Berlin offer different possibilities to store your luggage around the city.
You can find lockers at:
Alexanderplatz: lockers for luggage no more than 20kg, sizes: 42cm x 80cm x 28cm or 54.5cm x 80cm x 28cm, price: 1.50€ for 4 hours
S-Bahn Station Friedrichsstraße (Dorotheenstraße 30): standards lockers; price for 2 hours: 0.50€, price for 8 hours: 8€
Berlin Central Station: prices between 4€ and 6€ a day
Central Bus Station (ZOB): prices between 2€ and 5€ a day
