Enter values for 3 of the variables and press calculate on the remaining one. (Note that I use the more popular specific impulse with isp = ve / g)

Some other useful equations:

mdot = thrust / (isp * g) Dt = Math.sqrt(4 * ((mdot/(0.564 * chamber_pressure)) * 955.0071378899689) / Math.PI) [LOX/hydrocarbon engines] Mt = 0.316 * Math.pow(Mp, 0.6) [storable propellants in big tanks] Mt = 0.6321 * Math.pow(Mp, 0.56) [storable propellants in small tanks] Mt = 0.0152 * Mp + 318 [LOX - no insulation] Mt = 0.0694 * Mp + 363 [LH2 - no insulation]