Banks are businesses, and like all businesses, they exist to generate profit. Their revenue streams are more varied than most people realise, but all flow from the same fundamental position: banks sit at the centre of money flows and earn from every service they provide in that role.
Net interest income: the core business
The largest income source for most banks is net interest income — the difference between the interest rate they charge on loans and the rate they pay on deposits. This difference is the net interest margin (NIM).
If a bank lends at 5% and pays depositors 1%, that 4% spread on each pound lent is the gross margin on lending. Across a loan book of £100 billion, a 4% NIM generates £4 billion in gross lending income annually. The bank then incurs costs — staff, technology, credit losses, regulatory compliance — and the remainder is profit.
Why the NIM changes over time
The NIM is not fixed — it changes with interest rates and competition. When the Bank of England raises its base rate, banks typically raise their lending rates faster and more than they raise savings rates, initially widening the NIM. This is why bank shares often rise when rates increase. Over time, competition for deposits puts upward pressure on savings rates, gradually compressing the margin. When rates fall to near zero (as they did from 2009 to 2022), NIMs compress dramatically, significantly hurting bank profitability.
Fee and commission income
Fee income is the second major revenue stream. Banks charge for account maintenance, overdrafts, foreign exchange transactions, wire transfers, investment advice, and structuring financial products. For many retail banks, fees account for 30-40% of total revenue. Fee income is valuable because it is relatively stable — it does not fluctuate with interest rates the way lending income does.
Investment banks earn from trading (buying and selling financial instruments for their own account and for clients), underwriting (guaranteeing the success of a share or bond issuance for a fee), and advisory services (charging fees for advising on mergers and acquisitions, typically 0.5-2% of deal value). Goldman Sachs and Barclays' investment bank earn the majority of revenue from these activities rather than traditional lending.
A bank's core business model is elegantly simple: borrow cheap from millions of savers, lend at a higher rate to borrowers, and keep the spread — doing this profitably at enormous scale across millions of relationships simultaneously.
The cost side: what eats bank profits
Revenue is only half the story. Credit losses — borrowers who default on loans — are the biggest risk to bank profits. When economic conditions deteriorate, default rates rise and banks must set aside loan loss provisions, directly reducing profit. In the 2008 crisis, credit losses wiped out the capital of multiple major banks entirely.
The efficiency ratio — operating costs as a percentage of revenue — is a key metric for assessing bank management quality. A ratio below 50% indicates a highly efficient bank; above 70% suggests costs are eating most of the revenue. UK retail banks typically operate between 55-65%.
How 2008 changed banking forever
The 2008 financial crisis dramatically reduced bank profitability through higher capital requirements, tighter lending standards, increased compliance costs, and years of near-zero interest rates that compressed NIMs almost to zero. Banks responded by aggressively cutting headcount, investing in digital technology (reducing branch costs), and shifting toward fee-generating businesses less exposed to credit risk and interest rate cycles.