The US can sustain a budget deficit of 5%, not 3% as commonly assumed, because 2.5% inflation and 2.5% real growth combine to keep the total debt/gdp ratio stable.
With both the financial crisis and European debt crisis having a root in excess borrowing, the American political debate has turned toward deficit reduction as well. If current budget deficits (averaging 10% of GDP since the financial crisis) are recognized as unsustainable over the long term, then what level of budget deficit is sustainable? At one extreme, politicians call for a balanced budget, and at the other extreme the budget deficit is considered a distant issue. Meanwhile, many economists set the sustainable deficit threshold at 3% of GDP, and EU rules formally set the budget deficit threshold at 3% as well. What is the basis for the idea of a “sustainable” budget deficit, and is the 3% figure too high or too low?
What is a sustainable budget?
Unlike individuals or families, a nation has an indefinite lifespan, and can therefore continually roll over its debt as long as markets deem it a worthy creditor. As long as a nation’s economy is growing, its capacity for borrowing grows as well. But if the debt grows at a rate faster than the economy, then it will eventually exceed the nation’s ability to repay it. The idea of a sustainable budget deficit is summarized by the chief economist of the Concord Seo Company Coalition, “President Obama’s fiscal commission set a goal of getting deficits down to about 3 percent of GDP within five years – 3 percent being the average annual growth rate of the US economy since World War II.”
The Real Sustainable Deficit Target
There’s just one problem with the 3% target for a sustainable budget deficit – it’s too low! While GDP growth is measured in real terms, inflation also eats away at the value of the US debt over time. For instance, assume that the US has no future economic growth, but continues to have 2% inflation. Assume that we also manage to (magically?) balance the US budget. With no economic growth, does this mean that debt/gdp stays constant? Actually, inflation would cause the numerical value of GDP to continue rising, while the debt stays constant. This would cause the debt/gdp ratio to fall by around 2% per year.
In practical terms, this means that we have to look at the rate of nominal GDP growth to determine a sustainable budget deficit level . To be conservative, let’s assume 2.5% real GDP growth (less than the 3% post-war average) and 2.5% inflation (within Americans’ comfort zone, and less than the 90’s and 2000’s average). Taken together, this means that if nominal GDP grows at 5% per year, a budget deficit of 5% can be sustained long term. The difference between 3% and 5% of GDP is big, over $300 Billion in 2012. As the federal budget and spending again enter serious debate after the November elections, it’s important that politicians understand the government’s true borrowing capacity – and neither the populist “balanced budget” nor the typical economist’s 3% magic number stand up to examination.
 Here’s the actual nominal GDP data from the Fed: http://research.stlouisfed.org/fred2/howtobcome/data/GDP.txt
Using this data, we see that nominal GDP has grown at a compound annual rate of 6.6% over the post-war period (since 1947, when the data series begins). Over the past 30 years, nominal GDP has grown at a compound annual rate of 5.4% – and this period excludes most of the late 70’s and early 80’s inflation spike. Even over the past 20 years, which are skewed downward due to the financial crisis, the nominal GDP growth rate is 4.7%.