Filamentary transport across the scrape off layer is a key issue for the design and operation of future devices, such as ITER, DEMO and MAST-U, as it sets the power loadings to the divertor and first wall of the machine. Analysis has been performed on L mode filaments in MAST in order to gain an understanding of the spatial structure and attempt to reconcile the different scales of the filament width and the power fall off length (). The L mode filament heat flux arriving at the divertor has been measured using high spatial resolution (1.5 mm) infrared (IR) thermography. The filaments form discrete spiral patterns at the divertor which can be seen as bands of increased heat flux in the IR measurements. Analysis of the width and spacing of these bands at the divertor has allowed the toroidal mode number of the filaments to be determined (). The size of the filaments at the midplane has been determined using the target filament radial width and the magnetic field geometry. The filament width perpendicular to the magnetic field at the midplane has been found to be between 3 and 5 cm. Direct calculation of the filament width from midplane visible imaging gives a range of 4–6 cm which agrees well with the IR data.