Free Compound Interest Calculator for New Zealanders
Compound interest is interest earned on interest. The longer the term and the higher the rate, the bigger the gap between what you put in and what you end up with.
Your savings plan
Based on a $10,000 starting balance plus $500/month, growing at 6.0% for 20 years.
About our Compound Interest Calculator
$1 set aside today and left to compoundCompound interestInterest earned on the original principal plus all previously earned interest.View in glossary → at 6% becomes $10 in forty years. Add another dollar every year for those forty years and the same clock turns it into $164. The difference between the first scenario and the second is that interest earns interest on every dollar that has been put in along the way, not just on the original $1.
The calculator runs that same maths against the four levers you control: starting balance, monthly contribution, return rate, and time. Move any one of them and the future-value figure responds. Time is usually the most powerful, followed by contribution rate, with starting balance the smallest contributor for anyone investing for 20+ years.
For NZ savers, the most common application of this calculator is KiwiSaver projection (covered separately in the KiwiSaver page) or planning a non-KiwiSaver investment portfolio. The maths is identical; the difference is in the tax treatment, which the caveats and FAQs below cover.
How to use it
Enter the starting balance, monthly contribution, expected annual return, and number of years.
For the return rate, FMA-published KiwiSaver returns are a useful NZ-specific reference. Long-run balanced fund returns sit around 6-7% before fees, conservative around 3-4%, growth 7-9%. Subtract your fund’s annual fee for a more conservative projection.
The result panel shows the future value, your total contributions across the full period, the interest earned (future value minus contributions), and what percentage of the final balance is interest rather than principal.
Why use it
Compound interest is hard to intuit. Most people underestimate the long-run effect of small monthly contributions, and overestimate the effect of a large starting lump. The calculator makes the trade-off concrete: $400 a month for 30 years usually beats a $50,000 lump sum left to grow for the same period, even though $50,000 sounds bigger upfront.
Running the calculator three times with different return rates also disciplines optimism. A balanced KiwiSaver fund averaged 7% over the last decade; a growth fund averaged 9%. Using both as scenarios gives a high-low band for the final balance, and the gap between them is usually material across 20+ years.
The same maths drives the "save now vs save later" decision. Starting a $400-a-month contribution at 25 instead of 35 produces about 80% more at age 65, even though only 10 extra years of contributions have gone in. The early years are doing most of the work because they have the most compounding time.
The maths behind it
FV = P(1 + r/12)12n + M × ((1 + r/12)12n − 1) ÷ (r/12) P is the starting balance. M is the monthly contribution. r is the annual return rate, divided by 12 for monthly compounding. n is the number of years. The first term grows the starting balance forward through monthly compounding. The second term is the future value of an annuity, summing the future value of every monthly contribution. The calculator uses end-of-month contribution timing; if you contribute at the start of each month instead, the figure rises by one month of growth per contribution.
Worked example
Daniel, supermarket assistant manager in Hastings, starting with $5,000 and adding $400 a month from age 32 to 62.
Daniel sets up a regular saver at $400 a month, on top of the $5,000 he has put aside already. He picks an investment fund that has averaged 6% a year over the last decade.
Over 30 years to age 62, the $5,000 starter grows to about $28,700. The $400 monthly contributions, compounding alongside, add another $401,800.
Total balance at 62 lands near $430,500. Total cash put in over the 30 years is $149,000 ($5,000 + $400 × 360 months). Interest earned is the remaining $281,500, or 65% of the final balance.
Doubling the monthly contribution to $800 turns the final balance into $830,000. Halving it to $200 brings it down to $230,000. The starting balance contributes the same $28,700 in all three scenarios; the monthly contribution does most of the work.
Things to keep in mind
- Returns are not guaranteed. A 6% return is a long-run illustrative figure for a balanced fund. Real annual returns swing year to year, and a sequence of poor early-year returns can shorten the path to a target by years. The FMA publishes audited KiwiSaver returns by fund category as a NZ-specific reference.
- Inflation eats nominal returns. A 6% nominal return at 2.5% NZ inflation is closer to a 3.4% real return. The calculator works in nominal dollars; subtract your inflation assumption from the return rate to see the real-purchasing-power figure.
- Tax matters in NZ. RWTRWTResident Withholding Tax; tax taken at source from NZ-resident interest and dividends.View in glossary → on bank interest, PIE ratePIRPrescribed Investor Rate; the PIE rate the investor selects based on their income.View in glossary → on KiwiSaver and PIE funds, and FIFFIFForeign Investment Fund; non-NZ shares and certain offshore holdings taxed under special rules.View in glossary → rules on certain offshore investments all reduce the effective return. Use a post-tax return rate for a more accurate projection.
- Fees compound against you. Fund management fees of 1% a year reduce a 30-year final balance by roughly 25%, because the missing fees would otherwise have compounded too. The calculator’s rate input is pre-fee; subtract your fund’s fee for a more honest figure.
- Contribution timing. The calculator assumes monthly contributions made at the end of each month. Contributing fortnightly or weekly produces almost the same final balance over 20+ years; the small differences cancel out. Contributing once a year (rather than monthly) gives a slightly lower balance because each contribution sits idle longer before the first compounding period.
NZ-specific notes
FAQs
How is compound interest different from simple interest?
Simple interest is paid only on the original principal. Compound interest is paid on the principal plus all previously accrued interest. Over short periods (1-3 years) the difference is small. Over 20+ years, compound returns dwarf simple returns: at 6% over 30 years, a $10,000 lump grows to $57,400 with compounding versus $28,000 with simple interest.
What return rate should I use?
For long-run NZ savings projections, balanced KiwiSaver funds have averaged around 6-7% before fees and tax over the last decade per the FMA’s annual reports. Conservative funds sit closer to 3-4%, growth funds 7-9%. After fees and PIE tax, real-world returns to the investor are typically 1-2 percentage points lower than the headline figure.
Should I work in nominal or real dollars?
The calculator returns nominal future-value dollars (the actual figure on the statement). For a real-purchasing-power view, subtract your inflation assumption (commonly 2-3% for NZ) from the return rate before entering it. A 6% nominal return at 2.5% inflation is a 3.4% real return.
How does PIE tax affect KiwiSaver returns?
KiwiSaver funds are PIEs (Portfolio Investment Entities). PIE income is taxed at the investor’s PIR (10.5%, 17.5%, or 28%), capped so investors are not taxed worse than they would be on bracketed personal income. The published KiwiSaver returns are usually after the PIE tax has already been deducted at the investor level.
Why is the interest figure so much bigger than the contributions?
Compound returns are exponential, contributions are linear. Each contribution that goes in early has more time to earn returns, and the returns earn returns of their own. Over 30 years at 6%, a contribution made in year 1 grows to roughly six times its original value; a contribution made in year 25 only doubles.
What if I stop contributing partway through?
The accumulated balance keeps compounding until you draw on it, even with no new contributions. Stopping after 10 years on this calculator gives a balance after 30 years of $5,000 + 10 years of $400 contributions, all compounded for the full 30 years. Comparing "stop at 10 vs continue to 30" is a common test of how much extra the later contributions add.
References & sources
- Financial Markets Authority, "KiwiSaver annual report" (audited fund returns by category). fma.govt.nz
- Stats NZ, "Inflation and CPI". stats.govt.nz
- Inland Revenue, "Resident Withholding Tax (RWT)". ird.govt.nz
- Te Ara Ahunga Ora Retirement Commission, "Sorted goal planner and fund finder". sorted.org.nz