API Reference

dscr-ratio exposes four free functions. All take and return f64 (and one u32 term). No structs, no traits to implement, no features to enable.

dscr

pub fn dscr(noi: f64, annual_debt_service: f64) -> f64

Debt service coverage ratio = NOI ÷ annual debt service.

• Returns 0.0 when annual_debt_service <= 0.0 (avoids division by zero).
• noi is annual net operating income (rent minus operating expenses, before debt service and before taxes).

use dscr_ratio::dscr;

assert!((dscr(24_000.0, 20_000.0) - 1.2).abs() < 1e-9);
assert!((dscr(20_000.0, 20_000.0) - 1.0).abs() < 1e-9); // break-even
assert_eq!(dscr(100_000.0, 0.0), 0.0);                   // no debt → 0

monthly_payment

pub fn monthly_payment(principal: f64, annual_rate: f64, months: u32) -> f64

Monthly payment on a fully-amortizing fixed-rate loan (the standard US mortgage amortization formula).

• annual_rate is the nominal annual rate (e.g. 0.07 for 7%); it is divided by 12 internally.
• months is the loan term in months (360 = 30 years).
• Returns 0.0 when months == 0.
• When annual_rate == 0.0, returns the straight-line principal / months.

$$ M = P \cdot \frac{r(1+r)^n}{(1+r)^n - 1} \quad\text{where}\quad r = \frac{\text{annual rate}}{12},\;\; n = \text{months} $$

use dscr_ratio::monthly_payment;

// $100k, 7%, 30y → ≈ $665.30/mo
let m = monthly_payment(100_000.0, 0.07, 360);
assert!((m - 665.30).abs() < 0.1);

annual_debt_service

pub fn annual_debt_service(monthly: f64) -> f64

Annual debt service from a monthly payment — simply monthly * 12.0. This is the denominator of dscr.

use dscr_ratio::{monthly_payment, annual_debt_service};

let debt = annual_debt_service(monthly_payment(100_000.0, 0.07, 360));
// ≈ 7,983.61

verdict

pub fn verdict(ratio: f64) -> &'static str

Plain-language verdict for a DSCR value, matching common DSCR lender thresholds:

DSCR range	Return value	Meaning
ratio < 1.00	"fail"	Rent does not cover debt.
1.00 ≤ r < 1.15	"marginal"	Barely covers debt.
1.15 ≤ r < 1.25	"tight"	Meets most minimums.
1.25 ≤ r < 1.35	"good"	Best-terms threshold for most lenders.
ratio ≥ 1.35	"strong"	Comfortable cushion, best pricing.

use dscr_ratio::verdict;

assert_eq!(verdict(0.9), "fail");
assert_eq!(verdict(1.1), "marginal");
assert_eq!(verdict(1.2), "tight");
assert_eq!(verdict(1.3), "good");
assert_eq!(verdict(1.5), "strong");

Verdict vs. lender policy

These buckets reflect the most common DSCR lender thresholds, but every lender sets its own cutoffs and overlays. For a quote on a specific deal, run the numbers through the DSCRRadar DSCR loan calculator.