According to Bloomberg, a three-to-four week government shutdown would reduce fourth-quarter economic growth by as much as 1.4 percentage points depending on its length, as government workers are furloughed.  

A shutdown would slow the expansion because output lost when workers are furloughed subtracts from gross domestic product. The combined prospect of a budget standoff between the White House and Congress and haggling over the debt ceiling could have a bigger impact on the economy as businesses hold off on investment and households delay spending.