Latest amount & QC explanation
The amount of earnings required for a quarter of coverage (QC) in 2021 is $1,470. 'Quarter of coverage' is a legal term, but you may also see the term 'Social Security credit' (or just 'credit') used elsewhere. A QC is the basic unit for determining whether a worker is insured under the Social Security program. No matter how high your earnings may be, you can not earn more than 4 QC's in one year.

History
See historical series of earnings needed to earn one quarter of coverage, 1978-2021.

For years before 1978, an individual generally was credited with a quarter of coverage for each quarter in which wages of $50 or more were paid, or an individual was credited with 4 quarters of coverage for every taxable year in which $400 or more of self-employment income was earned. Beginning in 1978, employers generally report wages on an annual, instead of quarterly, basis. With this change to annual reporting, the law provided that a quarter of coverage be credited for each $250 of an individual's total wages and self-employment income for calendar year 1978 (up to a maximum of 4 quarters of coverage for the year). After 1978, the amount of earnings needed for a quarter of coverage changes automatically each year with changes in the national average wage index.

Determination of the quarter of coverage amount for 2021
The law specifies that the quarter of coverage (QC) amount for 2021 is equal to the 1978 amount of $250 multiplied by the ratio of the national average wage index for 2019 to that for 1976, or, if larger, the 2020 amount of $1,410. If the amount so determined is not a multiple of $10, it shall be rounded to the nearest multiple of $10.