The invention provides a computer software means for determining physical characteristics of an unrestrained animal by enabling a user to interact with one or more images of the unrestrained animal on a graphical user interface connected to a computer. Using the graphical user interface, the user is able to designate piecewise linear and circumferential measurements of selected animal features on the one or more images. The computer software program receives one or more selected images of an unrestrained animal, displays the one or more images on the graphical user interface, receives a user designated animal type and calibration measurements of animal size indicia, receives user selected measurements of animal physical characteristic from the one or more images and adjusts the measurements based on the calibration measurements of animal physical size, and displays the selected images, adjusted and computed measurements and estimated measurement error on the graphical user interface.