DeFi Lending and Utilization Curves

A bank sets deposit and loan rates by committee, adjusting them when it wants more or less of a given business. A DeFi lending protocol like Aave or Compound has no committee — it has a smart contract that reads one number, the utilization rate, and computes the interest rate from a fixed formula, live, every block.

The mechanics of a lending pool

Depositors put a token into a shared pool and receive an interest-bearing claim on it. Borrowers post a different token as over-collateralization and draw against the pool. Utilization is simply:

$$U = \frac{\text{total borrowed}}{\text{total deposited}}$$

In words: what fraction of the pool's assets are currently out on loan versus sitting idle, available for depositors to withdraw. The protocol then maps $U$ to a borrow rate using a kinked curve — flat and low for most of the range, then bending sharply upward near full utilization.

The kink exists because a lending pool has a liquidity problem a bank doesn't: there's no central bank standing behind it. If utilization hits 100%, a depositor wanting to withdraw simply cannot — every token is out on loan. The steep rate above the kink is meant to make borrowing expensive enough, and depositing attractive enough, that utilization gets pulled back down before it ever reaches that point.

Worked example

A stablecoin pool has $100 million deposited and $70 million borrowed, so $U = 70\%$. Below the kink at 80%, the model charges a base rate of 2% plus a slope of 8% times utilization: $2\% + 8\% \times 0.70 = 7.6\%$ borrow APR. Deposit APY is this borrow rate scaled down by utilization itself (since only the borrowed fraction earns interest, split with the protocol) minus a reserve factor — roughly $7.6\% \times 0.70 \times 0.90 \approx 4.8\%$.

Now suppose a large withdrawal pulls deposits down to $78 million while borrows stay at $70 million: $U = 70/78 \approx 89.7\%$, past the kink. The steep post-kink slope might push the borrow rate to 35% or more overnight — a signal to borrowers to repay and to new depositors that yield just got much richer, both of which pull utilization back down.

What this means in practice

The curve is a self-correcting thermostat, not a promise. It works because it makes staying near the danger zone expensive, but it does not guarantee liquidity returns fast enough in a genuine bank-run scenario, where everyone tries to withdraw at once regardless of rate. Traders watch utilization on major pools as a live signal of funding stress, similar to how a repo desk watches overnight funding rates spike around quarter-end.