Borrow Interest Rate

Mitigating liquidity risk through the borrow interest rate model

Aave’s interest rate algorithm is calibrated to manage liquidity risk and optimise utilisation. The borrow interest rates are derived from the Utilisation Rate

$U$

.

$U$

is an indicator of the availability of capital within the pool. The interest rate model manages liquidity risk in the protocol through user incentives to support liquidity:- When capital is available: low interest rates to encourage borrowing.
- When capital is scarce: high interest rates to encourage repayments of debt and additional supplying.

Interest Rate Model

Liquidity risk materialises when utilisation is high, and this becomes more problematic as

$U$

gets closer to 100%. To tailor the model to this constraint, the interest rate curve is split in two parts around an optimal utilisation rate $U_{optimal}$

. Before $U_{optimal}$

the slope is small, after it begins rising sharply.The interest rate

$R_t$

follows the model:

$if \hspace{1mm} U < U_{optimal}: \hspace{1cm} R_t = R_0 + \frac{U_t}{U_{optimal}} R_{slope1}$

$if \hspace{1mm} U \geq U_{optimal}: \hspace{1cm} R_t = R_0 + R_{slope1} + \frac{U_t-U_{optimal}}{1-U_{optimal}}R_{slope2}$

In the borrow rate technical implementation, the calculateCompoundedInterest method relies on an approximation that mostly affects high interest rates. The resulting actual borrow rate is as follows:

$Actual APY = (1+Theoretical APY/secsperyear)^{secsperyear}-1$

- When$U < U_{optimal}$the borrow interest rates increase slowly with utilisation
- When$U \geq U_{optimal}$the borrow interest rates increase sharply with utilisation to above 50% APY if the liquidity is fully utilised.

Both the variable and stable interest models, are derived from the formula above from the Whitepaper with different parameters for each asset.

Variable debt see their rate constantly evolving with utilisation.

Alternatively, stable debts maintain their interest rate at issuance until the specific rebalancing conditions are met. In V3 interest models are optimised by new rate strategy parameter **Optimal Stable/Total Debt Ratio** to algorithmically manage stable rate.

$if \hspace{1mm} ratio < ratio_{o}: \hspace{1cm} R_{t} = r_{0} + \frac{ratio - ratio_{o}}{1 - ratio_{o}}R_{base}$

Model Parameters

First, it’s crucial to distinguish assets that are used predominantly as collateral (i.e., volatile assets), which need liquidity at all times to enable liquidations. Second, the asset’s liquidity on Aave is an important factor as the more liquidity, the more stable the utilisation. The interest rates of assets with lower liquidity levels should be more conservative.

It is also key to consider market conditions (i.e., how can the asset be used in the current market?). Aave’s borrowing costs must be aligned with market yield opportunities, or there would be a rate arbitrage with users incentivized to borrow all the liquidity on Aave to take advantage of higher yield opportunities.

With the rise of liquidity mining, Aave adapted its cost of borrowing by lowering the *Uoptimal* of the assets affected. This increased the borrow costs that are now partially offset by the liquidity reward.

Variable Interest Rate Model Parameters

Variable rate parameters:

- $U_{optimal}$
- Base Variable Borrow Rate
- Variable Rate Slope 1
- Variable Rate Slope 2

Stable Interest Rate Model Parameters

Stable rate parameters:

- $U_{optimal}$
- Base Variable Borrow Rate
- Variable Rate Slope 1
- Variable Rate Slope 2
- Stable to Total Debt Ratio

The stable rate provides predictability for the borrower; however, it comes at a cost, as the interest rates are higher than the variable rate. The rate of a stable loan is fixed until the rebalancing conditions are met:

- 1.Utilisation Rate:$U_t > 95\%$
- 2.Overall Borrow Rate, the weighed average of all the borrow rates:$R_O < 25\%$

The assets that are most exposed to liquidity risk do not offer stable rate borrowing.

The base rate of the stable rate model corresponds to the average market rate of the asset.

V3 Interest rate Parameters

The interest rate parameters for V3 markets have been deployed with 3 interest rate strategies calibrated per cluster of assets that share similar risk profiles.

Rate Strategy Volatile One

Volatile assets need liquidity at all times and are thus calibrated at a low Optimal Utilisation Ratio

AAVE, BAL, CRV, DPI, GHST, LINK, SUSHI, WAVAX, WBTC, WETH, WFTM, WMATIC, WONE

Parameters

Value

Optimal Usage

45%

Base Variable Borrow Rate

0

Variable Rate Slope 1

4%

Variable Rate Slope 2

300%

Base Stable Borrow Rate

2%

Stable Rate Slope 1

7%

Stable Rate Slope 2

300%

Optimal Stable to Total Debt Ratio

20%

Rate Strategy Stable One

Low liquidity stablecoins have lower Optimal Utilisation Ratio than those with higher liquidity.

DAI

Parameters

Value

Optimal Usage

90%

Base Variable Borrow Rate

0

Variable Rate Slope 1

4%

Variable Rate Slope 2

60%

Base Stable Borrow Rate

2%

Stable Rate Slope 1

0.5%

Stable Rate Slope 2

60%

Optimal Stable to Total Debt Ratio

20%

Rate Strategy Stable Two

High liquidity stablecoins which are callibrated to lower rates to encourage borrow.

SUSD, USDC, USDT, EURS, JEUR, AGEUR

Parameters

Value

Optimal Usage

80%

Base Variable Borrow Rate

0

Variable Rate Slope 1

4%

Variable Rate Slope 2

75%

Base Stable Borrow Rate

1%

Stable Rate Slope 1

0.5%

Stable Rate Slope 2

75%

Optimal Stable to Total Debt Ratio

20%

When market conditions change, the interest rate parameters must be changed to adapt to utilisation on Aave’s market as well as to incentives across DeFi.

Supply rate

The borrow interest rates paid are distributed as yield for aToken holders who have supplied to the protocol, excluding a share of yields sent to the ecosystem reserve defined by the reserve factor. This interest rate is generated on the asset that is borrowed out then shared among all the liquidity providers. The supply APY,

$D_t$

, is:

$S_t = U_t ( SB_t S_t + VB_t V_t)(1-R_t)$

- $U_t$, the utilisation ratio
- $SB_t$, the share of stable borrows
- $S_t$, the average stable rate
- $VB_t$, the share of variable borrows
- $V_t$, the variable rate
- $R_t$, the reserve factor

The average Supply APY over a period also includes Flash Loan fees.

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Interest Rate Model

Model Parameters

Variable Interest Rate Model Parameters

Stable Interest Rate Model Parameters

V3 Interest rate Parameters

Rate Strategy Volatile One

Rate Strategy Stable One

Rate Strategy Stable Two

Supply rate