Catalyzes the formation of isopentenyl diphosphate (IPP), the universal five-carbon isoprenoid building block of all natural isoprenoids (PubMed:24327557, PubMed:26216978). Acts in parallel with the mevalonate (MVA) pathway and plays an important role in regulating the formation of both MVA and methylerythritol phosphate (MEP) pathway-derived terpenoid compounds by controlling the ratio of isopentenyl phosphate (IP) and dimethylallyl phosphate (DMAP) to isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP). Controls the levels of IP and DMAP that are competitive inhibitors of the farnesyl diphosphate synthase. Regulates the production of farnesyl diphosphate-derived terpenoids in the cytosol, and geranyl diphosphate-derived compounds in plastids (PubMed:26216978). ATP + isopentenyl phosphate = ADP + isopentenyl diphosphate kcat is 1.9 sec(-1) with isopentenyl phosphate (PubMed:24327557). kcat is 3.7 sec(-1) with isopentenyl phosphate (PubMed:26216978). kcat is 2.7 sec(-1) with dimethylallyl diphosphate (PubMed:26216978). kcat is 0.0039 sec(-1) with geranyl phosphate (PubMed:26216978). Belongs to the isopentenyl phosphate kinase family.