A three-dimensional tumor ruler and holder and methods of use thereof are provided. One embodiment provides a three-dimensional tumor ruler to simultaneously measure the length, width, and height of a tumor when positioned on or near the tumor. The tumor measurements can then be used to calculate tumor volume. Tumor volume can be used to accurately determine for example the dose of a drug or radiation to administer to a subject. In another embodiment, the tumor ruler can be used to position the tumor during administration of a drug or radiation therapy to maximize exposure of the tumor to the therapy.
