<P> By dimensional analysis these quantities are the ratio of a stock (with dimensions of currency) by a flow (with dimensions of currency / time), so they have dimensions of time . With currency units of US dollars (or any other currency) and time units of years (GDP per annum), this yields the ratio as having units of years, which can be interpreted as "the number of years to pay off debt, if all of GDP is devoted to debt repayment". Thus, 90% refers to a debt which would take 90% of a year's GDP to pay off . </P> <P> This interpretation must be tempered by the understanding that GDP cannot be entirely devoted to debt repayment--some must be spent on survival, at the minimum, and in general only 5--10% will be devoted to debt repayment, even during episodes such as the Great Depression, which have been interpreted as debt - deflation--and thus actual "years to repay" is debt - to - GDP divided by "fraction of GDP devoted to repayment", which will generally be 10 times as long or more than simple debt - to - GDP . </P> <P> The change in debt - to - GDP is approximately "net increase in debt as percentage of GDP"; for government debt, this is deficit or (surplus) as percentage of GDP . </P> <P> This is only approximate as GDP changes from year to year, but generally year - on - year GDP changes are small (say, 3%), and thus this is approximately correct . </P>

One way to measure national debt over time is relative to