Investing Amount Calculator

What actually drives how much you need to invest

The monthly figure this calculator gives you comes down to three levers: how much time you have, what return you expect, and what you're starting with. Change any one of them and the required contribution shifts, sometimes dramatically.

Timeframe is the biggest lever by far. Stretching a goal from five years to ten doesn't just halve the required contribution, it does better than that, because your earlier contributions have longer to compound. Starting earlier almost always beats contributing more later.

Return rate assumptions matter too, but be realistic. It's tempting to enter a high number to make the monthly figure look manageable, but an optimistic assumption can leave you short at the finish line, especially for shorter goals where there's less time to recover from a bad run. Contributing regularly, weekly, fortnightly or monthly rather than a single lump sum, also takes the pressure off trying to time the market. This is dollar cost averaging: you buy more units when prices are low and fewer when they're high, a habit rather than a decision you have to keep making.

What this looks like with real numbers

Say your goal is $500,000, you already have $10,000 saved, and you're weighing up a few different timeframes and return assumptions. Here's the monthly contribution each combination requires.

Starting balance $10,000, target $500,000, contributions made monthly.

Look at the 10-year rows first. Even with a better return, you still need to put away roughly $2,600 a month, a serious ask for most Australians on a median income. Now look at the 30-year rows: at 8% p.a. you need just $262 a month for the same $500,000. That difference isn't a better return, it's time. An extra 20 years cuts the required monthly contribution by more than 90%, because your $10,000 starting balance alone grows to roughly $109,000 over 30 years at 8%, before you've contributed an extra dollar.

Here's a second example: a $1,000,000 goal, starting with $20,000, over 30 years at 8% p.a. Your $20,000 grows to roughly $218,700 on its own, leaving about $781,300 for contributions to cover. Run that through the annuity formula and you land on approximately $524 a month, a realistic figure for many households, especially once you factor in salary growth over three decades.

Four mistakes that will throw your plan off

The calculator gives you a clean number. These are the things that quietly turn it into a plan that fails.

Assuming unrealistically high returns. Plug in 12-15% and the monthly figure looks easy, but that's not a realistic long-run assumption for a diversified Australian portfolio. The ASX has delivered roughly 10% p.a. in nominal terms over the long run, and once you strip out inflation of around 2.5-3%, the real return is closer to 7%, for an all-shares portfolio with no diversification or fees. A realistic diversified portfolio typically lands in the 6-8% range in nominal terms. Use 6% for conservative planning and 8% for a growth-oriented portfolio with a longer runway, not higher.

Ignoring inflation. Your $500,000 target sounds like a lot today. In 30 years, at 2.5% inflation, it buys roughly what $235,000 buys now. You can handle this by using a real return (nominal return minus inflation) so everything stays in today's dollars, or by setting a higher nominal target to account for the erosion. Either works, but ignoring it entirely doesn't.

Not stepping up contributions as income grows. Most people set a monthly figure and leave it there for years, missing an easy win. If your salary grows 3-5% a year, that's an opportunity to increase contributions rather than let lifestyle costs absorb all of it. Even a 5-10% annual step-up compounds meaningfully: starting at $500 a month and increasing 5% a year gets you to $638 by year five and $814 by year ten.

Treating the output as a guarantee. The calculator assumes a smooth, consistent return every year. Real markets don't work like that: the ASX fell roughly 38% in 2008 and rose over 23% in 2019. Use the output as a planning tool, not a contract, and revisit it every year to recalibrate against what actually happened.

Turning the number into a real plan

Set up an automatic transfer from your everyday account to your investment account the day after your pay lands, so you spend what's left rather than what you intended to invest. If you're paid fortnightly, consider splitting your monthly target across two fortnights to smooth the cash flow. Working out where that contribution fits inside your broader budget is easier with a framework like the 50/30/20 rule, which splits your take-home pay into needs, wants and savings.

If the required contribution looks too high, you've got three levers: extend the timeframe, lower the target, or accept a higher return assumption with eyes open to the risk that comes with it. Don't just inflate the return rate to make the number feel more comfortable. Clearing high-interest debt first is also worth doing before you ramp up contributions, since paying off a card at 20% p.a. is a guaranteed return most investing strategies won't beat, and keeping an emergency fund separate means you're never forced to sell at the wrong time to cover a surprise cost.

Don't forget superannuation, either. Your compulsory super contributions are already building long-term wealth even though they're not captured here. This calculator is best used for goals outside super, a house deposit, an investment portfolio, or financial independence before preservation age. For retirement-specific planning, a dedicated retirement calculator or a licensed financial adviser will give you a fuller picture.