Pay off debt or invest — how does the maths compare?
Repaying a debt that charges X% a year is a guaranteed, tax-free return of X% — it's interest that will now never be charged. Investing offers an uncertain return. So the comparison is rate vs rate: the known rate on the debt against the hoped-for rate on the investment.
Two facts reorder the queue before the comparison starts: an employer retirement match is an immediate gain on the matched amount that no ordinary debt rate approaches, and without a cash buffer, the next emergency tends to be funded by new borrowing — often at the most expensive rate in the house.
The guaranteed return nobody advertises
Every extra rand paid into a debt earns a return equal to that debt's interest rate — with certainty. On a debt charging 20% a year (an illustrative rate), R1,000 extra off the balance saves exactly R200 of interest over the next year. No market delivered that; the loan contract did.
That return also arrives tax-free. There is no tax on interest you never get charged.
Rate vs rate — the actual comparison
An investment competes with that certainty using a return that isn't promised. If the debt charges 20% and the investment is hoped to earn 10% (both illustrative), the maths is settled before it starts: the certain 20% wins in every market scenario.
The comparison only becomes interesting when the rates are close — a low-rate home loan against long-horizon investing, for instance. There the arithmetic alone can't decide it, because one side is guaranteed and the other is a range of possibilities. What the maths can do is make the trade-off exact instead of vague.
After tax, the gap usually widens
Returns on ordinary investments can be taxed — interest at your marginal rate once past the exemption, and capital gains when you sell. The debt-side return carries no tax at all. A like-for-like comparison therefore puts the investment's after-tax rate against the debt's full rate.
Retirement contributions complicate this in the other direction: they're tax-deductible within limits, which lifts the effective return on that specific kind of investing. It's the one place the investment side gets its own tax advantage.
Two caveats that come before the comparison
The rate-vs-rate contest assumes a clean starting line. Two situations change the arithmetic before it begins.
- The employer match: where an employer matches retirement contributions, the matched portion is an immediate gain before any market growth — a rand-for-rand match (illustrative) is a 100% gain on the day it lands. No ordinary debt rate approaches that.
- The emergency buffer: extra repayments sent into a debt are hard to get back in a crisis, and a household with no buffer tends to fund the next surprise with fresh borrowing. How big that buffer conventionally is, this library covers in "How big should an emergency fund be?"
What the maths can't capture
A guaranteed rate and a possible rate aren't the same kind of number, even when they're equal. Money paid into a bond or loan is also less reachable than money in an access account — and some people simply sleep better watching a balance shrink than watching a portfolio wobble. The calculation frames the decision; it doesn't feel it.
Terms used on this page
- marginal rate
- The tax rate on your next rand of income — the bracket the top slice of your income falls into.
- access account
- An account you can withdraw from quickly — same day to a few days — without penalties or selling anything first.
Reviewed July 2026