German Railways (DB) operates a train from Darmstadt to Frankfurt am Main every 20 minutes. Tickets cost $10 - $12 and the journey takes 18 min. S-Bahn also services this route every 20 minutes. Alternatively, FlixBus operates a bus from Darmstadt to Frankfurt am Main 4 times a week. Tickets cost $1 - $4 and the journey takes 21 min. Eurolines DE also services this route twice a week.
From Darmstadt Hbf train to Frankfurt(Main)Hbf. 18 min 9€. From Darmstadt, Darmstadt central station bus to Frankfurt, Frankfurt Süd station. 21 min 30€. From Darmstadt taxi to Frankfurt am Main. 25 min 91€. From Darmstadt drive to Frankfurt am Main. 25 min 5€. From Darmstadt rideshare to Frankfurt. 48 min 2€.