Experience Points
The Experience Point cost to gain a level is based on your total character level, not your level in a particular class, as shown in the Character Advancement table in "Character Creation." For example
The Experience Point cost to gain a level is based on your total character level, not your level in a particular class, as shown in the Character Advancement table in "Character Creation." For example, if you are a level 6 Cleric / level 1 Fighter, you must gain enough XP to reach level 8 before you can take your second level as a Fighter or your seventh level as a Cleric.
Source and licence
From System Reference Document 5.2.
Licence: SRD (CC BY 4.0).
This work includes material taken from the System Reference Document 5.1 ("SRD 5.1") by Wizards of the Coast LLC and available at dnd.wizards.com/resources/systems-reference-document. The SRD 5.1 is licensed under the Creative Commons Attribution 4.0 International License available at creativecommons.org/licenses/by/4.0/legalcode.
This work includes material from the System Reference Document 5.2.1 ("SRD 5.2.1") by Wizards of the Coast LLC, available at dndbeyond.com/srd. The SRD 5.2.1 is licensed under the Creative Commons Attribution 4.0 International License, available at creativecommons.org/licenses/by/4.0/legalcode.