Mortgage installments explained: P&I amortization plus the 1/12 escrow split

This post is for two audiences at once — partner banks reviewing the math behind mortgage installments, and renters (or their brokers) reading one line on a mortgage statement and trying to understand what it actually means. The two pieces of math that appear on every statement are the amortizing principal-and-interest monthly payment and the 1/12 escrow split, and the numbers the mortgage calculator renders match the spec end-to-end.

The monthly P&I is the standard amortizing fixed-rate formula — M = P · r·(1+r)^n / ((1+r)^n − 1) — where P is the principal in cents, r is the annual rate expressed as basis points divided by 10,000, and n is the term in months (30 years times 12 = 360). The worked example from the spec doc lands at $2,528.27/month on P = $400,000, r = 6.50%, n = 360 — the rate-times-n constant is what produces that figure, not a rounding shortcut. Cents rounding is absorbed by the last amortization row so the sum of principalPaid over n payments equals P exactly.

Escrow works the same way. The page takes E_annual — taxes plus insurance for the year, in cents — as the canonical input and derives the monthly escrow as E_monthly = round(E_annual / 12). On the spec doc worked example, E_annual = $4,800 → $400/moin escrow. The reason annual escrow is the input rather than monthly is that it lets banks reason at any scale and match it against a T&I statement — the monthly figure follows deterministically.

When a borrower opts into a cadence — weekly or twice-monthly — the monthly pool of P&I plus escrow is split across N rows with a per-row service fee applied on top. Weekly is four rows at 2.00% per row; twice-monthly is two rows at 1.50% per row. The BPS rates are the single source of truth in the brand-stripe-links config and match the numbers on /pricing. The spec doc worked example: 4 weekly rows of approximately $732.07 each, fee approximately $14.64, total per row approximately $946.71; 2 twice-monthly rows of approximately $1,464.14, fee approximately $21.96, total per row approximately $1,486.10. The cents-exact last-row-absorbs-remainder property mirrors splitRent, so per-row amounts sum exactly to the shared monthly pool.

The four canonical figures for partner reviewers are 2.00% per weekly row, 1.50% per twice-monthly row, $2,528.27monthly P&I plus $400 monthly escrow (a pooled $2,928.27 a month), and the live calculator that rederives those from any input. The full formula, the BPS source, and the worked numbers live in the mortgage installments spec doc.