The Debt Spiral Math: 8.5% Growth Against a 3.4% Blended Rate

I keep a sticky note above my desk that says "r minus g." It's the only equation in macro that's ever made me money, and it's the one almost nobody quotes when they're screaming about the debt. If an economy grows faster than the rate it pays on its debt, the ratio shrinks on its own. No austerity. No grand bargain. Just arithmetic.

Right now the US is winning that trade. And the market is pricing the opposite.

The Math Nobody Runs

Start with the setup everyone knows. The 10-year Treasury yield is above 5%. Federal interest costs have blown past $1 trillion on an annualized basis. Both numbers get treated as proof of a spiral.

Then there's the half that doesn't trend. The Bureau of Economic Analysis measured second-quarter growth at 8.5% annualized, before inflation. The average rate on outstanding federal debt sits near 3.4%. Growth is running two and a half times the cost of the debt.

Why the gap between 5% and 3.4%? Stock versus flow. The 5% is the marginal rate, what a new bond costs today. The 3.4% is the blended rate across roughly $30 trillion of outstanding paper, and a lot of that was issued when yields were under 2%. Average maturity runs about six years, so only a slice of the stack reprices annually. TD Securities has been making this point for a while now. Higher rates feed through as bonds roll off, not overnight. It's a slow bleed, not a cliff.

So why does every debt-spiral thread read like a countdown clock?

What It Means For Markets

Because the marginal rate still matters, even when it isn't the headline number. Every auction at 5% nudges the blended rate higher. Every nudge eats into the cushion.

Here's what matters: the direction of the blended rate, not the level of the 10-year. Treasury has been leaning on bills, short-dated paper that reprices fast. That's a bet on lower rates later. If the bet sours, the average rate climbs quicker than most models assume, and the cushion thins out.

For crypto, the read is pretty direct. A widening spread in the government's favor weakens the debasement thesis that's driven a lot of bitcoin and gold flows over the past two years. Positioning gets uncomfortable when the strongest pillar of a story gets quietly removed. I'd watch hard-asset flows for the first sign that large holders are rethinking their exposure.

My Take

The spiral isn't imminent. That's the honest answer, and it's an unpopular one. Frankly, the folks calling for a bond market collapse have been wrong for three years running, and the arithmetic explains why.

But I don't want anyone getting comfortable. The cushion here's growth, and growth is the fragile variable. Nominal GDP at 8.5% solves a lot of problems. Nominal GDP at 4% doesn't. If growth cools while the blended rate drifts toward 4.5%, the sign flips and the whole conversation changes.

From a risk perspective, that's the scenario worth modeling. Not default. Not a spiral. A simple crossing of two lines.

What to watch next: the Treasury's quarterly refunding statements, especially the split between bills and longer coupons, plus the blended interest rate in the monthly budget statement. Those two numbers tell you whether the math still holds. Everything else is noise.