Down Payment Calculator

Think of the down payment as the portion of the home you own the moment you close. You and the bank are co-owners on day one — you hold the share you paid for in cash, the bank holds the share it financed. The down payment percentage tells you how that ownership is split at the start. Put down 20% and you begin with 100% ownership structure where you hold a fifth and the lender holds the rest as secured collateral.

The cash you bring to closing determines everything that follows. A larger down payment means a smaller loan, which means lower monthly payments, less interest accruing each month, and — over a 30-year term — a significantly lower total repayment. The relationship is not proportional: because interest compounds across 12 payments each year, shaving the loan balance early has outsized effects over long terms.

The monthly payment calculation layers the interest rate and loan term on top of the loan amount. Once you know what you are borrowing, the amortization formula distributes that obligation evenly across the payment schedule — each payment blends principal repayment and interest, with interest front-loaded in early years. A higher down payment does not change the rate or the term — it simply reduces the base the formula operates on, which is why it is one of the most direct cost controls a buyer holds.

Use this calculator when you are comparing down payment options before making an offer, or when a lender has given you a rate quote and you want to see what different down payment sizes do to your monthly obligation. It is also the right tool when you are working backward from a target monthly payment to figure out how much cash you need at closing.

This calculator is appropriate for conventional fixed-rate purchase mortgages. It is less reliable for adjustable-rate mortgages (where the rate changes after a fixed period), interest-only loans, or scenarios where seller credits, closing cost financing, or grant programs affect the actual cash required at closing. In those cases, use the output here as a baseline and adjust with your lender for the specifics of the product.

Stop trusting this calculator as your final number once you have an actual loan estimate in hand. The loan estimate your lender provides by law includes all fees, taxes, insurance escrow, and PMI — the complete picture this tool intentionally omits to keep the down payment math clean and comparable. Use this tool to compare and decide; use the loan estimate to commit.

Mistake: treating the down payment percentage as the only lever. Many buyers fixate on hitting 20% without comparing what a slightly different percentage does to their monthly payment. The down payment directly sets the loan amount, and small percentage changes on high-priced homes translate to large dollar differences. Modeling a few percentages side by side with this calculator before making a decision takes under a minute and often reveals that a slightly lower percentage produces nearly the same monthly payment as 20% with far less cash tied up at closing.

Mistake: forgetting that the calculator shows only principal and interest. The monthly payment output here is the amortized loan obligation. It does not include property taxes, homeowners insurance, HOA fees, or — if your down payment is under 20% — PMI. Buyers who budget to the calculator number and neglect these line items routinely find their actual housing cost is higher than expected. A useful rule of thumb is to treat the payment shown here as the floor, not the ceiling.

Mistake: using a round-number interest rate that does not match the actual quote. Rate sensitivity is high: a difference of even half a percentage point changes the monthly payment by a meaningful amount on a large loan. Always enter the rate from your actual lender quote, not a round approximation. Lenders quote rates to two decimal places for a reason — small differences compound across 12 payments per year over decades.

The core formula has two steps. First, the down payment amount: multiply the home price by the percentage expressed as a decimal. For the example inputs of $400,000 at $80,000 target, that is home price times the percentage divided by 100. The loan amount follows immediately: subtract the down payment from the purchase price to get $320,000.

Monthly payment uses the standard fixed-rate amortization formula: Monthly Payment = P times [r(1+r)^n] divided by [(1+r)^n minus 1], where P is $320,000, r is the annual rate divided by 12 to get a monthly rate, and n is the loan term in years multiplied by 12 to get total payment count. For the example, that produces a monthly payment of $2,023.

Total interest emerges from multiplying the monthly payment by n (the total number of payments) and subtracting the original loan amount. The result — $408,142 for the example — reveals the true cost of financing. It is often the number that most surprises buyers: the cumulative interest on a 30-year mortgage frequently exceeds the original loan principal, which is the core reason that a larger upfront payment has compounding cost benefits over the life of the loan.

The amortization formula assumes a perfectly flat payment schedule with a constant rate — no prepayments, no rate resets, no refinancing. In practice, the single most powerful deviation from this model is prepayment: an extra principal payment early in the loan term eliminates years of future interest because it attacks the balance when the interest-to-principal ratio inside each payment is highest. The formula cannot capture this because it assumes you make exactly the payment it calculates, nothing more, for exactly n months.

The other hidden assumption is that your down payment represents fully liquid, unencumbered cash. For buyers weighing whether to invest surplus funds versus put them into a larger down payment, the relevant comparison is the guaranteed after-tax return of reducing mortgage debt versus the expected return on an alternative investment — a calculation that sits outside this tool but shapes whether maximizing the down payment is actually the optimal move.