A construction loan calculator is the tool you need before you sign anything with a builder, because building a house is financed completely differently from buying one. Instead of a single lump-sum mortgage, a construction loan releases money in stages as work is completed β foundation, framing, roofing, and so on β and for most of that period you're only paying interest on whatever has actually been drawn, not on the full approved amount. That distinction confuses a lot of first-time builders, and it's exactly what this calculator is built to clear up.
Arb Digital built this construction loan calculator as part of a free suite of lending and real estate tools because so many of the people who come to us for websites and marketing also happen to be builders, remodelers, and real estate professionals who field this exact question from clients every week. Plug in your numbers below and you'll see both halves of the picture: what the interest-only draw period costs you month to month, and what the permanent mortgage payment looks like once your home is complete and the construction loan converts (or is refinanced) into a standard loan.
What This Construction Loan Calculator Does
This tool models a typical "construction-to-permanent" loan, which is the most common structure lenders offer for owner-built or custom homes. During the build, you draw funds from your approved credit line as each phase of construction is inspected and signed off. You pay interest only on the amount actually disbursed β not the full loan balance β because undrawn funds haven't cost the lender anything yet. Once construction wraps up, the loan rolls into a permanent mortgage with its own rate and amortization schedule, and you start making regular principal-and-interest payments like any other homeowner.
The calculator estimates your average outstanding balance during the build based on the percentage of funds you tell it are typically drawn, works out the interest-only payment on that average balance, projects the total interest cost across the whole construction period, and then amortizes the full loan amount over your chosen permanent mortgage term so you can see the payment that follows. Between the two phases, you get a realistic sense of what the entire project costs to finance from groundbreaking to move-in day.
How to Use It
- Enter the total construction loan amount. This is the full amount your lender has approved, not what's currently drawn.
- Enter the construction-phase interest rate. Construction loans typically carry a variable rate tied to prime, and it's usually higher than a standard mortgage rate because the lender is taking on more risk during the build.
- Set the construction period in months. Most residential builds run 6 to 18 months depending on size, complexity, permitting, and weather delays.
- Estimate the average percent of funds drawn. You won't have 100% of the loan disbursed on day one β funds trickle out as milestones are hit. A rough average across the whole build is usually somewhere between 40% and 60%.
- Enter your expected permanent mortgage rate and term. This is the rate you expect once the loan converts to a standard mortgage after the certificate of occupancy is issued.
- Click Calculate to see your monthly interest-only cost, total construction-phase interest, and permanent payment side by side.
The Formula β How It's Calculated
During construction, interest accrues only on the drawn portion of the loan, so the calculator approximates your average outstanding balance as Loan Amount Γ Average Percent Drawn. Monthly interest during the build is then Average Balance Γ (Construction Rate Γ· 12), and total construction-phase interest is that monthly figure multiplied by the number of months in your build. Once the home is complete, the calculator switches to standard mortgage amortization on the full loan amount: Payment = P Γ r(1+r)^n Γ· ((1+r)^n β 1), where P is the loan principal, r is the monthly permanent interest rate, and n is the number of monthly payments in the permanent term. This is the same formula used by every conventional fixed-rate mortgage calculator, and the Consumer Financial Protection Bureau publishes a clear explanation of how amortization and interest-only periods interact in its guidance on construction and renovation loan options.
Why Construction Loans Cost More Than a Standard Mortgage
Lenders price construction loans higher than finished-home mortgages for a simple reason: risk. A half-built house is worth far less as collateral than a completed one, budgets on custom builds run over more often than they don't, and the lender has to monitor draws and inspect progress at every stage. That extra oversight and exposure shows up as a rate premium, typically one to two percentage points above what you'd pay on a conventional mortgage for the same borrower profile. Most construction loans also carry a variable rate tied to the prime rate, which means your interest-only payment can move during the build if rates shift β something a fixed-rate purchase mortgage never does.
There's also a structural difference worth understanding: interest-only payments during construction don't build any equity. You're simply covering the cost of borrowing while the asset is being built. Equity only starts accumulating once the loan converts to the permanent phase and your payments begin including principal. That's one reason experienced builders try to keep the construction timeline as tight as realistically possible β every extra month on-site is another month of interest-only payments with nothing paid down.
