The most common approach to measuring and understanding GDP is the expenditure method:

GDP = consumption + gross investment + government spending + (exports − imports), or,
GDP = C + I + G + (X-M).

"Gross" means depreciation of capital stock is not subtracted. If net investment (which is gross investment minus depreciation) is substituted for gross investment in the equation above, then the formula for net domestic product is obtained. Consumption and investment in this equation are expenditure on final goods and services. The exports-minus-imports part of the equation (often called net exports) adjusts this by subtracting the part of this expenditure not produced domestically (the imports), and adding back in domestic area (the exports).

Economists (since Keynes) have preferred to split the general consumption term into two parts; private consumption, and public sector (or government) spending. Two advantages of dividing total consumption this way in theoretical macroeconomics are:

• Private consumption is a central concern of welfare economics. The private investment and trade portions of the economy are ultimately directed (in mainstream economic models) to increases in long-term private consumption.
• If separated from endogenous private consumption, government consumption can be treated as exogenous,Fact: date=March 2008 so that different government spending levels can be considered within a meaningful macroeconomic framework.

GDP vs GNP

GDP can be contrasted with gross national product (GNP, or gross national income, GNI), which the United States used in its national accounts until 1992. The difference is that GNP includes net foreign income (the current account) rather than net exports (the balance of trade). Put simply, GNP adds net foreign investment income compared to GDP. United States GDP, GNP and GNI (Gross National Income) can be compared at EconStats 1.

GDP is concerned with the region in which income is generated. It is the market value of all the output produced in a nation in one year. GDP focuses on where the output is produced rather than who produced it. GDP measures all, disregarding the firms' nationality.

In contrast, GNP is a measure of the value of the output produced by the "nationals" of a region. GNP focuses on who owns the production. For example, in the United States, GNP measures the value of output produced by American firms, regardless of where the firms are located. Real GNP in the year 2007 was 3.2.