Determine a nation's total consumption by multiplying its disposable income by its marginal propensity to consume. Add this total to autonomous consumption (expenditure) and you will have found consumption.
Calculate a nation's total investments by adding its capital expenditures and inventories.
Total a country's government spending by adding all expenses at each level of government.
Subtract the country's total imports from its total number of exports. This will give you its net exports.
Add the nation's sum of consumption, total investments, total government purchases, and net exports using the numbers just calculated to find aggregate expenditures.