First Home Deposit vs Invest Calculator

Money being saved for a house deposit is not ordinary investment money, and the reason is a single feature: it has a date attached. Ordinary long-term investing survives a market fall because you can simply not sell, wait, and let the recovery happen. A deposit cannot do that. It is needed on a particular day, and a fall shortly before that day hits the entire balance with no time left to recover from it. That one difference inverts most of the usual advice about horizons, because a longer saving period does not reduce this risk, it increases the amount exposed to it. This page runs both paths honestly. It taxes cash at your marginal rate, since bank interest is subject to resident withholding tax, and taxes the invested path at prescribed investor rates, because comparing a gross fund return against an after-tax cash rate is the most common way this decision gets made badly. It tracks both against your actual deposit target, so you can see whether either path gets you there, which frequently turns out to be the more useful finding. And it tests the scenario that actually matters, which is a fall landing just before settlement, then shows the upside and the downside side by side rather than only the expected outcome. Where neither path reaches the target in time, the honest conclusion is usually that the return is not the lever worth pulling, and the amount saved each month or the date itself is.

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A Date Changes Everything

The standard argument for investing over cash is that time smooths out falls. It is a good argument, and it depends entirely on being able to wait.

A deposit cannot wait. It is needed when the purchase happens, and if the market falls the month before, the balance is what it is. There is no recovery period because there is no time left.

That is why this decision does not follow the usual rules, and why a longer saving period makes the exposure larger rather than smaller.

Worked Example: $150,000 Deposit In Four Years

Starting with $40,000.00 and saving $1,500.00 a month, contributing $112,000.00 in total.

In cash at 3.50% taxed at 33%, netting 2.35%: $119,337.32.

Invested at 7.00% less a 0.30% fee and PIE tax, netting 5.30%: $129,428.08.

Invested, after a 20% fall just before settlement: $103,542.47.

So the upside is $10,090.76 and the downside is $15,794.85. The downside is larger.

The Number Worth Remembering

On the worked example a fall of 7.80% at the wrong moment wipes out the entire expected advantage of investing.

Falls of that size are not unusual events. They happen in ordinary years without anything remarkable going on, which means the advantage is genuinely fragile rather than theoretically at risk.

Anything larger than that and the cash path would have been better, and you would be looking at a smaller deposit at exactly the moment you need it.

Neither Path Might Get You There

The most useful line on this page is often the shortfall column. On the worked example the deposit target is missed by $30,662.68 in cash and $20,571.92 invested.

When both paths fall short, the return is not the lever. Closing a $30,000 gap through investment returns over four years means taking a great deal of risk for an outcome that still may not arrive.

The levers that actually work are the monthly amount, the date, and the target itself. Our house deposit savings calculator works the required monthly saving backwards from a date, which is the more productive question.

When Investing A Deposit Does Make Sense

Two situations genuinely change the answer.

A soft date. If buying in four years means "four years, or six if the market is down", the money behaves more like ordinary investment money and a fall becomes survivable. The date being genuinely flexible is the thing that matters, not the length of it.

A deposit already secured. If you have the deposit and are saving beyond it, the surplus is not deposit money and does not carry this constraint.

Where the date is firm and the deposit is not yet there, the case for cash is strong regardless of what the expected return says. Our savings to investment switch calculator covers the general version of this trade-off, where the horizon is flexible.

Tax Both Sides The Same Way

A frequent error is comparing a gross fund return against an after-tax savings rate, which flatters investing considerably.

On the worked example the fund returns 7.00% gross and nets 5.30% after fees and PIE tax. The savings account pays 3.50% gross and nets 2.35% after tax at 33%.

The real gap is 2.95 percentage points, not the 3.50 the headline numbers suggest. Our PIE savings vs bank savings calculator handles the tax comparison in detail.

What This Page Leaves Out

KiwiSaver. The first home withdrawal has its own rules, and whether to move a KiwiSaver fund to a conservative option before buying is the same logic applied to a different account.

House prices moving. A rising market raises the deposit you need while you save, which is a real risk that works against saving slowly. It is not a reason to take market risk with the deposit, since your investments falling and house prices rising can easily happen at once.

The First Home Grant and other assistance, which change the target rather than the saving strategy.

Related NZ First Home Calculators

• House Deposit Savings Calculator: the monthly saving a date requires.
• Savings To Investment Switch Calculator: the same trade-off with a flexible horizon.
• PIE Savings vs Bank Savings Calculator: taxing both sides properly.
• Emergency Fund Placement Calculator: where short-term money belongs.
• Cash Drag Calculator: what holding cash costs when there is no date.
• First Home Buyer Guide: background on this topic.