Example result based on the prefilled values.
Portfolio Value
NZ$142,438.10
Total Invested
NZ$53,000.00
Growth
NZ$89,438.10
Continue your plan
Useful next calculations
When to use this calculator
- Before choosing between saving, investing, or increasing your monthly contribution.
- When you want to compare best-case, base-case, and cautious return assumptions.
- When you need a quick projection before making a longer-term portfolio decision.
- When you are deciding how many more years of contributions are needed to reach a specific target balance.
- When you want to see whether starting earlier versus contributing more each month produces a bigger outcome.
A realistic New Zealand planning example
Use these sample inputs as a quick scenario test, then change one variable at a time to compare outcomes.
Initial Investment (NZ$)
NZ$15,000
Monthly Contribution (NZ$)
NZ$250 per month
Expected Annual Return (%)
8
Years
10 years
After entering these figures, review portfolio value, total invested and growth together rather than in isolation — each metric tells a different part of the story. Then rerun the tool with one input adjusted to see which variable has the biggest effect on all three outputs before you settle on a plan.
How to read your results
Portfolio Value
Use this metric to compare scenarios side by side and understand how changes in the key inputs drive the final outcome. If the figure surprises you, isolate one variable at a time and rerun the calculation to identify which assumption is responsible.
Total Invested
This is the headline outcome of the calculation, but it is most useful when read alongside the supporting metrics below it rather than in isolation. Try changing one input at a time and watching how this total moves to understand which driver has the biggest impact.
Growth
Use this metric to compare scenarios side by side and understand how changes in the key inputs drive the final outcome. If the figure surprises you, isolate one variable at a time and rerun the calculation to identify which assumption is responsible.
Method & assumptionsAuthoritative sources
This calculator models the growth of index fund investments by applying a net annual return to a starting lump sum and regular contributions. The return rate you enter should reflect expected market growth minus the fund's Ongoing Charges Figure (OCF) and your platform fee — this gives a realistic net return rather than a gross headline figure.
The calculator assumes reinvestment of all dividends and a constant annual growth rate. In practice, index fund returns fluctuate significantly year to year. Many UK financial planners use 5–7% annually as a long-term real return assumption for global equity index funds, though past performance is not a guarantee of future results. The model does not account for tax on dividends or capital gains above annual allowances if held outside an ISA or SIPP wrapper. Holding index funds within an ISA or SIPP eliminates most ongoing UK tax considerations.
Common mistakes
- !Using an assumption that is not supported by a current local quote, bill, statement or official source.
- !Treating a generic estimate as a lender, provider, payroll or tax authority decision.
- !Mixing monthly and annual inputs without converting them consistently.
- !Forgetting location-specific taxes, fees, eligibility rules or payroll deductions where they apply.
- !Testing only one scenario instead of checking how a cautious assumption changes the result.
What to do next
- Run a second scenario with a cautious assumption so you can see the downside clearly.
- Compare the result with the related calculators below before making a decision.
- Check current local rules, eligibility and provider terms before applying or committing money.
- Keep a record of the assumptions so you can update the estimate when a quote, bill or pay figure changes.
- Use the result to prepare better questions for a lender, provider, adviser or employer rather than treating it as a final answer.
Frequently asked
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