What it means
At its simplest, interest is rent paid for the use of money. The lender gives up the use of funds for a period and takes the risk of not being repaid, and the interest rate is the price that compensates them for both.
That is why a well-secured mortgage costs far less than an unsecured working capital facility. Simple interest is calculated only on the original principal, so a fixed amount accrues each period.
Compound interest is calculated on the principal plus interest already added, so the balance grows on itself. Over one year the two are almost identical, but over several years the gap becomes substantial in either direction, whether you are the borrower or the saver.
In business accounts, interest paid appears as a finance cost below operating profit, which is why operating profit and net profit can move very differently when borrowing costs change. Interest received appears as finance income.
Analysts look at operating profit precisely because it strips out financing decisions and shows how the trading business itself is performing. How interest is charged matters as much as the headline rate.
Facilities differ in whether interest accrues daily, monthly or annually, whether it is charged on the drawn balance or the full facility, and whether unused commitments attract a separate fee. Two loans quoted at the same rate can produce noticeably different cash costs once these mechanics are applied.
Tax treatment adds another layer, because interest on business borrowing is normally deductible against taxable profit, subject to local restrictions on how much can be claimed. That makes the after-tax cost of debt lower than the headline rate, which is one reason debt often looks cheaper than equity funding.
It is also why finance teams compare funding options on an after-tax basis rather than on the quoted rate alone.
In practice
Real-world examples.
Example
A bakery takes a $50,000 equipment loan at 6% and budgets $3,000 of interest in its first year, which it shows as a finance cost below operating profit. Because the loan amortises, the interest charge falls each year as the balance reduces.
Example
A consultancy holds $400,000 of surplus cash in a notice account paying 3%, generating $12,000 of interest income a year. The finance director treats this as a genuine offset against the interest cost of the firm's overdraft facility when reporting net finance costs.
Example
A retailer using supplier credit realises that its early settlement discount of 2% for paying 30 days sooner is effectively worth far more than the 7% overdraft rate it would pay to fund the earlier payment. It draws on the overdraft to take the discount and improves annual margin as a result.
Think of it
“Interest is the price of money-what you pay to borrow or earn by lending.
Formula
Calculation
Simple interest = principal x rate x number of years. Compound interest = principal x ((1 + rate) raised to the number of years) minus principal.
Suppose a business borrows $50,000 for three years at 6% a year. On a simple interest basis, annual interest is 6% of $50,000 = $3,000, so three years costs 3 x $3,000 = $9,000.
On a compound basis the calculation runs year by year. Year one adds $3,000, taking the balance to $53,000. Year two adds 6% of $53,000 = $3,180, taking it to $56,180. Year three adds 6% of $56,180 = $3,370.80, taking the balance to $59,550.80. Total compound interest is $3,000 + $3,180 + $3,370.80 = $9,550.80, which is $550.80 more than the simple interest result on the same headline rate.Case study
Seen in the real world.
Fenwick Tooling is a fictional engineering firm used here for illustrative purposes. It borrowed $50,000 over three years to buy a second lathe and, when comparing offers, focused only on the 6% headline rate quoted by two different lenders.
The first offer charged simple interest on the original amount, costing $9,000 over three years. The second rolled unpaid interest into the balance each year, producing $9,550.80 of interest on identical terms otherwise. The $550.80 difference was small in isolation but represented a real cash outflow that the original budget had not captured.
The illustrative lesson Fenwick's owner drew was procedural rather than dramatic: from then on the company asked every lender for the total amount repayable in cash, not just the rate. That single question made offers comparable in a way that percentages alone never did.
Watch out
Common mistakes.
- Comparing loans on the headline rate alone, when compounding frequency, fees and whether interest is charged on the drawn or committed balance can change the cash cost materially.
- Treating interest as an operating expense, when it is a finance cost that sits below operating profit and should not be mixed into trading performance.
- Forgetting that interest on a reducing balance loan falls over time, which causes budgets built on a flat annual charge to overstate later years.
Questions
People also ask.
What is the difference between interest and an interest rate?
Interest is the money amount actually charged or earned, while the interest rate is the percentage used to work that amount out.
Is interest paid tax deductible?
Interest on genuine business borrowing is normally deductible against taxable profit, though most tax systems restrict how much a highly indebted company can claim.
Does compound interest always cost more than simple interest?
Over more than one period at the same rate, yes, because later charges are applied to a balance that already includes earlier interest.
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