Savings & Investment Calculator — Lump Sum + Monthly, Year by Year
Combine an initial lump sum with monthly contributions and see your compound growth broken down year by year — principal vs. investment returns at every step, not just a final number.
| Initial Investment |
JPY
|
|---|---|
| Monthly Contribution |
JPY
|
| Annual Return Rate |
%
|
| Investment Period |
years
|
| Year | Balance | Cumulative Principal | Investment Returns |
|---|---|---|---|
| {{ row.year }} | {{ fmt(row.balance) }} | {{ fmt(row.invested) }} | {{ fmt(row.returns) }} |
What Is a Savings Simulation?
A savings simulation works out what a plan of contributing a fixed amount every month, invested at compound interest, will be worth in the future. Unlike a bank deposit where interest simply piles up, the gains from investment are folded into the following year's investment base, so the longer the horizon the more the growth accelerates. Enter four things — an initial investment, a monthly contribution, an annual rate and a contribution period — and this tool shows the final total with its breakdown into principal and investment gains, in a year-by-year compound table and a chart.
Building a formula that mixes monthly contributions with a lump sum is fiddly by hand or in a spreadsheet. This tool handles the initial investment (the lump sum) and the monthly contribution (the ongoing part) together, breaking down how both grow under compounding into a yearly balance, cumulative principal and gains. Use it when planning contributions in a tax-advantaged account or setting a rough target for retirement or education funds. Everything is computed inside your browser — the amounts you enter are never sent to a server.
How to Use the Savings Simulation
- Enter the initial investment Enter any lump sum you already have. If you are starting contributions from nothing, leaving it at zero is fine.
- Enter the monthly contribution Enter the amount you can put towards investment every month. Running the figures on an amount you can comfortably sustain is the sensible approach.
- Enter the annual rate (expected return) Enter the annual return you anticipate, as a percentage. If you are unsure, the guide figures in the Tips section will help.
- Enter the contribution period Enter how many years you will contribute for. For retirement, the years remaining until you stop working; set it to suit your purpose.
- Check the result and the yearly table The final amount, the total invested and the gains appear as you type. Open «Year-by-year simulation» and you can follow the balance one year at a time.
Tips for getting more out of it
- This calculation includes the compound interest effect. The longer the period, the greater the impact.
- A 5% annual return is a rough historical average for indices like the S&P 500 and global stock index funds. It does not guarantee future results.
- The longer your investment period, the greater the benefit of compounding. Starting early is the single most important factor.
- Using tax-advantaged accounts (such as NISA in Japan, ISA in the UK, or 401(k)/IRA in the US) can make your investment returns and dividends tax-free.
- Even with zero initial investment, regular contributions alone can build significant wealth. A larger initial investment lets you benefit from compounding sooner.
When a Savings Simulation Comes in Handy
Working backwards from a target for education or a home
With a goal such as «I want a certain sum in ten years», vary the contribution and the rate and search for the combination that reaches it.
Comparing a lump-sum bonus against monthly contributions
Put the lump sum into the initial investment and the ongoing part into the monthly contribution, and see the combined compounding effect together.
Sensitivity analysis on the rate of return
Recalculate with only the annual rate changed — 3%, 5%, 7% — and compare how much the final amount moves when your assumption about returns changes.
Drafting a plan for a tax-advantaged account
Get a sense of how assets grow at a given pace of contribution within a tax-free allowance. For the details of the scheme itself, use the NISA simulator.
Checking interim milestones for retirement savings
Put the years remaining until retirement into the contribution period and you can see the balance year by year along the way. For a projection that includes living costs after you begin drawing down, use the retirement fund calculator.
Contribution and Compounding Terms
- Compound Interest
- A method that adds the gains from investment — interest, dividends and so on — to the principal and calculates the next period's gains on that total. Because gains earn gains, assets grow further ahead of simple interest the longer the period runs.
- Regular Investing
- Investing a fixed amount, or a fixed number of units, at regular intervals such as monthly or weekly, rather than committing a lump sum at once. It tends to buy less when prices are high and more when they are low, which evens out the purchase price.
- Principal
- In this tool, the total of the initial investment and the monthly contributions — that is, the money you yourself put in. It is kept separate from the gains, and the final amount minus the principal is the gain.
- Yield (Annual Rate)
- An indicator, expressed as a percentage, of how much return an invested amount earns in a year. Here you enter it as an assumption for estimating future results; it is not a guarantee of actual returns.
- Dollar-Cost Averaging
- Investing a fixed sum at fixed intervals into an instrument whose price moves, mechanically. Monthly contributions rest on this idea, and it spreads the risk of buying at a peak across time.
- Investment Gain
- The amount by which the value has grown beyond the principal you invested. In this tool's results it appears as «final amount − total invested», and the proportion tends to grow the longer compounding has run.
Frequently Asked Questions
Side Note — Why Compound Interest Was Called "the Eighth Wonder of the World"
The quote "compound interest is the greatest invention of mankind" is often attributed to Einstein, though its true origin is uncertain. Regardless, it is cited worldwide as a concise expression of the extraordinary power of compounding.
Investing $300 per month at a 5% annual return for 20 years turns a $72,000 principal into approximately $123,000. Over 30 years, it grows to roughly $250,000 — with investment returns adding more than $120,000 on top.
Compound interest works against you just as powerfully with high-interest debt. Revolving credit at high rates means you are continuously paying compound interest as a borrower. In wealth building, make compounding work for you.