Cash vs Invest Calculator

Holding a sum in cash and holding it in investments are the same decision made twice: once about return and once about certainty. This calculator prices only the first, showing what the difference in assumed growth rates comes to across a chosen period.

At the defaults, 50,000 compounding at 3% reaches 90,305.56 after twenty years while the same sum at 7% reaches 193,484.22. The gap is 103,178.66, which is more than twice the original amount. Cash is not standing still in that comparison; it is simply growing at less than half the rate.

What the figure does not say is which side is preferable, because the two rates are not the same kind of number. A cash rate is broadly known in advance and the balance does not fall. An investment return is an assumption with variation around it, including periods where the balance is lower than it started. Comparing them as though both were certain is what the arithmetic here does, and it is the main thing to hold in mind when reading the output.

Quick example

With an amount of 50,000, a cash rate of 3%, an investment return of 7% and a twenty-year horizon, the result is 103,178.66. The supporting rows show a cash value of 90,305.56, an investment value of 193,484.22, a return gap of 4.00% and the horizon.

Which inputs matter most

The two rates do not carry equal weight, which is easy to miss because the result card reports a single return gap. Raising the investment return from 7% to 8% widens the gap by 39,564, or 38%. Raising the cash rate from 3% to 4% narrows it by only 19,251, or 19%. The investment side moves the answer roughly twice as hard, because it is compounding from a larger base every year.

That also means the spread between the rates does not determine the result on its own. Adding a percentage point to both, so 4% against 8% with the same four-point spread, produces 123,492 rather than 103,179. The levels matter as well as the difference between them.

Amount scales the gap exactly proportionally, so doubling it doubles the answer and changes nothing else. The horizon is the input with the most leverage per unit: extending from twenty years to twenty-one adds 10,835 to the gap, more than a full percentage point on the cash rate removes.

What is happening under the hood

Each amount is compounded annually at its own rate for the number of years given, and the cash result is subtracted from the investment result. That is the whole calculation.

Two properties follow from the structure. Annual compounding is assumed on both sides, so a cash account paying monthly and an investment reinvesting quarterly are both approximated rather than modelled. And because the result is a difference between two exponential curves, it is not proportional to time: five years gives 12,164, ten years 31,162, twenty years 103,179 and thirty years 259,250.

Why the number matters

An opportunity cost is invisible by nature. Money in a savings account does not feel like it is costing anything, because the balance only ever rises, and nothing on a statement shows the alternative that was not taken. Putting a figure on it is the only way to make the trade-off visible enough to weigh.

The figure is not an argument for one side. It is one half of a comparison whose other half is certainty, liquidity and the ability to spend the money at short notice without regard to what markets did that month. Neither half is worth anything without the other in view.

Where to go next

This calculation rarely sits alone in a planning exercise. The compound interest calculator covers the growth mechanics on their own, the emergency fund calculator sizes the cash that is held back from the comparison in the first place, and the bucket retirement strategy calculator handles the same question across several time horizons at once.

Why the gap widens rather than grows

The gap between holding cash and investing widens geometrically, not steadily, because both sides compound at different rates. At the defaults, 50,000 over twenty years reaches roughly 90,300 at 3% and about 193,500 at 7%, a difference near 103,200. Over ten years the same rates produce a gap closer to 31,200, so about 70% of the twenty-year difference accrues in the second decade.

That shape has a practical consequence. A short horizon produces a small gap, which is why the calculation says little about money that may be needed within a few years, and a long horizon produces a large one, which is why the same 4-point rate difference looks trivial at five years and dominant at thirty.

What the comparison flattens

The comparison is nominal and deterministic on both sides, which flattens two differences that matter.

The first is inflation, and it applies to both sides rather than one. Neither figure is adjusted, so both are overstated in purchasing-power terms. The arithmetic is stark at the defaults: with inflation also at 3%, the 90,305.56 of cash after twenty years buys exactly what 50,000 buys today, a real return of nothing at all. The investment side would be worth about 107,100 in today's terms on the same assumption, so the real gap is roughly 57,100 rather than 103,200. Central banks publish the consumer price measures these adjustments are made against.

The second is uncertainty. A cash rate is set and broadly knowable; an investment return is an expectation with dispersion around it, and the order in which returns arrive changes the outcome for anyone drawing on the money. Securities regulators worldwide require that distinction to be stated in anything sold as an investment, and it applies equally to a calculator projecting a smooth curve. The single figure here is a central case, not a floor.

Tax is the third simplification. Interest and investment gains are frequently taxed under different rules and at different rates, and tax-advantaged accounts change both sides again, so the after-tax gap can be materially different from the nominal one in either direction.