Calculation Methodology
The exact formulas and limits behind every figure on this site.
Last materially reviewed: July 2026 · Maintained by the Stonehaven team · Corrections: office@stonehavencre.com
Deterministic code, not estimates by judgment
Every number comes from deterministic, unit-tested code. No figure is AI-generated or adjusted editorially. Fixtures with independently verified values run before any change ships.
The residential DSCR convention we model
For 1-4 unit rentals we model the common convention: DSCR = eligible monthly rent ÷ monthly PITIA, where PITIA is principal and interest plus one-twelfth of annual property taxes and insurance plus monthly association dues. Principal and interest uses the standard amortization formula at the entered rate and term; interest-only uses loan × monthly rate; zero rate amortizes straight-line. Output models the entered assumptions, not any program.
Investor cash flow is deliberately different
The investor view subtracts vacancy from rent, then management (on collected rent), maintenance and capital-expenditure reserves (on gross rent), plus taxes, insurance and dues, to reach estimated NOI; investor DSCR divides NOI by principal-and-interest debt service. More conservative than the lender formula; never mixed.
Target-ratio sizing
Rent needed at a target ratio is PITIA × target. Maximum housing expense is rent ÷ target; maximum principal and interest subtracts taxes, insurance and dues from that figure; the illustrative maximum loan inverts the amortization formula at the entered rate and term. Sizing figures are illustrative, not program limits.
SBA structure calculator (/sba-loan-calculator)
504: borrower injection of 10% of total project cost, plus 5% if the property is special-purpose and 5% if the business is under two years old, capped at 20%; a bank portion at 50% of project cost; a CDC portion equal to the remainder. Each portion's payment uses the standard amortization formula at the user-entered rate and term. 7(a) is a single amortizing loan for project cost minus the entered down payment. Rates are always user estimates. Rent comparison = estimated total payment minus entered current rent.
Commercial sizing calculator (/commercial-loan-calculator)
Commercial DSCR is annual NOI divided by annual debt service. The DSCR-limited loan converts NOI ÷ 12 ÷ target DSCR into a maximum payment and inverts the amortization formula at the entered rate and term. The LTV-limited loan is value × maximum LTV. The supported loan is the lesser; the calculator names which limit binds. Resulting DSCR is recomputed from the supported loan's payment. Implemented in js/sba-calc.js and js/commercial-calc.js with fixture tests.
Mortgage payment & affordability calculator (/mortgage-calculator)
The payment view computes principal and interest with the standard amortization formula at the user-entered rate and term, then adds annual taxes ÷ 12, annual insurance ÷ 12, monthly HOA, and mortgage insurance as a user-supplied annual percent of the loan ÷ 12. Affordability multiplies gross monthly income by the user-adjustable housing and total-debt ratios (defaults 28/36), takes the lesser after subtracting monthly debts, subtracts the tax/insurance/HOA estimate to reach maximum principal and interest, inverts the amortization formula into a loan amount, and grosses up by the down-payment percentage into an illustrative price. It reports which ratio binds.
Refinance break-even calculator (/refinance-calculator)
Break-even months = closing costs ÷ (current monthly payment − new monthly payment); net figure = monthly savings × expected months in the home, minus costs. If the new payment is not lower, the calculator says so. Implemented in js/residential-calc.js with fixture tests.
HELOC calculators (/heloc)
Available line = home value × user-entered maximum combined loan-to-value, minus current mortgage balance, floored at zero. Draw-period payment is interest-only on the drawn balance (balance × annual rate ÷ 12); repayment-period payment is full amortization over the user-entered repayment years; the step-up is reported. Rates are user estimates. Implemented in js/heloc-calc.js with fixture tests.
What these calculators do not determine
Eligibility, approval, pricing, leverage limits, reserves, rent-evidence treatment, closing timelines. The lender decides those in underwriting. Stonehaven arranges financing and performs preliminary analysis.
Prepayment explainer
The prepayment module multiplies the entered balance by the step percentage of an illustrative structure for the chosen exit year. Structures vary by lender and state; final loan documents control.