How to Calculate Construction Loan Interest
Knowing how to calculate construction loan interest is one of the most practically useful skills in residential development finance. The math is straightforward — and understanding it reveals why your effective cost of capital is almost always lower than the rate on your term sheet suggests.
How to Calculate Construction Loan Interest — The Formula and the Logic
Construction loan interest is calculated differently from a conventional mortgage and understanding the difference is essential to modeling your project economics accurately. A conventional mortgage accrues interest on the full loan amount from day one of the loan term. A construction loan accrues interest only on the amount that has actually been disbursed — the portion of the loan that has been advanced to the borrower through draws. The portion of the loan sitting in the holdback, waiting to be drawn as construction progresses, does not accrue interest until it is advanced.
This distinction has a significant impact on your actual carrying cost. On a $1 million construction loan at 10.99%, the stated annual interest cost would be $109,900 if you calculated interest on the full balance from day one. But if the loan starts with only the acquisition funding disbursed and the construction holdback is drawn gradually across 10 months, your actual total interest cost is typically 40% to 55% of what the stated rate implies. Knowing how to calculate construction loan interest precisely is what allows you to model your projects accurately and evaluate financing options honestly.
The Construction Loan Interest Formula
Daily Interest Cost = Outstanding Balance x Daily Rate
Monthly Interest Payment = Outstanding Balance x (Annual Rate / 360) x Days in Month
Total Project Interest = Sum of all monthly payments across the full loan term
Important: Construction loans use a 360-day year for interest calculation — not 365. This is standard across private construction lending and is specified in your loan documents. The outstanding balance changes each time a draw is funded, so each payment period must be calculated against the balance for that specific period.
The 360-day year convention is not unique to construction lending — it is used across commercial real estate finance generally. On a practical level it means your daily interest rate is slightly higher than if calculated on a 365-day basis, but the difference is small and consistent across all lenders using the same convention. What matters is understanding that interest accrues on actual days elapsed, against the actual outstanding balance, using the 360-day year as specified in your loan documents.
Three Worked Examples at Different Draw Stages
The best way to understand construction loan interest calculation is to work through it at different stages of a real project. Here are three examples showing exactly how to calculate your monthly interest payment at the beginning, middle, and end of a construction loan draw schedule.
Example 1 — Month 1, Only Acquisition Funded
Project Inputs
The Calculation
Example 2 — Month 6, Three Draws Funded
Project Inputs
The Calculation
Example 3 — Month 11, Project Nearly Complete
Project Inputs
The Calculation
How Interest Grows Across the Full Draw Schedule
The table below shows how interest accrues across each phase of a typical six-draw construction project, using the $900,000 loan example from above at 10.99%.
| Phase | Draw Amount | Cumulative Disbursed | Monthly Interest |
|---|---|---|---|
| Acquisition at closing | $350,000 | $350,000 | $3,312/mo |
| Draw 1 — Foundation (15%) | $82,500 | $432,500 | $4,092/mo |
| Draw 2 — Framing (25%) | $137,500 | $570,000 | $5,397/mo |
| Draw 3 — Mechanical (20%) | $110,000 | $680,000 | $6,438/mo |
| Draw 4 — Drywall (15%) | $82,500 | $762,500 | $7,219/mo |
| Draw 5 — Finish Work (20%) | $110,000 | $872,500 | $8,256/mo |
| Draw 6 — Final (5%) | $27,500 | $900,000 | $8,513/mo |
The total interest cost for this project depends on how long each phase takes. On a 12-month project with roughly two months per draw phase, total interest across the full project term is approximately $72,000 to $80,000 — significantly less than the $98,910 that applying 10.99% to the full $900,000 for 12 months would suggest. That gap is the practical benefit of interest accruing only on disbursed amounts.
Your Effective Interest Rate Is Lower Than You Think
The effective interest rate on a construction loan — the actual cost of capital expressed as a percentage of the total loan amount over the full term — is almost always meaningfully lower than the stated annual rate. This is one of the most important and least understood aspects of construction loan economics.
Why the Effective Rate Is Always Lower Than the Stated Rate
The stated rate on a construction loan — 10.99%, for example — is the annual rate applied to the outstanding balance. But the outstanding balance starts well below the total loan amount and grows gradually as draws are funded. On a project where the construction holdback is disbursed evenly across 10 months, the average outstanding balance across the loan term is approximately 65% to 70% of the total loan amount.
On a $900,000 construction loan at 10.99% with even monthly draws over 12 months, total interest paid is approximately $74,000. As a percentage of the full $900,000 loan amount over 12 months, that effective rate is approximately 8.2% — not 10.99%. The difference reflects the fact that you were not paying 10.99% on $900,000 for 12 months. You were paying it on an average outstanding balance of roughly $600,000 to $650,000.
This is why the rate comparison between a private construction loan at 10.99% and a bank construction loan at 7% is less dramatic in practice than it appears on paper. When you calculate the effective cost of both — accounting for the average outstanding balance, the draw timing, and the project term — the gap narrows significantly before you even factor in the operational costs of the bank’s slower process.
Use Our Free Construction Loan Interest Calculator
Rather than working through these calculations by hand for every project you evaluate, use our construction loan interest calculator to model your specific project’s interest cost in seconds. Enter your loan amount, rate, acquisition amount, draw schedule, and build timeline and get a month-by-month interest projection that you can use directly in your project pro forma.
Construction Loan Interest Calculator
Model your exact project — loan amount, draw schedule, build timeline. Get your month-by-month interest cost and total carrying cost in seconds. Free to use, no signup required.
Common Questions About Construction Loan Interest
At Mayflower Venture Partners, we walk every borrower through their specific interest cost projections before closing so there are no surprises. We lend throughout Massachusetts, Connecticut, Rhode Island, New Hampshire, and Maine. Use our construction loan interest calculator to model your next project, and call us when you are ready to move forward.
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