Determining the Bond work indexes of a sample involves tests using laboratory scale grinding circuits. A unit weight is determined by achieving maximum packing of the prepared ore sample; the unit weight is then fed into the circuit, with undersized product being replaced by fresh feed at the commencement of each cycle. The grind times for each cycle are determined by the percentage of undersize product with relation to an ideal period product (IPP). Upon obtaining a consistent IPP, the undersize product of the final cycle is sized to obtain a work index.