Why your average Aave rate differs from the current rate

The borrow rate you see on Aave is a snapshot of one moment. What a loan actually costs depends on every moment in between, including the ones nobody was watching. In September 2026 we measured how big that difference can be, and it was larger than we expected.

The spike we found

While we were building the USDC borrow rate tracker for the Ethereum Core market, the Aave app showed a steady 4.43%. To check our numbers we sampled the contract every five minutes across several days. Most of each day looked exactly as expected. Then, around midnight UTC, this happened:

For roughly 40 minutes the rate sat near 15%, more than three times its normal level, then dropped straight back. We saw the same pattern on each day we sampled. During those minutes the pool was almost fully borrowed, which put the rate on the steep part of the interest rate curve.

Forty minutes is less than 3% of a day, but at more than three times the price it matters. Written as APRs, the normal rate was 4.34% and the spike rate was 14.3%:

(1,400 min x 4.34% + 40 min x 14.3%) / 1,440 min = 4.62% APR

That is about 4.7% as an APY. Borrowers were paying roughly 4.7% while the screen said 4.43% almost all day.

Why checking the rate now and then misleads

Suppose you note the rate four times a day. Two things can happen, and both give the wrong answer:

• You miss the spike. Every reading says 4.43%, so your average says 4.43%. Too low.
• One reading lands on the spike. Your average becomes (15.4% + 3 x 4.43%) / 4, about 7.2%. Far too high.

The true figure, about 4.7%, is one that simple sampling almost never produces. Sampling more often helps, but there is a better tool built into the protocol.

The borrow index: a running interest meter

For every asset, Aave keeps a number called the variable borrow index. It started at 1 when the asset was listed, and it grows every second at whatever the borrow rate is in that second. Your debt is a fixed "scaled" amount multiplied by this index, so when the index grows by 1%, every borrower's debt has grown by 1%. There is more on this in aTokens and debt tokens.

Because the index accumulates second by second, it cannot miss anything. A 40 minute spike leaves its mark on the index even if nobody looked at the rate during those minutes.

Working out the rate borrowers really paid

Read the index at two moments, and the growth between them is the exact interest charged in that period:

APR = ln(index 2 / index 1) / seconds between readings x 31,536,000

Convert that to an APY and it is directly comparable with the rate in the app. For 19 September 2026 this method gave 4.77% for USDC, against the 4.43% displayed for most of that day.

The method has a useful side effect. If a reading is missed, nothing is lost, because the next reading still covers the whole gap. Our tracker saves the index every six hours and builds each day's average from it.

What this means if you borrow

The current rate tells you what you are paying this minute. The realized average tells you what the loan has cost so far. For budgeting, the average is the number to use, for example in the borrow cost calculator.