Construction Loan Calculator

A construction loan is a short-term financing product designed to fund the building of a new home or major renovation project. Unlike a traditional mortgage — where you borrow a lump sum secured against an existing property — a construction loan releases funds in stages called draws as your builder completes defined milestones. Once the building phase ends, the loan either converts to a permanent mortgage (in a construction-to-permanent product) or is paid off by refinancing into a conventional home loan.

Construction loans typically carry higher interest rates than permanent mortgages because they represent greater lender risk. The collateral (your unfinished home) has uncertain value until construction is complete, and the loan is disbursed over time rather than as a single secured advance. Most lenders charge interest only during the building period, and that interest accrues only on the portion of the loan that has actually been drawn — not the full approved amount.

Because draws happen progressively, the outstanding balance grows each month. Early in the construction period you owe little, so interest charges are small. By the final months, most of the loan has been drawn and monthly interest is at its peak. This calculator models that draw schedule precisely: it divides the total loan into equal monthly draws, accumulates the outstanding balance, and charges interest each month on the cumulative amount drawn.

Understanding the true cost of a construction loan requires looking at both phases: the construction interest that accrues during the build, and the long-term mortgage payments that follow. Many borrowers focus only on the monthly payment after conversion and underestimate the thousands of dollars in construction-phase interest that compound before the permanent loan even begins.

This construction loan calculator models the two-phase financing structure of new-build projects. You enter your land cost, total construction budget, down payment percentage, construction loan interest rate, build duration in months, the expected permanent mortgage rate, and the long-term loan term. The calculator then computes every number you need to understand your full financing picture.

Phase 1 — Construction draw period: The loan amount (total project cost minus down payment) is divided into equal monthly draws. Each month's interest charge equals the cumulative drawn balance multiplied by the monthly interest rate. These monthly charges are summed to produce the total construction-phase interest.

Phase 2 — Permanent loan: Once construction is complete, the full loan amount converts to a standard amortizing mortgage. The calculator applies the standard mortgage payment formula using your permanent rate and term to give you a monthly payment and the total interest you will pay over the life of that loan.

The loan-to-value (LTV) ratio is also calculated as the loan amount divided by total project cost, expressed as a percentage. Lenders closely monitor this figure — most require an LTV of 80% or below (i.e., a minimum 20% down payment) to avoid private mortgage insurance and qualify for the best rates.

The draw schedule is the backbone of a construction loan. It maps out when and how much of your approved loan amount will be released to your builder. Lenders generally require independent inspections before each draw is authorized, confirming that work has been completed to a defined stage — foundation poured, framing complete, mechanical rough-in done, drywall installed, and so on.

This calculator assumes equal monthly draws, which is a reasonable approximation for planning purposes. In practice, builders and lenders often use milestone-based draws that can be uneven — a large initial draw for site preparation and foundation, smaller mid-project draws, and a final draw upon completion. However, the equal-monthly-draw model closely reflects the arithmetic average and gives a reliable estimate of total construction-phase interest.

Because interest is charged only on the outstanding balance, the monthly interest charge starts small and grows each month. In the first month you pay interest on just one month's draw; by the final month you pay interest on the full loan amount. This is fundamentally different from a regular mortgage where the full balance is outstanding from day one.

One practical implication: if your construction runs over schedule, your construction-phase interest costs increase. Every extra month adds an interest charge on the full (or near-full) drawn balance — typically the most expensive months of the construction period. Delays are one of the most significant and underestimated financial risks in new construction.

Illustrative example based on $320,000 loan at 7.5% over 12 months.

Many modern construction loans are structured as construction-to-permanent (C2P) products, also called single-close or one-time-close loans. With this structure, you apply once, lock your permanent rate at closing, and the loan automatically converts to a conventional mortgage when construction is complete. This saves on closing costs compared to a standalone construction loan that must be refinanced at project completion.

The alternative is a two-close loan: a standalone construction loan that you pay off or refinance into a separate permanent mortgage after completion. Two-close loans offer more flexibility — you can shop for the best permanent rate closer to your move-in date — but they require going through underwriting and closing twice, adding both cost and complexity.

This calculator models the two-phase economics regardless of which structure you use. The construction interest represents phase one costs, and the permanent loan calculations represent phase two. Whether you use a C2P product or two separate closings, the interest arithmetic is identical — the main difference is legal/administrative, not financial.

When comparing lenders, look beyond the interest rate to the total cost across both phases. A lender with a slightly higher construction rate but a meaningfully lower permanent rate can be less expensive overall. Use the total interest figure this calculator produces to make apples-to-apples comparisons between loan offers.

Construction loans have stricter qualification requirements than conventional mortgages because of the additional risk lenders take during the building phase. Understanding these requirements helps you prepare before approaching a lender.

• Credit score: Most lenders require a minimum score of 680–720 for construction loans. The best rates generally go to borrowers with scores above 740.
• Down payment: Expect to put down at least 20% of the total project cost (land plus construction). Some lenders allow 10–15% with private mortgage insurance, but 20% is the norm for favorable terms.
• Debt-to-income ratio (DTI): Lenders typically cap DTI at 43–45%, calculated using the projected permanent loan payment, not the lower interest-only construction payment.
• Builder approval: Your general contractor must be licensed and insured. Most lenders require detailed construction plans, a signed contract, and a fixed-price bid before they will approve the loan.
• Contingency reserve: Many lenders require a cost contingency of 5–10% built into the loan amount to cover change orders and unexpected expenses.
• Cash reserves: Having 6–12 months of mortgage payments in reserve after closing strengthens your application considerably.

Because construction loans are more complex to underwrite, working with a lender experienced in new construction — rather than a general retail mortgage lender — can significantly smooth the approval and disbursement process.

Construction loan costs are influenced by variables you can control. A few strategic decisions before and during construction can meaningfully reduce the total interest you pay.

Minimize the construction period. Construction-phase interest grows every month the project runs. Choosing a builder with a proven track record for on-time delivery is as financially important as negotiating the contract price. Every month of delay on a large loan can cost thousands of dollars in additional interest.

Increase your down payment. A larger down payment directly reduces the loan amount, which reduces both construction-phase interest and permanent loan payments. Going from 20% to 25% down on a $400,000 project saves $20,000 in principal — and substantially more in lifetime interest on the permanent mortgage.

Compare construction rates aggressively. Because construction loans are less commoditized than standard mortgages, there is more variation in rates across lenders. Local banks, credit unions, and regional lenders that specialize in new construction often offer more competitive terms than large national lenders.

Lock your permanent rate early if rates are rising. With a construction-to-permanent product, you lock your permanent rate at the initial closing — before construction begins. In a rising-rate environment this can save significant money. In a declining-rate environment, a two-close structure lets you capture a lower rate near completion.