About NoiMath

Last updated November 13, 2026 · NoiMath

NoiMath is a small, independent site that does one thing: real estate maths, in the browser, without asking for anything first. No account, no email capture, no paywall, no lead form.

What this site is

A set of calculators for underwriting residential investment property: cap rate, cash-on-cash return, rental cash flow, DSCR, BRRRR, fix-and-flip, the 70% rule, and a deal analyzer that combines them. Every tool shows its formula, documents its accounting conventions, and can be shared as a link with the inputs embedded.

The written guides explain the concepts behind the numbers — what NOI includes, what a DSCR loan requires, where the 70% rule comes from, how a BRRRR refinance works. They cite their sources and link to the calculator that applies the idea.

How the numbers are built

All calculation logic lives in one place and is covered by automated tests. The site has a documented golden example — a deliberately marginal deal with negative cash flow — that acts as a snapshot contract: if a formula changes, the test fails and the documentation has to be updated with it. Boundary cases are tested too: zero interest, all-cash purchases, interest-only loans, negative NOI, and deals where the refinance returns all of the capital.

Accounting conventions are not chosen per page. NOI, capex treatment, vacancy and the percentage bases are defined once on the methodology page and used everywhere. That is the main reason the tools agree with each other.

What this site is not

• Not financial, investment, legal or tax advice.
• Not a broker, lender or property manager. We do not sell leads.
• Not a property database. Every number comes from what you type.
• Not an appraisal or a valuation service.
• Not gated. There is no account and no email capture.

Who runs it

NoiMath is maintained by an independent developer who invests in residential rental property and got tired of calculators that hid the number behind a signup form. The tools here are the ones used to underwrite actual deals.

How it is funded

The site is supported by display advertising. Advertising has no influence on the formulas, the defaults, or the order results are presented in. There are no paid placements, and no affiliate link is added to a page without a disclosure next to it.

Corrections

If a formula here is wrong, it matters more than a typo. Send the correction through the contact page with the tool name and the inputs you used. Corrections are fixed and the methodology is updated when a convention was unclear.