The invention relates to a user interface (300) for measuring an object viewed in an image computed from image data, the user interface comprising an image unit (310) for visualizing the image data in the image, a deployment unit (320) for deploying a caliper (21) in an image data space, a scaling unit (330) for scaling the caliper (21) by a scaling factor in a direction in the image data space, a translation unit (340) for translating the caliper (21) in the image data space, and a caliper unit (350) for visualizing the caliper (21) in the image, wherein the caliper (21) comprises a knot for measuring the object, and wherein the object is measured based on the scaling factor. The caliper (21) comprising the knot, which determines the shape of the caliper (21), is a simple reference object of known geometry and size. Looking at the image data and the caliper (21) visualized in the image, the user may easily place the caliper (21) in the image data space and adjust its size to match the size of the measured ob
