How to Calculate the Right Down Payment Percentage (And Why Most Buyers Get It Wrong)

The 20% Rule Is Not a Law

The 20% down payment figure entered the cultural mainstream because it eliminates private mortgage insurance. That is its only mathematical basis. Lenders invented PMI to protect themselves, not buyers. The 20% threshold has no bearing on what optimizes your net worth.

The actual question is simpler and more demanding: what return can your capital earn outside the home, and what does it cost you to leave it inside?

When the answer to the first question exceeds the answer to the second, putting less down is the correct financial move. When borrowing costs exceed your expected returns, you put more down. The right percentage shifts every time rates move, every time asset valuations change, and every time your personal tax situation changes.

Most buyers never run this calculation. They pick a number because a relative told them to, or because 20% sounds responsible. Neither justification holds up.

The Core Calculation

Four inputs determine your optimal down payment percentage.

1. Your mortgage rate (after tax)

If your rate is 6.85% and you itemize deductions at a 32% marginal rate, your after-tax cost of borrowing is approximately 4.66%. If you take the standard deduction, you pay the full 6.85%.

2. Your expected return on invested capital

Use a conservative figure. The S&P 500 has returned roughly 10.4% annually over the last 30 years, but expected forward returns are lower. A reasonable planning assumption sits between 6% and 8% for a diversified equity portfolio. Use your actual allocation, not an index.

3. PMI cost

PMI typically runs 0.5% to 1.5% of the loan amount annually, depending on credit score, loan-to-value ratio, and lender. On a $450,000 loan, that is $2,250 to $6,750 per year, paid until you reach 20% equity.

4. Time to PMI cancellation

At current appreciation rates and a standard amortization schedule, most buyers reach 20% equity in 6 to 9 years without making extra payments. With extra principal payments, that window shortens. Run the exact timeline before assuming PMI is permanent.

Worked Example 1: $500,000 Home, Strong Investor

Consider a buyer purchasing a $500,000 home. They have $150,000 available, which is enough for either a 20% down payment ($100,000) or a 10% down payment ($50,000).

Scenario A: 20% down

• Loan amount: $400,000
• No PMI
• Monthly principal and interest at 6.85%: approximately $2,626
• Capital deployed into the home: $100,000
• Capital remaining for investment: $50,000

Scenario B: 10% down

• Loan amount: $450,000
• PMI at 0.9% annually: $337 per month
• Monthly principal and interest at 6.85%: approximately $2,955
• Additional monthly cost vs. Scenario A: $666
• Capital deployed into the home: $50,000
• Capital remaining for investment: $100,000

The question is what that extra $50,000 does over the 7-year PMI window.

At a 7% annual return, $50,000 grows to approximately $80,425 over 7 years. That is $30,425 in gains. Over the same 7 years, PMI costs this buyer $337 per month, totaling roughly $28,308 before any tax consideration. The investment gains exceed the PMI cost by approximately $2,117 before taxes.

This buyer comes out ahead by putting less down, but only marginally. The decision is close. What tips it decisively is the buyer's actual investment discipline. A buyer who does not invest the difference gains nothing and loses the PMI cost entirely.

Worked Example 2: $750,000 Home, Higher Rate Environment

A buyer in a higher-cost market purchases a $750,000 home. They have $225,000 liquid. Options: 20% down ($150,000), 15% down ($112,500), or 10% down ($75,000).

20% down

• Loan: $600,000
• No PMI
• Monthly P&I at 7.1%: approximately $4,030

15% down

• Loan: $637,500
• PMI at 0.75%: $398 per month
• Monthly P&I: approximately $4,282
• Total additional monthly cost: $650
• Extra capital available: $37,500

At 7% over 6 years, $37,500 grows to approximately $56,250. Total gain: $18,750. Total PMI paid over 6 years: approximately $28,656. The math flips. Putting 15% down costs this buyer roughly $9,900 more than putting 20% down, because the PMI exceeds the investment return on the retained capital.

At a 10% expected return, the retained $37,500 grows to $66,544 over 6 years, a gain of $29,044. That barely clears the PMI cost. The margin is thin enough that a single bad market year erases the advantage.

