How loan maths actually works
What you’ll actually pay back
A loan is a deal: the lender gives you a lump sum today, and in exchange you give them a stream of equal monthly payments for an agreed number of months. The total of those payments is more than the lump sum — the difference is interest, which is what the lender earns for letting you have the money now.
The amortisation formula
The maths behind every standard SA loan is the same. Each monthly instalment is calculated so that, over the full term, the loan reduces to zero. Early payments are mostly interest because the outstanding balance is high. Late payments are mostly capital because the outstanding balance has shrunk.
P = (r × PV) ÷ (1 − (1 + r)^−n)
PV = the loan amount, r = the monthly interest rate (annual rate ÷ 12), n = the number of months. The calculator above does this for you, and shows the total interest as a separate line so you can see the cost in Rand, not just percentage.
Rate vs term: what changes what
Two levers, very different effects.
- Rate changes the slope. A higher rate means more interest accumulates each month on the same balance.
- Term changes how long that slope runs. A longer term means more months of accumulating interest, even at the same rate.
Shortening the term usually beats negotiating the rate down. A R250,000 loan at 15% over 60 months costs about R107,000 in interest; the same loan over 36 months costs R62,000. The shorter loan’s instalment is higher (R8,664 vs R5,949), but you save R45,000 of interest — money that stays with you.
Fixed vs linked rates
Two ways an SA loan can be priced.
- Fixed rate— the rate is locked for the full term. Your instalment never moves. Easy to budget; less attractive when interest rates drop because you don’t benefit.
- Linked rate— the rate is set as prime (the Reserve Bank’s benchmark) plus or minus a margin. When the SARB hikes or cuts, your instalment changes the following month. Common on vehicle and mortgage finance.
For linked loans, always stress-test. Plug today’s effective rate into the calculator, see the instalment. Then plug in the rate +2% or +3%. If your budget can’t take the higher number, the loan is too big for you — even though you can “afford” it today.
The first lever almost no one uses
Once a loan is running, the highest-impact thing you can do is pay more than the minimum instalment, early. Every extra Rand goes to capital. Every Rand of capital you knock off now means you stop paying interest on it for the rest of the term.
On a 60-month loan, an extra R500 a month from month one typically shortens the term by 8–12 months and saves R8,000–R15,000 in interest, depending on the rate. The same R500 extra starting in month 50 saves almost nothing — the interest has already been paid. Front-load the extra payments if you have a choice.
The Loan Payoff Calculator models this directly — enter the loan plus an extra payment, see the new payoff date and the interest saved.
The fees this calculator misses
Interest is the biggest cost on a loan, but not the only one. South African business loans also carry an initiation fee (one-off, often capitalised into the loan), a monthly service fee (added to every instalment), and sometimes compulsory credit life insurance. All four costs are governed by the National Credit Act, with caps the NCR publishes.
For the full all-in number — interest plus every fee — use the Business Loan True-Cost Calculator. On a small loan, fees can add nearly 20% on top of the loan amount before you pay a cent of interest.