How do I qualify for US Social Security, and how is the amount worked out?

Qualifying: forty credits

The Social Security Administration puts the threshold in one sentence. "You must earn at least 40 Social Security credits to be eligible for Social Security benefits."

Since 1978 you can earn four credits a year and no more, so forty credits is ten years. But it is not ten years of full-time work, and this is the part people miss.

Credits are bought with earnings, not with time. "In 2026, you earn one Social Security and Medicare credit for every $1,890 in covered earnings each year. You must earn $7,560 to get the maximum four credits for the year." Someone who earned $7,560 in three weeks of a year and nothing after has the same four credits as someone who worked every week of it.

So a short American stint in the middle of a New Zealand or Australian career can be worth more than it looks. Ten summers is ten years, as far as qualifying goes.

The other half of that sentence matters just as much:

The number of credits does not affect the amount of benefits you receive.

Forty credits and four hundred credits get you through the same door. What happens on the other side is a different calculation entirely.

The amount: an average, then a formula

Two steps, and they are worth keeping apart in your head.

Step one is an average. Your past earnings are adjusted, or indexed, to what they would be worth at today's wage levels, using the national average wage index. The Administration explains why: indexing "ensures that a worker's future benefits reflect the general rise in the standard of living that occurred during his or her working lifetime". Then it takes "up to 35 years of earnings", chooses "those years with the highest indexed earnings", adds them up and divides by the number of months. The result is your average indexed monthly earnings.

Step two is the formula. That monthly average is sliced in three, and each slice is paid at a different rate.

Your average indexed monthly earnings, sliced in three

90%first $1,28632%next $6,46315%everything above $7,749

For someone first becoming eligible in 2026, the Administration's own wording is that the benefit is the sum of:

(a) 90 percent of the first $1,286 of his/her average indexed monthly earnings, plus (b) 32 percent of his/her average indexed monthly earnings over $1,286 and through $7,749, plus (c) 15 percent of his/her average indexed monthly earnings over $7,749.

The percentages are fixed by law. The two dollar amounts are not: they are called bend points and they move each year with national wages. For 2026 they work out at $1,286 and $7,749.

The shape of that is the whole point. The first $1,286 of your monthly average earns six times what the dollars above $7,749 earn. Social Security replaces a much larger share of a modest income than of a large one, and that is deliberate.

The Administration's own example: a person with maximum taxable earnings every year since age 22 who retires at 62 in 2026 would have an average indexed monthly earnings of $14,358, which produces a primary insurance amount of $4,216.90. A lifetime at the very top of the earnings scale, and the formula returns under a third of it.

Why thirty-five years is the cross-border problem

Here is the arithmetic that catches people who worked in more than one country.

The number of years in that average is not set by how long you worked. It is set by your age. The Administration's manual defines computation years as "the base years having the highest indexed or unindexed earnings and equal in number to the elapsed years less drop-out years", and divisor months as "the number of months in the computation years". For retirement, elapsed years are counted from after 1950 up to the year you turn 62, and five are dropped. For anyone retiring now that leaves thirty-five.

Thirty-five years of months go into the divisor whether or not you have thirty-five years of American earnings. If twelve of your working years were in New Zealand or Australia, the American total is spread across the same thirty-five years as if you had been there the whole time.

That is the single biggest reason a cross-border worker's US benefit comes back smaller than they expected. It is not a penalty and nothing is being taken off. The average is simply being taken over a working life you spent partly somewhere else.

There is no way to fill those years from the other side, because the United States has no social security agreement with New Zealand. With Australia there is one, in force since October 2002, which is a different position entirely.

One piece of good news, though, and it is recent. A foreign pension used to cut a US benefit under the Windfall Elimination Provision. That was repealed back to January 2024, so a New Zealand or Australian pension no longer reduces what the formula above produces.

When you take it changes it again

The formula gives you the amount at full retirement age. For anyone born in 1960 or later that age is 67.

You can start at 62. The Administration's table is exact about the cost: for someone born in 1960 or later, "a $1000 retirement benefit would be reduced to $700", a reduction of 30 per cent. That is not a temporary reduction while you wait. It is the rate you are on.

Or you can wait. "Social Security retirement benefits are increased by a certain percentage for each month you delay starting your benefits beyond full retirement age", at 8 per cent a year for anyone born in 1943 or later, and "the benefit increase stops when you reach age 70."

So between the earliest and the latest date, the same work record produces payments that differ by roughly three quarters. It is the largest single lever most people have over the number, and it costs nothing to pull except time.

Two practical notes the Administration attaches to the waiting. If you delay past 65, sign up for Medicare separately at 65, because delaying that "may cost you more money" later. And if you start between full retirement age and 70, some of the credits earned in the year you start are not added until the following January.

What this guide does not tell you

It does not tell you your number. That depends on your actual earnings record, which only the Administration holds, and its own estimate through a my Social Security account is the only one worth acting on.

It leaves alone spousal and survivor benefits, disability, Medicare, and how much of a benefit is taxable in the United States.

It also says nothing about what happens to the payment once you leave the country, which is a separate rule and a strict one: whether it continues at all depends on your citizenship.