The takeaway: at 7%+ mortgage rates, the case for putting less down weakens considerably. The PMI cost rises, the opportunity cost of borrowed money rises, and forward investment returns need to be higher to justify the math.

PMI Is Not the Only Cost of a Larger Loan

Buyers who focus only on the PMI premium miss a second cost: the interest on the additional loan principal.

In Worked Example 2, the buyer who puts 15% down instead of 20% borrows an extra $37,500. At 7.1%, that additional principal costs approximately $223 per month in interest during the first year alone. Over a 6-year PMI window, before any principal paydown, that additional interest totals roughly $13,350. Add that to the $28,656 in PMI, and the total incremental cost of the 15% scenario approaches $42,000 against an investment gain of $18,750 to $29,000.

The numbers tighten further. This is why the 15% option in a high-rate environment rarely wins.

Credit Score Changes Everything

PMI pricing is not flat. A buyer with a 760 credit score might pay 0.5% annually. A buyer with a 680 credit score pays closer to 1.4% on the same loan-to-value ratio. On a $400,000 loan, the difference is $3,600 per year, or $300 per month.

At that PMI rate, the calculus shifts dramatically. A buyer with a 680 score on a $500,000 purchase, putting 10% down, pays $466 per month in PMI rather than $300. The breakeven investment return required to justify putting less down rises accordingly. At a 6.85% mortgage rate and 1.4% PMI, a buyer needs to earn over 9% annually on their retained capital just to break even. That is a more aggressive return assumption than most conservative allocations justify.

Before finalizing a down payment, get your exact PMI quote from at least two lenders. The difference between pricing tiers can change the optimal decision by several percentage points of down payment.

How Rate Buydowns Factor In

Some buyers face a third option: use excess capital to buy down the mortgage rate rather than increase the down payment or keep the funds invested.

At current origination costs, one discount point typically costs 1% of the loan amount and reduces the rate by 0.20% to 0.25%. On a $450,000 loan, one point costs $4,500 and saves approximately $56 per month. The payback period runs roughly 80 months, or 6.7 years.

If the buyer plans to hold the property for less than 7 years, the buydown destroys value. If they plan to hold for 15+ years, it generates material savings. The down payment percentage interacts directly with the buydown decision because both compete for the same pool of capital.

Run all three options simultaneously. The CalcMoney Mortgage Calculator models rate buydowns, PMI timelines, and investment opportunity cost in the same interface.

The Liquidity Floor You Cannot Cross

One constraint overrides every return calculation: post-closing liquidity.

After closing, most financial planners recommend holding 6 to 12 months of total housing costs in accessible reserves. For a buyer with a $4,500 monthly payment, that means keeping $27,000 to $54,000 liquid after down payment and closing costs.

A buyer who maximizes their down payment and ends up with $15,000 in reserves has optimized for the wrong variable. A single major repair, a job disruption, or a medical event eliminates their cushion entirely. The financially correct down payment percentage is one that preserves this floor with margin to spare.

No PMI savings justify housing yourself into illiquidity.

H3: When 20% Is Clearly the Right Answer

Three situations favor the maximum down payment regardless of opportunity cost arithmetic.

First, when the buyer has no consistent investment history and would not invest the difference. The theoretical return on retained capital is worth nothing if it stays in a savings account at 4.5%.

Second, when the buyer is within 5 years of retirement and cannot tolerate sequence-of-returns risk on the retained capital.

Third, when the buyer's DTI ratio is borderline and the smaller loan amount makes approval materially more secure. Financial stress at closing costs more than any PMI calculation captures.

Run the Numbers Before You Commit

The right down payment percentage for a $400,000 home in 2026 is not the right percentage for a $750,000 home, a different credit score, a different rate environment, or a different investment discipline. The answer is personal and quantitative, not conventional.

The CalcMoney Mortgage Calculator lets you input your specific rate, PMI quote, credit score tier, and expected return on capital. It outputs the breakeven timeline, the total cost comparison across down payment scenarios, and the liquidity position at each option.

Run the numbers before you wire the funds. The difference between scenarios can exceed $40,000 over a 7-year horizon. That gap is worth 20 minutes of calculation.

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Open the CalcMoney Mortgage Calculator and model your down payment scenarios now →