The paper introduces AxCaliber, a novel noninvasive method for measuring the diameter distribution of axons using diffusion MRI. Axon diameter is crucial for understanding nerve function, as it directly affects conduction velocity and latency. Traditional methods for obtaining this information are invasive and limited in scope. AxCaliber employs a model of water diffusion within restricted cylindrical axons to estimate the diameter distribution within nerve bundles. The method is validated by comparing the diameter distributions measured using AxCaliber with those obtained from histological techniques on sciatic and optic nerve tissue specimens. The results show a high correlation between the two methods, with mean axon diameters of 3.74 μm for the optic nerve and 6.3 μm for the sciatic nerve. AxCaliber is also applied to porcine spinal cord MRI data, allowing for pixel-by-pixel measurement of axon diameters and segmentation of the spinal cord into distinct regions based on their diameter distributions. This approach has potential applications in longitudinal studies of nerve growth and the diagnosis of disorders affecting specific axon populations in the central and peripheral nervous systems.The paper introduces AxCaliber, a novel noninvasive method for measuring the diameter distribution of axons using diffusion MRI. Axon diameter is crucial for understanding nerve function, as it directly affects conduction velocity and latency. Traditional methods for obtaining this information are invasive and limited in scope. AxCaliber employs a model of water diffusion within restricted cylindrical axons to estimate the diameter distribution within nerve bundles. The method is validated by comparing the diameter distributions measured using AxCaliber with those obtained from histological techniques on sciatic and optic nerve tissue specimens. The results show a high correlation between the two methods, with mean axon diameters of 3.74 μm for the optic nerve and 6.3 μm for the sciatic nerve. AxCaliber is also applied to porcine spinal cord MRI data, allowing for pixel-by-pixel measurement of axon diameters and segmentation of the spinal cord into distinct regions based on their diameter distributions. This approach has potential applications in longitudinal studies of nerve growth and the diagnosis of disorders affecting specific axon populations in the central and peripheral nervous systems.