There are books of tables published for these conversions covering all ranges of specific gravity and temperature. However this graph will solve your problem for most of the fluids you will come across in aviation.
www.fisherregulators.com/technical/tables/gravity.pdf
Measure the density in a flask using a hydrometer. ( for Jet A1 it should have a range of 0.75 to 0.85, this is stamped on the paper inside the glass.) At the same time read the temperature of the Jet A1 and apply both numbers to the graph to give you the corrected SG.