The article cites the government's $637 billion debt, which is the net of its financial and non-financial assets, but its interest costs are actually calculated on its gross interest-bearing debt, which includes its outstanding bonds and treasury bills, plus its pension liabilities  (veterans' and civil service pensions). Moreover, it issues bonds and T-bills on behalf of itself and most of its Crown corporations. And right now, the average interest rate it is paying is less than 3 per cent (see its Fiscal Monitor at Dept. of Finance). The gross interest-bearing debt is now close to $1 trillion, so one can readily see how much how much the federal government's interest expense would grow on the basis of any given increases in the rates it has to pay.