RentScope — free rental property calculator

Live: https://lowlune.github.io/rentscope/

RentScope is a fast, private, browser-only rental property calculator. Enter a property's price, rent and loan terms to get monthly cash flow, cap rate, cash-on-cash return, DSCR, levered IRR, and a multi-year projection with exit analysis.

• No signup, no tracking, no server — all math runs in your browser.
• Shareable URLs: every input is saved in the query string.
• CSV export of the full projection.

Free calculators

• Rental property calculator — cash flow, cap rate, cash-on-cash, DSCR, IRR, multi-year projection
• Cap rate calculator
• DSCR calculator
• Cash-on-cash return calculator
• BRRRR calculator
• NOI calculator
• Gross rent multiplier (GRM) calculator
• Break-even occupancy calculator
• 1% rule calculator
• 50% rule calculator
• House hack calculator
• 1031 exchange calculator
• Rental property depreciation calculator
• Seller financing calculator
• Hard money loan calculator

Each landing page's worked example is computed at build time by the same tested engine — the numbers on the page come from the code, not from a spreadsheet.

Accuracy & testing

The math lives in pure, dependency-free engines with assertions:

node test-calc.mjs              # 24 assertions  — core underwriting engine
node test-1031.mjs              # 29 assertions  — 1031 exchange (boot, deferred gain)
node test-depreciation.mjs      # 47 assertions  — 27.5/39-yr MACRS, mid-month, recapture
node test-sellerfinancing.mjs   # 1,070 assertions — owner-carried notes + balloons
node test-hardmoney.mjs         # 2,201 assertions — hard-money / bridge loans (points, interest-only)

Develop

python3 -m http.server 8000   # then open http://localhost:8000

Files

• index.html — the free calculator app
• calc.js, exchange1031.js, depreciation.js, sellerfinancing.js, hardmoney.js — pure, tested engines
• app.js — UI wiring, chart, CSV export
• *-calculator.html — SEO landing pages with build-time worked examples
• styles.css

RentScope Pro

RentScope Pro is an offline, self-contained HTML underwriting model. It adds after-tax cash flow (depreciation, interest, marginal tax), capital-gains and Section 1250 recapture on exit, amortization schedules, 5×5 sensitivity tables, and a printable report. It is a separate paid download; this repository contains only the free web tool.

Related

• FeeScope — free reseller fee & profit calculator.

Disclaimer

Educational estimation only — not financial, tax, or investment advice. Assumptions and simplifications are documented in the engine source.

MIT licensed (free web tool only).