A standard US 30-year fixed-rate mortgage amortises monthly using M = P × [r(1+r)^n] ÷ [(1+r)^n − 1]. The Freddie Mac Primary Mortgage Market Survey publishes the weekly national rate, and the FHFA sets the annual conforming loan limit ($766,550 in 2024, higher in designated high-cost areas).

The US 30-year fixed-rate mortgage is a uniquely American instrument. Most developed economies don't offer it because the prepayment optionality embedded in the borrower's right to refinance creates a risk profile no commercial lender wants to hold. The reason it exists in the US is securitisation through Fannie Mae and Freddie Mac, who absorb that prepayment risk through MBS pricing.

This calculator handles the loan products US homebuyers actually face:

• Conforming 30-year fixed — up to the FHFA's annual conforming loan limit, backed by Fannie/Freddie, lowest available rates, 20% down avoids PMI
• FHA — 3.5% minimum down with 580+ FICO; upfront 1.75% MIP plus annual 0.55-1.05% MIP, with MIP for the life of the loan if down payment is under 10%
• VA — for eligible veterans, active duty, and surviving spouses; 0% down, no PMI, with a one-time funding fee of 1.4-3.6% (waivable for disabled veterans)
• USDA Rural Development — 0% down in designated rural areas, income limits apply
• Jumbo — above the conforming limit, tighter qualification (700+ FICO, 20%+ down, 6-12 months reserves)
• ARMs (5/1, 7/1, 10/1) — fixed for the initial period, then resets annually against SOFR + margin

Beyond P&I, the calculator handles the rest of PITI and additional carrying costs: - Property tax — varies widely, from 0.3% (Hawaii) to over 2% (New Jersey, Illinois) of assessed value annually - Homeowners insurance — $1,200-2,500/year typical, materially higher in CA, FL, and coastal areas - PMI — 0.3-1.5% per year of the loan balance if down payment is under 20% on a conforming loan - HOA dues — where applicable, typically $200-800/month for condos and planned communities

The 28%/36% rule remains the lender baseline: housing should sit under 28% of gross monthly income, total debt under 36%. For loan-program details, the Consumer Financial Protection Bureau's mortgage guides and the FHFA's annual conforming loan limits page are the authoritative federal references.