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API Reference

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

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%); 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 amortization_schedule::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);

// 0% interest → straight-line
assert!((monthly_payment(12_000.0, 0.0, 12) - 1_000.0).abs() < 1e-9);

period

pub fn period(balance_before: f64, annual_rate: f64, payment: f64) -> (f64, f64, f64)

A single amortization period. Returns (interest, principal_paid, remaining_balance_after).

  • interest = balance_before * (annual_rate / 12).
  • principal_paid = (payment - interest), clamped so the final payment never overpays the balance.
  • remaining_balance is floored at 0.0.
use amortization_schedule::{monthly_payment, period};

let p = monthly_payment(100_000.0, 0.07, 360); // ≈ 665.30
let (interest, principal_paid, remaining) = period(100_000.0, 0.07, p);
// interest ≈ 583.33, principal_paid ≈ 81.97, remaining ≈ 99,918.03

total_interest

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

Total interest paid over the full life of the loan: (monthly_payment * months) - principal.

use amortization_schedule::total_interest;

let ti = total_interest(100_000.0, 0.07, 360); // ≈ 139,508.89
assert!(ti > 130_000.0 && ti < 140_000.0);

schedule

pub fn schedule(principal: f64, annual_rate: f64, months: u32) -> Vec<(f64, f64, f64)>

The full amortization schedule as a Vec of (interest, principal_paid, remaining_balance) tuples — one entry per period, in order, stopping once the balance reaches zero.

use amortization_schedule::schedule;

let s = schedule(100_000.0, 0.07, 360);
assert_eq!(s.len(), 360);
assert!(s.last().unwrap().2 < 1.0); // final remaining balance ≈ 0

Final period rounding

The last principal_paid is clamped to the remaining balance, so the schedule always closes exactly at 0.0 rather than overshooting by a few cents of float error. For a polished printable table with extra payments, use the AssetLoanCalculator amortization tool.