A fast method of calculation of the flux surfaces and magnetic fields external to a tokamak plasma in air-core tokamaks is presented. The plasma is represented by a set of wires with given current and position. Central solenoid, stray field compensation and shaping coils are represented by sets of wires with given current and location. Radial and vertical field coils are represented by wires with fixed location but variable total current. The radial and vertical force balances are satisfied by fitting the currents in the vertical and radial field coils, to obtain correct values for the force integrals over a fixed boundary, external to the plasma. The calculation was implemented in a code which is suitable for general design work on magnetic configurations, divertor geometry, and poloidal field systems. The code is interactive, and has a turnaround time of about 2 minutes.