Why the same mistakes keep happening
Most rent vs buy decisions errors are not arithmetic errors — they are assumption errors. The formula is right, but the input is idealized: a round number, an optimistic rate, a best-case month. Below are the mistakes behind most bad results, grouped by the calculator where they bite.
Rent vs Buy Cost Comparison
Comparing rent to P&I only — taxes, maintenance, and insurance add 30–45% to the real monthly cost.
Counting appreciation at full value and forgetting the ~7% round-trip cost of selling.
Giving the down payment a 0% return in the comparison — that cash has a market rate.
Break-Even Horizon Calculator
Using the optimistic stay ("we will be here forever") instead of the realistic one.
Assuming appreciation that your specific metro has not delivered over the last full cycle.
Ignoring that renters earn returns on the unspent down payment — it is not idle cash.
Cash to Close Calculator
Budgeting only the down payment and meeting closing costs at the table.
Spending the last dollar on the down payment, leaving zero reserve for year-one surprises.
Assuming gifts need no paperwork — undocumented deposits can stall or kill an approval.
Rent Equivalent Calculator
Comparing rent to the mortgage P&I alone and concluding ownership is "the same monthly cost."
Forgetting that the equivalent-rent figure assumes you stay long enough to earn the appreciation credit.
Using the ratio to judge a whole city from one luxury listing.
The habit that fixes all of them
Write down the assumption you are least sure about every time you run a number. If the answer matters, test it: change that one input by ±20% and see whether the decision flips. If it flips, the assumption — not the math — is your real problem, and it deserves the research time.