The effects of varying the fin thickness on the steady-state rate of heat loss from the fin array have been measured experimentally for free-convection conditions. The duralumin fin array had a base area of 500 mm (vertical) × 190 mm (horizontal), and the fins protruded 65 mm proud of, and perpendicularly out from, the base. The experiments were performed with five different fin thicknesses, namely 1, 3, 6, 9 and 19 mm, for base temperatures of 20·0 (±0·1)°C and 40·0 (±0·1)°C above that of the ambient environment, which was maintained at 20 (±0·2)°C. There is an average optimal uniform fin thickness equal to 3·0 ± 0·5 mm, corresponding to the maximum rates of heat loss, for the range of conditions stated when the uniform separations between the adjacent fins exceeds 20 mm. For 20 mm ≤ s ≤ 50 mm, the optimal fin thickness decreases slightly as either (i) the fin separation or (ii) the base temperature is reduced.