How Building 36 Financial Calculators Taught Me Personal Finance from Scratch
I am a regular guy from Panipat, Haryana. I work a 9-to-5 job in digital marketing. I have a B.Tech in Computer Science. And until January 2026, I knew almost nothing about personal finance.
Not “I knew a little.” I mean genuinely nothing. I did not know the difference between SIP and lumpsum investing. I could not tell you what an emergency fund was supposed to look like. I had no idea how EMI interest actually worked, even though I had seen my family take loans.
That is embarrassing to admit publicly. But I think a lot of Indians in their 20s are in the same boat. We get engineering degrees. We get jobs. We start earning. And nobody teaches us what to actually do with the money.
So in early 2026, I decided to learn personal finance from scratch.
Not from a course. Not from a certification. Just by reading, calculating, and figuring things out on my own. And that is where the calculators started.
Learning by Building: The First Calculator
The first calculator I built was a simple EMI calculator. I wanted to understand how a home loan EMI is actually calculated. I found the formula online, but reading it on paper did not click. So I thought, let me just build an interactive tool where I can slide the loan amount, change the interest rate, adjust the tenure, and see the EMI change in real-time.
I built it. And suddenly the formula made sense to me in a way that no article ever explained.
Then I thought, what about SIP? Everyone keeps saying “start a SIP” but nobody explains what the actual returns look like over 10, 20, or 30 years. So I built a SIP calculator.
Then came the retirement calculator. The emergency fund calculator. The income tax calculator. The FD and RD calculator. One by one, every time I learned something new about money, I turned it into a calculator.
Not because I was planning to build a website, but because building the tool was how I learned the concept.
The Birth of MonuMoney.in
After building 8 calculators, I realized something interesting. Every time I searched for a financial calculator on Google, the experience was terrible.
Most sites bombard you with popups. Some force you to create an account before showing results. Almost all of them show outdated tax slabs or interest rates. And the design on most of them looks like it was built in 2010.
I thought, I already have these calculators. Why not put them on a proper website where anyone in India can use them for free, without any of that nonsense?
That is how MonuMoney.in was born.
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The idea was simple: Build every financial calculator an Indian might need. Make them completely free. No login required. No popups. No spam. Just type your numbers and get your answer.
I started with 8 calculators and planned out 36 total, covering everything from EMI and SIP to PPF, NPS, gratuity, income tax comparisons (old vs. new regime), credit card payoffs, and even a FIRE (Financial Independence, Retire Early) calculator.
(You can see all 36 calculators here: monumoney.in/calculators)
The Power of “Learning by Building”
Here is the part that surprised me the most: Building these calculators taught me more about personal finance than months of reading articles ever could.
When you have to code the income tax slab logic, you actually understand how tax works. When you build a compound interest calculator, you viscerally feel the difference between starting to invest at 25 versus 35.
There is a concept in education called “learning by building.” I did not plan for it, but that is exactly what happened. Every calculator I built became a personal finance lesson I will never forget.
Shocking Financial Truths I Discovered
Some numbers genuinely shocked me while building these tools:
- The Cost of Delaying SIPs: If you start a SIP of just Rs 5,000 per month at age 25 with a 12% annual return, you will have roughly Rs 1.76 crore by age 55. But if you start the same SIP at age 35, you will have only about Rs 50 lakhs. Same monthly amount. Same return rate. 10 years of delay cost you over Rs 1.25 crore. I did not read this in an article. I saw it on my own calculator. And it hit differently.
- The Reality of Home Loan Interest: A Rs 50 lakh home loan at 8.5% for 20 years means you pay back roughly Rs 87 lakhs total. You pay Rs 37 lakhs just in interest! But if you prepay even Rs 1 lakh per year extra, you save over Rs 8 lakhs in interest and close the loan 3 years early. Again, I did not learn this from a YouTube video. I learned it by building the loan prepayment calculator.
What’s Next?
I am still building. 8 calculators are live and working right now. The remaining 28 are planned, and I am building them one by one. The PPF calculator is next, then compound interest, then the in-hand salary calculator that converts your CTC to actual take-home pay.
Stop Reading, Start Calculating
If you are someone who has been meaning to understand your own finances better but never found the right starting point, my advice is this: stop reading and start calculating. Plug in your own real numbers. Your actual salary. Your actual loan amount. Your actual monthly savings. The moment you see your own numbers on screen, personal finance stops being abstract and starts being personal.
That is the whole point of MonuMoney. Not to give you generic advice, but to give you tools to figure out your own answers.
If you want to check them out or have a suggestion for a calculator I should build next, I would genuinely love to hear from you.
— Monu Kumar MonuMoney.in
