Alfvén wave turbulence models lie at the heart of many investigations into the winds and extreme-ultraviolet and X-ray emission from cool, solar-like stars. The models provide insights into mass loss, magnetic spin down and exoplanet habitability. Yet they currently rely on ad hoc estimates of critical parameters. One critical but unknown parameter is the perpendicular correlation length, which controls the turbulent heating rate and, hence, has a role in determining the properties of coronal plasma. Here, using the Coronal Multi-channel Polarimeter, we measure the correlation length of Alfvénic waves at the base of the corona. The measurements are an order of magnitude closer to the Sun than previous estimates for the perpendicular correlation length. Our analysis shows the values are broadly homogeneous through the corona and have a distribution sharply peaked around 7.6–9.3 Mm. The measured correlation length is comparable to the expected scales associated with supergranulation. The results provide a stringent constraint for Alfvén wave turbulence modelling.