It frequently happens in the use of radon “seeds” therapeutically that the total dose given is required in equivalent milligram-hours. Thus a seed of 2 m.c. is left in the patient for ten days and we require to know the equivalent dose in milligram hours. The amount may obviously be calculated by taking the percentage destruction of the radon during the time of exposure, and thence by multiplication by the original strength of seed the total millicuries destroyed is obtained. From this result, by multiplying by the factor of conversion (= 132) the dose in milligram-hours is finally calculated. The process is somewhat lengthy in practice, even with a graph from which the percentage decay may be read directly. Moreover the value of the conversion factor is involved, and it seemed preferable to construct a slide-rule arrangement capable of reading the doses directly from the original strength of “seed” and time left in the patient. A simple device which has been found useful at the Caner Hospital was therefore constructed, and it is in the hope that some such arrangement may be useful to others that this description is published.