New on FinoLib · Volume I: Global Economy
My latest article is now live on FinoLib, the free finance library I write: GDP Formula Explained: Components and Trade Deficits, from Volume I: Global Economy. Read the summary below for the substance of the topic, then continue to FinoLib for the complete treatment.
The idea in plain words
An economy’s books rest on identities. Y = C + I + G + NX, and national saving minus investment equals net exports, so a trade deficit is a saving shortfall financed by selling assets abroad. A tariff that does not change saving or investment cannot close a trade deficit.
Key points
■US GDP in 2025 was about $30.86 trillion: consumption $20.87 trillion (67.6%), investment $5.56 trillion (18.0%), government $5.36 trillion (17.4%) and net exports −$0.92 trillion (−3.0%).
■The US current account deficit was about $1.18 trillion, or 3.8% of GDP, in 2025, larger than net exports because income and transfers are added.
■A wider budget deficit with private saving unchanged lowers national saving and, unless investment falls, widens the external deficit: the twin deficits.
■In the small-economy example, a $50 billion gap between spending and income is financed by foreigners buying $50 billion of the country’s bonds, shares or property.
A closer look
The earlier chapters of this Part met the pieces one at a time: GDP, trade, exchange rates and reserves (Sections 0.10 and 1.7), the balance of payments. This chapter joins them into one set of books. The equations below are identities: they hold in every economy in every year because of how the items are defined. They do not tell you why something happened, but they rule out stories that cannot add up, and that is most of what a careful reader of economic news needs.
Ledger one: who buys the output. Everything an economy produces in a year (Y) is bought by households (C, consumption), by businesses building new capital (I, investment), by government (G) or by foreigners, net of what the country buys from them (NX, net exports). So Y = C + I + G + NX. Two words carry special meanings. Investment here means new machines, buildings, software, homes and inventories, not buying shares; a share purchase only changes who owns existing capital. Government purchases mean goods and services the state buys, such as salaries and equipment; a Social Security payment is a transfer, not G, and it shows up in C when the retiree spends it.
A worked example
Worked Example: Why a Trade Deficit Is a Saving Gap
A small economy produces $1,000 billion a year. Households spend $650 billion, government buys $200 billion and collects $180 billion in taxes, and firms invest $200 billion. Step Formula and substitution Result National saving S = Y − C − G = 1,000 − 650 − 200 $150 billion Private saving Y − T − C = 1,000 − 180 − 650 $170 billion Government saving T − G = 180 − 200 −$20 billion (a deficit) Net exports NX = S − I = 150 − 200 −$50 billion Check C + I + G + NX = 650 + 200 + 200 − 50 $1,000 billion = Y Households save $30 billion more C = 620, so S…
The Simple Version
A whole economy keeps books, just like a company. Everything produced is bought by someone, every dollar saved is lent to someone, and every dollar that crosses a border is recorded on both sides. A few accounting identities, rules that are true by definition, tell you what has to give when anything changes.
When This Breaks: A Peg That the Economy Stops Supporting
A peg with open capital survives as long as markets believe the central bank will defend it. When the economy needs lower rates than the anchor country, or the currency looks overvalued, speculators face a one-way bet: if the peg holds they lose little, if it breaks they win big. Every defense costs reserves, and reserves are finite.
Recap
An economy’s books rest on identities: Y = C + I + G + NX, and national saving minus investment equals net exports, so a trade deficit is a saving shortfall financed by selling assets abroad. The impossible trinity limits a country to two of a fixed exchange rate, open capital and its own interest rates.
What the full article covers
The full article includes worked examples, a recap, a reveal-on-click quiz and the official sources behind every figure.
Volume I: Global Economy
Read the full article on FinoLib
The complete article, GDP Formula Explained: Components and Trade Deficits, with every worked example, diagram, quiz and source, is free to read in Volume I, Global Economy on finolib.com.
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