The cost of a seat reservation depends on the type of train and the level of comfort.

Second clas reservations for domestic trains are usually between 3-10 euro. International trains can be more expensive, for example the Thalys. Supplements for night trains can range from 3 euro for a reclining seat to over 100 euro for a private sleeping cabin.

For an overview of fees, see these pages:

Reservation fees for domestic and international are also printed on the back of the InterRail map that you get with every Global Pass. Bear in mind that the information on this website is usually more up-to-date, as fees can change throughout the year.

Sometimes it's worth researching alternative routes that don't require a reservation. See the FAQ How to avoid reservation fees?