Invest or Pay Off Debt Calculator
Should you invest a lump sum or use it to pay off debt?
Enter a lump sum amount, your debt's annual interest rate, your expected investment return, and a time horizon. The calculator shows what each option is worth at the end of the period — helping you decide which gives better financial results.
Invest or pay off debt — understanding the financial trade-off
The decision between investing money and using it to pay off debt is one of the most common financial dilemmas. Both options improve your financial position, but they do so differently, and in different market and interest rate conditions one is often substantially better than the other. The core analytical framework is straightforward: paying off debt at a given interest rate is mathematically equivalent to earning that rate as a guaranteed, risk-free return. Investing offers potentially higher returns but with uncertainty and risk.
This calculator takes both options and projects them forward over your chosen time horizon. The debt payoff option shows the compound value of eliminating a debt burden at a given interest rate — what the compounded debt cost would have been, which you now avoid. The investment option shows what the lump sum would grow to if invested at your expected return rate. The comparison makes the financial difference between the options concrete.
When to pay off debt first
As a general principle, paying off debt is the better financial choice when the debt's interest rate exceeds your expected investment return. High-interest consumer debt — credit cards typically carry rates of 18–30% annually, depending on the issuer — almost always warrants prioritisation over investment. No diversified investment portfolio reliably earns 18% or more annually over multi-year periods. Paying off credit card debt is the highest guaranteed return most people can access.
Personal loans, car loans, and other consumer debt typically carry interest rates of 5–15%. At these rates, the comparison becomes more nuanced and depends on what investment return you realistically expect. If your investment portfolio historically earns 8% and your loan costs 6%, investing arguably offers a better expected return — but the loan rate is certain while the investment return is not. Many financial advisors recommend paying off debt in this middle range as a matter of risk preference, not pure mathematical optimisation.
When to invest instead
Investing is generally preferable to debt repayment when the debt interest rate is well below your expected investment return, the debt is long-duration and low-cost (such as a mortgage at 3–4%), and you have employer matching available in a workplace pension scheme that you would forfeit by diverting cash to debt repayment. Employer matching is effectively a 50–100% immediate return on contributions, which almost always exceeds the benefit of debt repayment regardless of the interest rate.
Tax-advantaged investment accounts add a further layer of complexity. Contributions to a pension, ISA, or 401(k) may be made from pre-tax income, reducing the effective cost of investment and potentially improving the investment versus debt-payoff calculation substantially. If your marginal tax rate is 40%, the effective after-tax cost of a pension contribution is 60% of the face value, which changes the break-even interest rate at which debt repayment becomes preferable.
The risk dimension
This calculator presents a pure financial comparison of expected outcomes, but the risk dimension matters too. Debt repayment offers a guaranteed outcome. Investing offers an expected outcome that may be higher or lower than modelled. If the difference in projected outcomes is small, the risk-adjusted case for debt repayment is often stronger — a guaranteed outcome close in value to an uncertain outcome with higher variance is generally preferable, all else equal. Conversely, if the investment case is dramatically better in expected terms, the risk premium may be worth accepting.
Combining both approaches
In practice, many financial planners recommend a hybrid approach: maintain minimum payments on lower-interest debt while investing surplus cash, while aggressively paying off high-interest debt. This approach eliminates the most expensive debt quickly while not completely forgoing investment growth. The exact threshold above which debt becomes "high interest" for this purpose is a matter of personal preference and risk tolerance, but rates above 8–10% are commonly treated as high-priority targets for payoff before or alongside investment.
