A novel dispersion equation describing the instability of low velocity, liquid jets is derived using an integro-differential approach. The explicit form of the derived equation enables much easier prediction of the most unstable wavenumber and disturbance growth rate than does Tomotika’s implicit, complex dispersion relationship. Good agreement is observed with all limiting solutions to Tomotika’s relationship and with numerical solutions to non-limiting cases. The influences of density ratio, viscosity ratio and Ohnesorge number on liquid jet instability are examined.