What Is a Construction Loan Calculator?
Unlike a standard mortgage where you receive the full amount at closing, a construction loan releases money in stages called draws as building milestones are reached. You only pay interest on the funds actually drawn, so the balance — and your interest cost — grows over the build period. This calculator estimates the total interest you will pay during construction before the loan converts to permanent financing.
How to Use It
Enter the total construction loan amount, the annual interest rate, and the construction term in months. Then choose an average drawn-balance factor: 50% models evenly spaced draws, 40% a slow start, and 60% a front-loaded schedule (large early draws for site work and materials). The result shows estimated total interest, the average drawn balance, average monthly interest, and the combined cost.
The Formula Explained
Because draws happen gradually, the average outstanding balance over the build is roughly the loan amount times a factor f (about 0.5 for even draws). Interest is then that average balance multiplied by the annual rate and the fraction of a year the loan is open:
$$I = L \cdot f \cdot \frac{r}{100} \cdot \frac{m}{12}$$
This is a simple-interest approximation. Actual interest depends on exact draw timing and any interest reserve, but the average-balance method is the standard quick estimate lenders use.
Worked Example
Suppose you borrow $300,000 at 7.5% over 12 months with even draws (factor 0.5). Average balance = \(300{,}000 \times 0.5 = \$150{,}000\). Interest = \(150{,}000 \times 0.075 \times 1 = \mathbf{\$11{,}250}\). Average monthly interest = \(11{,}250 \div 12 \approx \$937.50\), and total cost = \(300{,}000 + 11{,}250 = \$311{,}250\).
FAQ
Why only half the loan? You draw funds gradually, so on average about half the balance is outstanding during the build — that is why interest is far less than on the full amount.
Is this exact? No. It is an estimate. Real interest depends on your exact draw schedule; ask your lender for an amortized draw breakdown for precise figures.
Does this include the permanent mortgage? No. This covers only the interest-only construction phase. After completion the loan typically converts to a standard mortgage with separate terms.
