Your goal
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Enter your goal and click Calculate.Calculate the exact monthly amount needed to reach your investment goal.
Your goal
This calculator works backwards from your future goal. You provide three things: (1) the amount you want at the end, (2) when you want it by, and (3) the rate of return you expect. The calculator solves for the monthly contribution needed to bridge the gap.
Mathematically, it's the future value of an annuity formula rearranged. The result is sensitive to all three inputs, but most sensitive to the time horizon — small changes in years can dramatically change the required monthly amount.
If you don't know what you should be investing each month, start with the 50/30/20 rule: 50% of after-tax income on needs (rent, food, transport), 30% on wants (entertainment, dining, hobbies), 20% on financial goals (saving, investing, debt repayment).
The 20% bucket is what you have available to invest. For someone earning NZ$70,000/year after tax (~$4,800/month take-home), that's about $960/month. You don't have to hit 20% from day one — even 5–10% gets you started.
Now compare what happens if you start 5 years later (20-year horizon, same goal): the required monthly jumps to ~$1,920/month — 53% more, just for waiting 5 years.
Before you commit to a monthly investment amount, consider the alternative: paying down high-interest debt. The simple decision rule:
Both work, but they're not equivalent. Investing more has a roughly linear effect — double the contribution, roughly double the final value (over the same period). Investing longer has an exponential effect because of compounding. Reaching the same goal:
| Goal | Time | Monthly required (at 7%) |
|---|---|---|
| $500,000 | 15 years | $1,580 |
| $500,000 | 20 years | $960 |
| $500,000 | 25 years | $610 |
| $500,000 | 30 years | $405 |
The takeaway: time is the cheapest form of contribution. If you can start today with a smaller amount rather than waiting until you can afford "the right amount," start now.