A quorum exists when a majority of the members are present, which is 218 members in the House of Representatives provided no vacancies exist, and 51 members in the Senate, which has 100 representatives. The size of a quorum depends on the size of the organization and is usually established in the founding documents of the organization.
The size of the quorum in any organization may change when vacancies open up due to retirement or death of members. Depending on the organization, quorums may be necessary before significant votes and actions can take place. Members who form a quorum may make decisions for their organizations, even in the absence of all members.