Debt sustainability: why the number that matters isn't the debt level

TL;DR

Debt sustainability isn't about how much you owe — it's about whether your economy grows faster than your interest bill. The key variable is the interest-growth differential (r minus g). When growth exceeds interest rates, debt ratios shrink on autopilot. When it doesn't, even modest debt becomes a trap.

What it means (plain English)

Governments borrow constantly. The question isn't whether they should — it's whether the debt is on a stable path. Three things determine that:

• Debt-to-GDP ratio: the stock of debt relative to the size of the economy. This is the headline number, but it's the least informative on its own.
• Primary balance: government revenue minus spending, excluding interest payments. If you're running a primary surplus, you're generating cash to service debt. If not, you're borrowing to pay interest — which compounds.
• Interest-growth differential (r - g): if the interest rate on your debt (r) is lower than your GDP growth rate (g), the debt ratio naturally falls over time even without surpluses. If r exceeds g, you need ever-larger surpluses just to stabilize. The IMF's World Economic Outlook publishes fiscal projections that let you track r-g dynamics across countries.

Common misconception

"Any country with debt above 100% of GDP is in trouble." Japan has been above 200% for years and borrows at near-zero rates because it has a captive domestic savings pool, its own central bank, and debt denominated in its own currency. Meanwhile, countries borrowing in dollars with volatile growth and thin domestic savings can blow up at 40%. Context is everything. The Debt-Trade Spiral shows how external deficits accelerate this dynamic.

Headline translation

When you read: "Country X's debt hits record high," ask: what's happening to r minus g? A rising debt ratio during a growth boom with low rates is very different from a rising ratio during stagnation with rising rates. The trajectory matters more than the snapshot.

A concrete example

Italy's debt-to-GDP has hovered around 140% for years. When ECB rates were near zero and nominal growth was positive, the debt ratio was roughly stable — uncomfortable but manageable. The moment rates rose faster than growth, Italy needed primary surpluses just to keep the ratio from exploding. The arithmetic is mechanical and merciless — you can compare Italy and Japan's fiscal trajectories on MacroVedia. Aging Economies explains why demographics make this calculus harder over time.

The variables that actually matter

• Currency denomination: borrowing in your own currency means you can't technically default (you can always print). Borrowing in someone else's currency means you can.
• Maturity structure: long-dated debt gives you time. Short-dated debt means you're constantly rolling over at whatever rate markets demand.
• Who holds it: domestic savers are sticky. Foreign portfolio investors are not.
• Growth prospects: a country that can plausibly grow 5% has more room than one stuck at 1%.

If you only remember one thing...

Don't look at the debt level. Look at r minus g. That single differential tells you more about fiscal sustainability than any headline number ever will.