Construction-to-Permanent vs. Two Separate Loans
Most residential borrowers use what's called a single-close construction-to-permanent loan, which is what this calculator models. You close once, the loan funds the build in draws, and it automatically converts to a permanent mortgage when construction is finished β no second closing, no second set of closing costs, and no risk of your financing falling through between phases if your financial situation changes mid-build. The alternative is a two-close structure, where you take out a standalone construction loan, pay it off in full when the home is done, and then apply separately for a new purchase mortgage. Two-close loans give you more flexibility to shop the permanent rate at the end, but they mean two closings, two sets of fees, and a second underwriting review with no guarantee your rate or approval will look the same a year later. For most owner-occupants building a primary residence, the single-close structure this tool estimates is the more common and generally lower-hassle path.
What Affects Your Draw Schedule and Average Balance
The "average percent drawn" figure in this calculator is really a stand-in for your draw schedule, and it matters more than most first-time builders expect. A typical schedule might release 10% at foundation, 20% at framing, 20% at mechanicals and drywall, 20% at interior finish, and the remainder at final completion. If your project front-loads big draws early β say a large land or site-prep disbursement in month one β your average outstanding balance across the whole build will run higher than a project where draws are spread evenly, and your total interest bill will be higher too, even with an identical loan amount and rate. Ask your builder for a projected draw schedule before you finalize your loan terms, and use it to refine the average-drawn percentage you enter here rather than guessing.
- Longer builds cost more in interest β every month added to the schedule is another month of interest-only payments on the outstanding balance.
- Front-loaded draws raise your average balance β land purchase and site work at the start of a project pull a larger share of funds early.
- Weather and permitting delays are common β build in a buffer when estimating your construction period rather than using the builder's best-case timeline.
- Rate locks matter β ask whether your permanent rate is locked at closing or floats with the market until conversion, since that changes your risk during a long build.
Check the numbers on a bridge loan or a straight land purchase before you commit, or explore all of our free calculators. Arb Digital also builds fast, high-converting websites and content for builders, lenders, and real estate professionals.
Try the Bridge Loan Calculator All Free ToolsCommon Mistakes to Avoid
- Assuming interest accrues on the full loan from day one. You only pay on what's drawn β using the full loan amount for your interest estimate massively overstates your construction-phase cost.
- Ignoring rate variability. Construction loans are usually variable-rate; a rate that rises mid-build increases your interest-only payment without warning.
- Underestimating the timeline. Permitting delays, material shortages, and weather routinely push builds past their original schedule, and every added month costs real interest.
- Forgetting the conversion step. Some construction loans require you to formally qualify for the permanent mortgage again at conversion β get that requirement in writing at closing.
- Not comparing single-close and two-close options. The convenience of one closing can be worth less than a materially better permanent rate found by shopping separately.
Related Free Tools From Arb Digital
Building or buying raw land before you build? Run the numbers with our Land Loan Calculator. Need short-term financing to bridge the gap between two properties, such as covering the down payment on new construction while your current home sells? Try the Bridge Loan Calculator. Once the permanent mortgage is in place, check your ongoing costs with our Mortgage Calculator or see whether it makes sense to tap built-up equity later using the Home Equity Loan Calculator or Second Mortgage Calculator. And before locking in a budget, run our House Affordability Calculator to sanity-check what payment you can comfortably carry. Browse the rest of our free online tools hub at /tools/.
Frequently Asked Questions
Interest is charged only on the portion of the loan that has actually been disbursed at any given time, not the full approved amount. As your builder completes milestones and draws more funds, your outstanding balance β and therefore your interest cost β grows throughout the project.
Typically no. Most construction loans are interest-only during the build, meaning your payment covers only the interest on the drawn balance. Principal payments begin once the loan converts to a permanent mortgage after construction is complete.
It's a single loan that funds the building process in draws and then automatically converts into a standard amortizing mortgage once the home is finished, avoiding a second closing and a second round of closing costs.
Lenders take on more risk financing an unfinished property with less collateral value and more uncertainty around cost overruns and delays, so they typically price construction loans one to two percentage points above a comparable finished-home mortgage.
You may need to cover a funding gap out of pocket, seek an increase to your approved loan amount from the lender, or in some cases pause work while financing is renegotiated. This is why most lenders and builders recommend budgeting a contingency of 10β20% above the estimated build cost.
Some lenders offer a rate lock at closing that holds through the construction period and conversion; others let the permanent rate float with the market until the home is complete. Ask specifically which structure your lender uses, since it changes your interest rate risk over a long build.
This tool provides general estimates for educational purposes only and is not financial, tax, legal, or medical advice. Figures are illustrative; consult a licensed professional for decisions.
