Construction Mortgage Calculator
Building a home means paying for two loans that behave nothing alike: an interest-only construction loan that grows in draws while the house goes up, and a permanent mortgage that only starts when you move in. Most calculators show you one phase or the other. This one runs the whole sequence, from land price to the last permanent payment, on a single screen.
Enter the project costs, both interest rates, and how the builder will draw the money. You get the construction interest under a realistic draw pattern, the interest-only payment during the build, the converted permanent loan and its monthly payment, plus the loan-to-value and debt-to-income ratios a lender will check.
Estimates for planning. The construction phase is modeled on an average drawn balance for the draw pattern you pick; a real draw schedule bills interest on the exact balance each day. Taxes, insurance, and closing costs are not included.
Quick Answer
Key takeaways
- You only pay construction interest on money actually drawn. The draw pattern is worth real money: on this loan it swings the interest bill from $9,520 to $27,200.
- The payment that matters during the build is interest-only, about $1,133 a month here, far below the permanent payment that follows it.
- Rolling construction interest into the permanent loan trades a higher monthly payment for $13,600 less cash needed during the build.
- Lenders judge the finished picture: this example lands at 74.1% LTV and 21.1% DTI, inside the usual guidelines on both.
How to use the construction mortgage calculator
Work top to bottom. The first three fields set the project size: land, build contract, and your cash down, which together fix the loan amount. The next block sets the two rates and the two clocks, the build period in months and the permanent term in years. Then choose the structure: a construction-to-permanent loan converts with one closing, a construction-only loan needs a second closing later, and the draw pattern tells the calculator how fast the builder pulls money, which drives the interest estimate.
The last two fields are optional checks. The expected appraised value produces your loan-to-value ratio and the equity you hold the day the home is finished, and gross monthly income produces the debt-to-income ratio a lender computes from the permanent payment. If the finished home is really what you are deciding on, the buy a home calculator covers the purchase side of the same decision, and the affordability ratio calculator tests the payment against your wider budget.
The formulas behind both phases
Construction phase: Average balance = loan x draw factor (0.50 even, 0.65 front-loaded, 0.35 late, 1.00 upfront). Construction interest = average balance x construction rate x (months / 12). Interest-only payment = average balance x construction rate / 12.
Permanent phase: Permanent loan = construction loan + construction interest (if rolled in). Monthly payment = P x r(1 + r)^n / ((1 + r)^n - 1), with r the monthly permanent rate and n the term in months.
The draw factor is the honest simplification at the heart of this tool. A lender bills interest on the balance outstanding each day, and a month-by-month draw schedule would need a dozen inputs nobody has before the build starts. Averaging the balance by draw pattern captures the economics: money drawn early costs more interest than money drawn late, which is why two identical loans with identical rates can produce very different interest bills. The permanent phase is the standard amortization formula the Consumer Financial Protection Bureau loan estimate guidance teaches borrowers to read, applied to the converted balance.
Worked example: a $400,000 build, start to finish
The calculator is pre-filled with this case, and every figure below is what it returns.
| Measure | Result | Reading |
|---|---|---|
| Total project cost ($50,000 + $350,000) | $400,000 | Land plus build contract |
| Construction loan ($400,000 - $80,000) | $320,000 | 80% of project cost |
| Construction interest (even draws, 12 mo, 8.5%) | $13,600 | On a $160,000 average balance |
| Interest-only payment during build | $1,133.33/mo | The check you write while building |
| Permanent loan (interest rolled in) | $333,600 | $320,000 + $13,600 |
| Permanent payment (6.5%, 30 years) | $2,108.58/mo | Principal and interest |
| Grand total outlay | $839,088.41 | Down payment plus all 360 payments |
| LTV / DTI / equity at completion | 74.1% / 21.1% / $116,400 | Against a $450,000 appraisal |
The draw pattern is a price, not a detail
Run the same $320,000 loan at 8.5% for 12 months under each draw pattern and the interest bill alone changes by more than $17,000:
| Draw pattern | Average balance | Construction interest |
|---|---|---|
| Late draws (35%) | $112,000 | $9,520 |
| Even draws (50%) | $160,000 | $13,600 |
| Front-loaded draws (65%) | $208,000 | $17,680 |
| Full amount upfront (100%) | $320,000 | $27,200 |
That spread is why builders and lenders negotiate draw schedules, and why a calculator that silently assumes the full loan is outstanding for the full build (the full upfront row) overstates the interest on a normal project by about double. Front-loaded draws happen when expensive work lands early, such as a full materials purchase or a manufactured home set; late draws happen when the costly finishes pile up at the end. Pick the pattern that matches your builder contract, not the one that flatters the estimate. Before signing, pressure-test approval itself with the approved mortgage calculator.
