HOW TO USE
🎛️ Step 1 — Enter your plan (left panel)
There are four main inputs, each with a number box + a matching slider — drag or type, they stay in sync:
| Input | What to enter | Example |
|---|---|---|
| Initial deposit | Money you start with today | $10,000 |
| Monthly contribution | What you’ll add every month | $300 |
| Annual interest rate | Expected yearly return (%) | 7% |
| Years to grow | How long you’ll invest | 20 years |
Tip: start with realistic numbers — the S&P 500’s long-term average is roughly 7–10%, and a typical high-yield savings account is 4–5%.
🪙 Step 2 — Pick your settings
- Currency — choose $ / € / £ / ₹ / ₨. The whole app reformats instantly.
- Compound frequency — how often interest is reinvested: Annual, Quarterly, Monthly, or Daily. More frequent = slightly more growth.
- Inflation toggle — switch it on and set an inflation rate (e.g. 3%) to see what your money will actually be worth in today’s terms, not just the raw number.
📊 Step 3 — Read the results (top bar)
Four big cards update live as you change anything:
- Final balance — what you’ll end up with
- Total contributions — the money you put in
- Interest earned — the free money (the “magic of compounding”)
- Growth rate — your % return on contributions
📈 The chart
- Green area = total value over time
- Blue line = your contributions (it’s straight-ish; the gap between the two lines is your interest growing)
- Hover anywhere on the chart to see the exact balance and interest for that year
🔍 Extra insights below
- “How compounding frequency matters” — instantly shows your final balance if you’d compounded annually vs. daily, so you can see exactly what more frequent compounding is worth.
- Year-by-year breakdown — a table of contributions, interest, and balance for each year. Use “Show all” for every year or keep the 5-year summary.
♻️ Reset
Hit the Reset button to jump back to the default example.
Quick example to try: $0 deposit, $500/month, 8%, 30 years, monthly compounding → you’ll see roughly $745K, of which only ~$180K is your own money. That’s the compounding effect.
WealthGrowth
Final balance
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Total contributions
$0
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Interest earned
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Growth rate
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Your plan
Growth projection
How compounding frequency matters
Annually
$0
Quarterly
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Monthly
$0
Daily
$0
Year-by-year breakdown
| Year | Contributions | Interest | Balance |
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The Day I Did the Math, and It Kind of Ruined My Week
I built a compound interest calculator. But before I tell you about it, I have to tell you why, because the “why” is honestly the more interesting part.
A couple years back I was doing what I always do — promising myself I’d “get serious about saving” in January, losing interest by February, and forgetting about it by March. Rinse, repeat, every single year.
Then a friend sent me one of those links with no explanation. Just a link and a message that said: “type in your age.”
It was a compound interest calculator.
So I did. I put in what I had saved, what I could realistically put away each month, and a return rate that wasn’t optimistic, just average. And I looked at the number at the other end.
The first thing I felt wasn’t excitement. It was regret. This horrible, specific kind of regret, because the number staring back at me wasn’t “you’re doing fine.” It was “you’ve been throwing away time, and time is the one thing this whole equation runs on.”
Nobody explains it in a way that actually hits
Here’s the thing about compound interest: it’s been explained to me roughly four hundred times, and it never stuck. Every explanation is the same. “Interest on your interest.” “Your money makes money.” “The snowball effect.” Fine. Sure. I nodded along like everyone else.
But nobody ever showed me my own numbers. That’s the difference. When you see a chart of your money — not some hypothetical stranger’s money — and you watch the line do that thing where it starts slow and then goes almost vertical at the end, it stops being a finance concept and becomes a thing you feel in your stomach.
That chart is the whole point. A compound interest calculator without a graph is just a math formula wearing a nice jacket.
So I built one for myself
I’m not a financial advisor and I’m not going to pretend to be one. I just wanted a tool that showed me the thing I wished someone had shown me years ago. I called it WealthGrowth. It’s not fancy. It does exactly what it needs to do and nothing more.
You put in four things: what you’re starting with, what you can add each month, an interest rate, and how many years. That’s it. It does the rest.
But I put a few things in there that I couldn’t find elsewhere, or that were hidden behind paywalls or buried in menus.
The chart that shows the gap. This is the part that actually got through to me. The calculator draws two lines — one is your money, one is what you put in yourself. The space between them is the interest. And here’s the thing you need to see: for the first few years, that gap is basically nothing. It’s embarrassing, honestly. Then it widens. Then it widens faster. By year twenty it’s not a gap anymore, it’s a chasm. That visual is worth more than any article about the magic of compounding.
The frequency thing. Annual, quarterly, monthly, daily — everyone knows more frequent compounding is better, but better by how much? I added a little panel that just shows you all four at once, so you can see whether it’s worth caring about. Spoiler: it matters less than you’d think, but more than you’d want to ignore.
The inflation switch. This one I added because I got burned. Every calculator I tried showed me a nice big number and made me feel rich, then someone pointed out that in thirty years, that number won’t buy what it buys today. So there’s a toggle. Flip it, type in an inflation rate, and it shows you what your money is actually worth in today’s dollars. It’s less fun. It’s also the truth.
The year-by-year table. Because sometimes you don’t want a smooth line, you want to see the actual numbers, year by year, line by line, and watch the “interest” column quietly overtake the “contributions” column. That’s the exact moment the whole thing clicks.
The math, for the people who skipped that class (me)
I’m going to write the formula, but I promise it’s the last math you’ll see here.
The idea is simple. Every period, your money earns interest. Then that interest gets added to your pile. Then next period, your interest earns interest too. Over and over.
It’s why time is the most important input, not the interest rate. Everyone obsesses over getting 8% instead of 7%, when the real question is whether you started at 25 or 45. You can’t make up a decade with a better rate. You just can’t. The calculator will show you that, plainly and a little cruelly.
What I learned, and what I’d tell past-me
If I could send one message back to my twenty-year-old self, it wouldn’t be “invest smarter.” It would be: start. Even a stupidly small amount. Even an amount that feels pointless. Because the calculator taught me the one rule that actually matters: the money you put in during your twenties does more work than the money you put in during your forties, not because it’s bigger, but because it had more time.
That’s the whole trick. That’s the entire secret. There isn’t another one.
The tool is free, it runs in your browser, and it doesn’t ask for your email or send your data anywhere. Type in your numbers. Watch the line. Sit with whatever you feel.
Then do me one favor: don’t do what I did and wait.