There are two ways the Constitution can be formally amended. One of the ways requires a vote of two-thirds of both houses of Congress to propose an amendment and then have three-fourths of the state legislatures vote to approve it. All 27 amendments have followed this process.
Congress consists of two parts. These parts are the House of Representatives and the Senate. There are 435 members in the House of Representatives. In order for a proposed amendment to pass the House of Representatives, two-thirds of the members must vote for the amendment. That means 290 members of the House of Representatives must vote for the proposed amendment. In the Senate, each state has two members. Therefore, there are 100 members of the Senate. In order for a proposed amendment to pass the Senate, 67 members must vote for it to pass. Thus, if 290 members of the House of Representatives and 67 members of the Senate vote to pass a proposed amendment, it then goes to the state legislatures. 38 state legislatures must vote for the proposed amendment for it to take effect.