Even though Kythnos is a relatively small and quiet island, it has several ferries connecting it to the rest of the Cyclades and the major ports of the mainland.
Kythnos is connected with Lavrio port of Athens. The distance between the two ports is roughly 25 NM (*) which makes Lavrio the ideal departure point for Kythnos. The trip lasts 1:40 - 2:40 hrs, depending on the ferry and costs a minimum of €16. The ferry companies Goutos Lines and Hellenic Seaways and the vessels used carry both passengers and vehicles.
Kythnos is connected with Piraeus port of Athens. The distance between the two ports is roughly 49 NM. The trip lasts approximately 3 hrs costing €25 minimum. The ferry company is Zante Ferries and the vessel used carries both passengers and vehicles.(*) A Nautical Mile (NM) is equivalent to 1,852 meters or 6,076 feet.
Kythnos is an ideal holiday destination if island hopping is your thing. It connects with most other Cycladic islands. However, as the ferry connections are not on a daily basis, you need to make plans ahead so that you miss nothing.
From Kythnos you can travel to Kea (Tzia) by ferry. Hellenic Seaways is the company connecting the two islands using a car ferry and the distance is just 17 NM. It takes roughly 1:20 hrs at €7.00 minimum cost to get to Kythnos.
Syros is located east of Kythnos, at a distance of 26 NM. The car ferry of Hellenic Seaways arrives at Ermoupoli, the capital of Syros (and the capital and administrative center of the whole Cyclades) after 2:25 and the trip will cost you €9.
Naxos, the biggest Cycladic island, is located southeast of Kythnos, at a distance of 50 NM, so, it is a rather long trip that will take you nearly 6 hours to get to the port of Naxos Town. It will cost you €18. Again, it is Hellenic Seaways with a car ferry that makes this route.
Likewise, to get from Kythnos to Paros (a distance of 40 NM) will take you nearly 4:30 hours and it will cost you €14.5.
Ios and Santorini can also be accessed from Kythnos (58 NM and 78 NM respectively). Depending on the company, it may take you from 5:50 to 9:00 hours to get to Ios and from 7:30 to 10:45 hours to get to Santorini (Thira). However, due to their great distance, they are rarely combined with a holiday on the island of Kythnos.
Kimolos and Milos are connected with Kythnos via the same route. Car ferries of Zante Ferries and Hellenic Seaways offer connections that depart either in the morning or in the afternoon. The distances are 37 NM (the trip takes 5:15 to 10:45 hours) and 40 NM (the trip takes 4:00 to 12:00 hours) respectively.
Serifos, located just 16 NM from Kythnos, is an excellent destination for island hopping. The trip takes only 1:30 via the car ferry of Zante Ferries company. It costs €16.50 per person (adult).