The rent-versus-buy decision as a race between two compounding curves: the homeowner's equity (house value minus remaining loan) against the renter's investment account (down payment plus the monthly saving invested). The tool amortises the loan month by month, grows the house and the rent at their own rates, and compounds the investment return.
Both sides are computed to the same horizon with the same math, so the answer is a number, not a debate. The input assumptions — appreciation, investment return, loan rate — matter more than the arithmetic, which is why each is an explicit field.
Frequently asked questions
Why does the buy side start behind?
Early mortgage payments are mostly interest, and the purchase costs are paid up front — the equity curve is flat before it steepens. That is exactly the short-holding trap the comparison exposes.
What assumptions does it NOT model?
Taxes, maintenance, transaction friction and rent control are not itemised — the quick judgement sticks (price-to-rent ratio, the 5% rule) cover those; treat the output as the clean core comparison.
中文说明
把“租房还是买房”变成两条复利曲线的赛跑：房主的权益（房屋市值减剩余贷款）对阵租客的投资账户（首付款加上每月省下的差额定投）。工具逐月摊销贷款、让房价与租金按各自速率增长、按投资收益率复利。
两边在同一期限内用同一套数学计算，所以答案是数字而不是争论。增值率、投资收益率、贷款利率这些假设比算术本身更关键——因此每一项都是显式输入。
常见问题
为什么买房一方开局落后？
房贷前期还款大半是利息、交易成本又是先付的——权益曲线先平后陡。这正是对比暴露出的“持有期太短”陷阱。
哪些假设没有建模？
税费、维护、交易摩擦与租金管制未逐项建模——速判标尺（租售比、5% 法则）覆盖这些；应把输出当作干净的核心对比。