GrowthCalc – Compound Interest Calculator

Free, static compound interest calculator. Pure HTML/CSS/JS – no build step, no dependencies. Deployed as-is to Vercel.

Live URL: https://growthcalc-tool.vercel.app/ (placeholder – replaced after first deploy)

Features

Free

• Initial investment, monthly contribution (start/end of month), annual contribution increase
• Annual rate with daily / monthly / quarterly / annual compounding (exact per-period math, simulated month by month)
• 1–50 years (slider + number), 8 currencies via Intl.NumberFormat
• Optional inflation toggle → "in today's money" values
• Big final balance, total contributions, total interest, interest %, doubling time (rule of 72 + exact)
• Hand-built inline SVG stacked bar chart (contributions vs interest) with hover tooltip
• Yearly and monthly breakdown tables
• Copy summary, share link (inputs in URL), reset, localStorage persistence
• SEO content + FAQ with JSON-LD WebApplication and FAQPage

Pro (gated with isPro(), one-time licence)

• Compare up to 3 saved scenarios (free: 1) – comparison table + overlay lines on the chart
• Goal calculator: "reach X in Y years – what monthly contribution?" (bisection solver)
• CSV export of the yearly/monthly table
• Printable PDF report (print stylesheet + window.print())

Local run

npx serve .

Then open the printed URL (e.g. http://localhost:3000). cleanUrls means /privacy and /terms work without .html on Vercel; serve handles that too.

Monetization setup

All switches live in config.js:

1. AdSense – set ADSENSE_CLIENT to your ca-pub-… id. Ad slots (.ad-slot[data-slot]) render real units; until then they show a dashed placeholder. Add the slot ids in AdSense if you use manual units.
2. Pro checkout – create a product on Lemon Squeezy / Gumroad / Stripe Payment Links and paste the URL into PRO_CHECKOUT_URL. Set PRO_PRICE to match. Buyers paste the licence key from their receipt into the Pro modal (activateLicense in monetize.js is a simple length check – wire it to your provider's licence API before going live).
3. Google Analytics – set GA_ID (G-XXXXXXX). Events tracked: copy, share, download (csv/print), scenario_save, goal_solve, pro_click, begin_checkout, affiliate_click.
4. Affiliate brokers – fill AFFILIATE_LINKS.broker1 / broker2 with { name, url }. Partner cards render below the tool only when url is non-empty; the whole section stays hidden otherwise. Links use rel="sponsored noopener".

After the first deploy, replace the growthcalc-tool.vercel.app placeholder in index.html, privacy.html, terms.html, robots.txt, sitemap.xml and config.js with the real domain (e.g. sed -i 's/growthcalc-tool.vercel.app/growthcalc.app/g' *.html *.txt *.xml *.js).

Files

index.html      main tool page (SEO-complete)
styles.css      shared base.css + app styles (incl. print report)
app.js          calculator logic, chart, tables, scenarios, goal solver, export
monetize.js     shared theme / toast / GA / AdSense / Pro helpers (do not edit per app)
config.js       window.APP_CONFIG
privacy.html    privacy policy (AdSense requirement)
terms.html      terms of use
robots.txt, sitemap.xml, vercel.json, favicon.svg

Math

Effective monthly rate rm = (1 + r/n)^(n/12) − 1 where n is periods per year (365/12/4/1). Each month: interest = balance × rm; the monthly contribution is added before (start) or after (end) interest. Contribution grows by the annual increase at each new year. Inflation-adjusted value = balance ÷ (1 + i)^years.

Disclaimer: educational tool, not financial advice.