments and used in Fig. 2 and 3 in the main paper, all data are three-column files featuring time (s), position (nm), and force (pN); 2) OpticalStretching.zip, containing data from Optical-tweezer experiments shown in Fig. 4a in the main paper, all data are two-column files featuring time (s) and position (nm) traces; 3) OpticalSensing.zip, containing data from Optical-tweezer experiments shown in Fig. 4b in the main paper, all data are one-column files featuring position (m) traces, sampling frequency 25kHz; 4) OpticalMicroscopy.rar, containing data from Optical-microscopy experiments shown in Fig. 4c in the main paper, all data are one column files featuring position (nm) traces, sampling frequency 2kHz.