Net Interest Margin is a bank's net interest income (interest earned on loans and investments minus interest paid on deposits and borrowings) divided by its average earning assets — the core profitability metric for the traditional lending business.
Why It Matters
NIM is the single number that best explains why bank stocks move with interest rate expectations — a steepening yield curve typically widens NIM (banks borrow short, lend long) while a flat or inverted curve compresses it, directly hitting the largest revenue line for most banks. It's the first metric analysts check in every bank earnings call.
How It Works in Practice
- 1Net interest income = interest income (from loans, securities) − interest expense (on deposits, borrowings)
- 2Average earning assets = the average balance of loans, securities, and other interest-generating assets over the period
- 3NIM = net interest income ÷ average earning assets, typically annualized and reported quarterly
- 4NIM varies by business mix — a bank heavy in commercial lending has a different NIM profile than one focused on low-yield, low-risk securities
Common Pitfalls
NIM compression can happen even as absolute net interest income grows, if earning assets grow faster than the margin on them — the ratio and the dollar amount can tell different stories
Deposit costs (deposit beta — how much banks raise deposit rates as market rates rise) vary enormously by bank and franchise strength, making NIM sensitivity to rate cycles hard to generalize across the industry
Comparing NIM across banks without adjusting for asset mix (commercial vs. consumer, fixed vs. floating rate) can be misleading