A device (2) for measuring the tension of a belt (4) including a support (6) to releasably hold the belt (4), an hydraulic piston and cylinder arrangement (30), a threaded shaft (38) and a second hydraulic piston and cylinder arrangement (22). The two arrangements (30, 22) are connected by an inextensible tube (32). The arrangement (30) is controlled by rotating the shaft (38) and exerts pressure on the arrangement (22) which in turn exerts pressure on the belt (4) through the hemispherical head (30). When the tension of the belt (4) is being measured the shaft (38) is tightened to a predetermined volume to eliminate air bubbles. The shaft (38) is then rotated through 360° which moves the hemispherical head (30) by a known amount against the belt (4). The tension of the belt (4) can be determined from the reading on the pressure gauge (40).