Equal installment (annuity, 等额本息) keeps the payment constant: M = P × i × (1+i)^n / ((1+i)^n − 1), where P is the principal, i the monthly rate (annual ÷ 12) and n the number of months. Equal principal (等额本金) repays a fixed slice of principal P/n each month plus interest on the remaining balance, so the first payment is the highest and each following payment decreases by (P/n) × i; its total interest is P × i × (n+1) / 2.
Equal installment gives a predictable, lower initial payment but more total interest; equal principal costs more up front yet saves interest over the full term — this tool shows both plans side by side with the exact difference. All calculations run locally in your browser; nothing about your finances is uploaded.