How I built a free financial calculator suite for India in vanilla JS (no frameworks)

#javascript #opensource #react #software

I got frustrated with existing Indian finance calculators.
They're slow, cluttered, require logins, and most don't
understand Indian-specific products like step-up SIPs,
floating rate EMI tracking, or the old vs new tax regime.

So I spent a few weeks building one from scratch.

What it does

Home Loan EMI with full month-by-month amortisation table
Prepayment tracking — add a lump sum on any month and see exact interest saved
SIP, Step-up SIP and Lumpsum projections
FD & RD maturity calculator
Old vs New Tax Regime side-by-side comparison
Goal-based SIP planner with inflation adjustment
SWP (Systematic Withdrawal Plan) calculator
Multi-loan EMI comparison with Best Value highlighting

Tech used

Pure vanilla JS — no React, no Vue, no build tools
Chart.js for all visualisations
Single HTML file — entire app is one file
Hosted free on Netlify
Mobile responsive with CSS Grid and Flexbox

Interesting challenges

Step-up SIP calculation was trickier than expected —
you can't use the standard SIP formula, you have to loop
month by month and increment the contribution every 12 months.

Prepayment tracking required rebuilding the entire
amortisation schedule dynamically on every input change,
recalculating every future month's opening balance.

Old vs New tax regime — the slab structures are
completely different so I wrote two separate tax functions
and run both on every calculation.