How I Built a SIP Calculator (And How It Actually Works)
I’m a computer science student, not a certified financial expert. I want to say that upfront, because it matters for everything that follows. A few months ago, I got tired of finance calculators online that buried a simple answer under sign-up forms, pop-up ads, and cluttered design. So I decided to build my own.
This is the story of how I built it, the formula behind it, and what I learned along the way.
Why a SIP Calculator
A Systematic Investment Plan, or SIP, is a way of investing a fixed amount every month into a mutual fund instead of putting in one lump sum. It’s one of the most common ways people in India start investing, and the math behind it, while not complicated, isn’t something most people want to do by hand every time they want to check a number.
I picked this as my first calculator because the formula is well-documented, the use case is genuinely useful, and I use mutual funds myself, so I actually wanted this tool to exist.
How I Actually Built It
I’ll be honest about the process. I have some background in coding, but not deep financial software experience, so I leaned on AI tools to help me move faster, especially for the front-end layout and some of the trickier JavaScript logic. What I made sure to get right myself was the actual math. I didn’t want a calculator that looked good but gave wrong numbers, so I tested the formula against known, correct outputs before trusting it.
That distinction mattered a lot to me: I’m not a financial advisor, and I don’t want anyone mistaking this for professional advice. What I can vouch for is that the calculation itself follows a standard, publicly documented formula, implemented carefully.
How the Calculator Actually Works
The formula behind SIP growth is a compound interest formula, adapted for monthly contributions instead of a single lump sum:
FV = P × [((1 + r)^n — 1) / r] × (1 + r)
Where:
- FV is the future value, the total amount your SIP grows into
- P is the amount you invest every month
- r is the monthly rate of return, which is your expected annual return divided by 12
- n is the total number of months you stay invested
Here’s the part that actually matters: every monthly contribution starts compounding from the moment it’s invested, not just at the end. That means money you put in during year one has far more time to grow than money you put in during year fourteen. This is exactly why starting early changes the outcome so much, even if the monthly amount is identical.
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I also added an optional inflation adjustment, because a raw future value number doesn’t tell you much on its own. ₹50 lakh in fifteen years doesn’t have the same purchasing power as ₹50 lakh today, and I wanted the calculator to be honest about that rather than just showing an impressive-looking number.
Where You Can Actually Try It
Rather than just walking through the formula here, I’d rather you see it in action. I built the full calculator, with a year-by-year growth table, a visual breakdown of invested amount versus returns, and a downloadable PDF report, on my site here:
👉 wealthcalculatory.com/the-perfect-sip-calculator/
Go put in your own numbers. Try a few different monthly amounts and timeframes and watch how much the outcome shifts, especially when you stretch the duration out. That’s the part spreadsheets never quite make intuitive, and it’s the entire reason I wanted this tool to exist in the first place.
What I Learned Building This
A few things stuck with me from this project:
- Getting the math right is harder than making it look right. It’s easy to build something that visually works but quietly produces wrong numbers. I spent more time verifying outputs than I did on the design.
- Small UX decisions change how people actually use a tool. Adding the inflation toggle, for example, wasn’t originally part of my plan. I added it after realizing a raw maturity number without context was almost misleading.
- Being upfront about what I am and am not matters. I’m a student who built a tool, not a financial professional giving advice. I say that clearly on the site, and I’m saying it here too.
What’s Next
This SIP calculator was my first one, but it’s part of a larger project. I’ve since built out a small collection of other free calculators covering lumpsum mutual fund investing, home and car loan EMIs, step-up SIPs, savings growth, and a few others, all built the same way: formula-first, no sign-up required, nothing stored or tracked. You can see the full set here: wealthcalculatory.com
If you’re someone learning to code, or just someone trying to get a clearer picture of your own SIP numbers, I hope this was useful. And if you spot anything that looks off in the calculator itself, I’d genuinely want to know. You can reach me through Email
Note: Disclaimer: I’m not a financial advisor, and nothing in this post or the calculator should be taken as investment advice. The numbers are projections based on the inputs you provide and an assumed rate of return, not guarantees. Please consult a certified financial advisor for decisions specific to your situation.
Email: najiullahkhan1122@gmail.com
