By extending a technique used to derive the spatial density in star formation regions in our own Galaxy, we provide the first true measurements of the spatial density in starburst galaxies. We show how the relative intensity of the 1(10)-1(11) and 2(11)-2(12) K-doublet transitions of H2CO can provide a very accurate densitometer for the active star formation environments found in starburst galaxies. Our technique is applied to a sample of nineteen IR-bright galaxies which exhibit various forms of starburst activity. In the five galaxies of our sample where both H2CO transitions were detected we have derived accurate spatial densities. We also use H2CO to estimate the dense gas mass in our starburst galaxy sample, finding similar mass estimates for the dense gas forming stars in these objects.