Interest is the cost of borrowing. But "cost" isn't straightforward. The same nominal interest rate can mean very different amounts of money depending on how it's calculated and when it's applied. As a borrower, understanding how interest works is the difference between a manageable loan and one that spirals out of control.
Simple interest: the straightforward calculation
Simple interest is calculated only on the original loan amount. If you borrow £10,000 at 5% simple interest per year, you pay £500 per year. Simple interest is rare in modern lending; most formal loans use compound interest.
Compound interest: the default method
Compound interest is calculated on the original loan amount plus any accumulated unpaid interest. It's more expensive for the borrower but more common in UK lending. Over time, compound interest is significantly more expensive than simple interest, especially over longer periods or with larger loans.
Most loan agreements specify the compounding frequency: monthly, quarterly, or annually. Monthly compounding is more expensive than annual compounding because interest is calculated more often.
Fixed interest rates: predictability
A fixed interest rate stays the same throughout the loan term. You know exactly what your monthly repayment will be and aren't exposed to interest rate shocks. Fixed rates are standard for mortgages and long-term business loans in the UK.
Variable interest rates: flexibility with risk
A variable interest rate changes with market conditions, typically linked to the Bank of England base rate. When base rates change, your interest rate and monthly payment change with them. This is cheaper initially but riskier for the borrower. If the Bank of England raises base rates, your loan payments could jump significantly.
Comparing the true cost: APR
When comparing loans, ignore the headline interest rate. Instead, look at the APR (Annual Percentage Rate), which includes the interest rate plus all fees expressed as an annual percentage. This is the true cost of borrowing.
If you're reviewing a loan offer and unsure about the interest terms or whether it's competitive, see a sample report from QuickLegalCheck showing how we analyze loan terms and flag hidden costs.