What Is Net Interest Margin? A Plain-English Guide for Canadian DIY Investors

Net interest margin (NIM) measures how much a bank earns on its lending and investments after paying for the money it borrows. For Canadian bank investors it’s one of the three or four numbers that move earnings most — along with loan growth, credit losses and expenses.

Educational glossary entry — not a recommendation. Worked examples are hypothetical; the RBC figure is from its 2025 annual report.

Direct answer

NIM = net interest income ÷ average interest-earning assets, expressed as a percentage. Net interest income is interest earned on loans and securities minus interest paid on deposits and other funding. If a bank earns $16 billion of net interest income on $1,000 billion of average earning assets, its NIM is 1.6%. Small moves matter: on that balance sheet, a 0.05-point (5 basis point) change in NIM is $500 million of annual revenue. For scale, Royal Bank of Canada reported an all-bank NIM of 1.62% on average earning assets for fiscal 2025.

What moves NIM

Driver	Pushes NIM up	Pushes NIM down
Policy rates	Rising rates when assets reprice faster than deposits	Falling rates when loan yields reset down faster than deposit costs
Deposit mix	More chequing and low-rate savings deposits	Shift to GICs and high-interest savings
Asset mix	More credit cards and business loans	More low-spread mortgages and liquid securities
Mortgage renewals	Maturing low-rate mortgages renewing at higher rates	Renewing into lower rates
Competition	Rational pricing	Deposit or mortgage price wars
Hedging	Gains as hedges roll	Hedges locking in older rates

Reading NIM in Canadian bank results

Check	Why it matters
Which NIM? All-bank vs a segment (e.g., Canadian personal banking)	Segment NIMs are usually much higher than the all-bank figure, which includes low-spread trading and treasury assets
Definition	Banks differ: some use average earning assets, some exclude trading assets; compare a bank against its own history first
Basis points, not percent	A “4 bp” move sounds tiny but can be hundreds of millions of dollars
Volume × margin	Net interest income growth = asset growth and NIM change; one can hide the other
Management’s outlook	Look for NIM guidance and rate-sensitivity tables in the MD&A

NIM isn’t profit. A bank can widen NIM by lending to riskier borrowers and then give it back in provisions for credit losses. Read NIM together with credit and efficiency — see bank operating metrics to track.

Related reading

• Financials – Banks: industry structure and how money is made
• Valuation for banks: P/B, ROE, and why P/E alone fails
• What is ROE?
• How Royal Bank of Canada (RY) makes money
• Living sector snapshot: Banks · dated brief: TD

Sources

• Royal Bank of Canada, 2025 Annual Report, Table 1 (net interest margin on average earning assets, net: 1.62% for fiscal 2025) — rbc.com.
• Each bank’s quarterly report to shareholders and supplementary financial information (NIM definitions and rate-sensitivity disclosures).

Media credits

• Hero: “Time lock.agr.jpg” (time lock on a bank vault door) by ArnoldReinhold, CC BY-SA 4.0, via Wikimedia Commons. Cropped and resized to 1280×720. Real photograph, not AI-generated.

By Andrew · stocksummarizer.com