What competing calculators miss
The top construction loan tools reviewed for this page handle the permanent mortgage well and the construction phase loosely. Three gaps repeat.
Miss 1: full-balance interest. Several rivals charge interest on the entire loan for the entire build, the most expensive assumption possible. The draw pattern selector here models how money actually leaves the account, and the table above shows what that honesty is worth.
Miss 2: no interest-only payment. Total construction interest is a budgeting abstraction; the monthly interest-only figure is the payment a family actually has to carry alongside rent or an existing mortgage during the build. This tool shows both.
Miss 3: no lifetime total, no lender ratios. A monthly payment alone hides the cost of the whole structure. The grand total outlay puts the down payment and all 360 permanent payments on one line, and the LTV and DTI readouts show the two ratios the lender will judge, before the lender does.
Frequently Asked Questions
It runs in two phases. During the build you borrow in draws as work is completed and pay interest only on the money actually drawn. When the home is finished, the construction loan converts into a permanent mortgage with regular principal and interest payments over the term you chose, 30 years in the worked example.
Because the full loan is not outstanding from day one. Money leaves the account in draws, so interest accrues on a growing balance. This calculator models that with a draw factor: even draws average 50% of the loan outstanding, front-loaded draws 65%, late draws 35%, and a full upfront draw 100%.
Instead of paying the construction interest in cash during the build, it is added to the loan balance that converts to the permanent mortgage. In the worked example the permanent loan becomes $333,600 instead of $320,000, which raises the monthly payment slightly but avoids $13,600 of out-of-pocket cost during construction.
A construction-to-permanent loan (one-time close) uses a single closing and converts automatically, locking your permanent rate terms up front. A construction-only loan (two-time close) needs a second closing and a new permanent loan at completion, which adds fees and leaves your final rate exposed to whatever the market does during the build. The loan type selector changes the analysis notes; the payment math is the same.
Many lenders want the loan at or under 80% of the finished appraised value to avoid mortgage insurance, and a housing debt-to-income ratio comfortably under the common 43% ceiling used in qualified mortgage rules. The worked example lands at 74.1% LTV and 21.1% DTI, inside both guidelines.
No. It models the land cost, construction cost, down payment, construction interest, and the permanent principal and interest payment. Property tax, homeowners insurance, permits not already inside the construction budget, and closing costs are separate and should be budgeted on top.
Interest keeps accruing on the drawn balance, so every extra month adds roughly one month of interest-only cost at the current draw level. A longer construction period in the calculator shows the effect directly: at the worked example balance, each additional month costs about $1,133 in interest before conversion.
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References
- Consumer Financial Protection Bureau, Owning a Home toolkit and Loan Estimate explainer: how loan amount, rate, and term combine into the principal and interest payment on the permanent phase. CFPB: Understanding Your Loan Estimate. Accessed October 2026.
- Consumer Financial Protection Bureau guidance on construction loans and one-time versus two-time closing structures for building a home. CFPB: Owning a Home. Accessed October 2026.
- Federal Housing Administration, HUD 203(k) and construction financing resources on draw schedules, inspections, and interest charged on disbursed balances. HUD: 203(k) Rehabilitation Mortgage Insurance. Accessed October 2026.