WASHINGTON (REUTERS) - The United States government's over US$4 trillion (S$5.5 trillion) annual budget, the world's largest, relies heavily on individual wage earners whose taxes and retirement benefits are deducted from every pay cheque, leaning particularly on the top 20 per cent of income earners.

Corporations pay just a fraction of what individuals do into the federal spending pool, which funds the military, transportation safety, veterans benefits, regulatory agencies and programmes like Nasa.

A New York Times investigation published on Sunday (Sept 27) shows that President Donald Trump paid just US$750 in federal taxes during the years straddling his 2017 inauguration, and none at all for 10 of 15 years before then. Mr Trump dismissed the report as "fake news".

Mr Trump reported income of at least US$594 million for 2016 and early 2017, and assets worth at least US$1.4 billion, in a financial disclosure in June 2017.

HOW THE U.S. BUDGET IS FUNDED

Individuals, whether they are self-employed or earn a pay cheque from a small business or a giant corporation, foot most of the federal government's bills.

Of the US$3.46 trillion in receipts taken in by the US Treasury during fiscal 2019, nearly half came from the US$1.72 trillion in individual income taxes collected.

In addition, US$1.24 trillion in Social Security and Medicare taxes were paid by individuals, bringing their share to 85 per cent.

Taxes paid by corporations last year totaled US$230 billion, or just 6.6 per cent of the total in 2019. The remainder of federal revenues are made up from customs duties on imported goods, excise taxes such as those on gasoline, estate taxes and other miscellaneous taxes and fees.

WHO PAYS THE MOST?

W2 wage earners - those with a regular pay cheque from a business, government entity or non-profit - make up the largest share of tax revenue through income tax and social insurance withholdings, like the payroll tax that funds Social Security, the government retirement programme.

Those withholdings can reduce Americans' take-home pay by nearly 40 per cent, depending on income, and have made up about 73 per cent of total revenue over the past two fiscal years through August.

Despite high unemployment because of the coronavirus pandemic, withheld income taxes in 2020 have fallen by less than 1 per cent from 2019 levels, partly due to higher earnings early in the year and aid to small businesses that kept pay cheques coming to many idled employees over the spring and summer months.

Self-employed people, including many business owners, and those paying capital gains or other taxes not withheld from their pay cheques, make up the second-largest category, funding around 19 per cent of the total tax revenue during fiscal 2020 through August.

TAXES BY INCOME LEVEL

The US tax system has been described as "progressive", meaning that the share of taxes paid rises with income, through seven tax brackets.

The very poorest, those making under US$9,875 a year, are taxed at a 10 per cent rate, while the wealthiest, or those making US$518,400 and over are taxed at a 37 per cent rate. But what the wealthiest pay is often much lower due to a variety of complicated tax loopholes, which can benefit hedge fund managers, private equity firm partners and real estate investors.

The 2017 tax cuts passed by Republicans and signed by Mr Trump largely kept that relationship intact, but shifted more of the tax burden to wealthy and upper-middle-class wage earners. But income deductions for mortgage interest and state and local taxes paid, the biggest middle-class tax breaks, were reduced considerably.

America's 37 per cent top marginal tax rate is lower than the top rates for many wealthy and developing countries, including the OECD average of 41.2 per cent, according to KPMG.

Corporations got a major tax cut in the Republican bill, when their tax rate before deductions was slashed to 21 per cent from 35 per cent.

IS THE UNITED STATES COLLECTING ENOUGH TAX?

No. Even before the coronavirus pandemic sparked a deep recession this year, the federal budget deficit in fiscal 2019 was US$984 billion and was forecast in January to top US$1 trillion in fiscal 2020, which ends on Wednesday (Sept 30).

Massive spending to keep businesses and households from collapsing is expected to drive the fiscal 2020 deficit to US$3.3 trillion, dwarfing the previous record of US$1.4 trillion in 2009 and making up the largest share of gross domestic product since the end of World War II.

If action is not taken to shrink deficits in future years through spending cuts or tax increases, the Congressional Budget Office has warned that federal debts as a percentage of GDP will double by 2050.