Construction loan interest works differently from a standard mortgage - and the difference catches a lot of builders and developers off guard when the final financing cost comes in higher than budgeted. With a mortgage, you borrow the full amount on day one and pay interest on the full balance from the start. With a construction loan, you draw funds in stages as the project progresses, and you only pay interest on what you've drawn. The catch is that each draw increases the balance, so interest accrues on a rising number every month. The total interest cost depends on your draw schedule and construction timeline, not just your rate and loan amount.
Missing this in a project budget can leave a 2–5% gap between your projected and actual financing costs - on a $400,000 loan, that's $8,000–$20,000 that wasn't in the plan. This guide explains how construction loan interest accrues, walks through the two calculation methods used by the BidFlow Construction Loan Interest Calculator, and shows a full draw schedule example month by month.
How construction loans differ from permanent mortgages
Understanding the structure is the prerequisite to understanding the math. Construction loans are short-term instruments - typically 6 to 18 months - designed to fund a build in progress. They convert to a permanent mortgage (or get paid off in full) once the certificate of occupancy is issued.
| Feature | Standard Mortgage | Construction Loan |
|---|---|---|
| Disbursement | Full amount at closing | In draws as work is completed |
| Interest basis | Full loan balance from day 1 | Only the amount drawn to date |
| During construction | Full P&I payments begin immediately | Interest-only payments on drawn balance |
| Total interest cost | Predictable from day 1 | Depends on draw schedule and timing |
| Rate type | Fixed or ARM | Usually variable (prime + spread) |
| Loan term | 15–30 years | 6–18 months (converts at completion) |
The interest-only-on-drawn-balance feature is the source of the budgeting confusion. A builder who estimates interest by applying the annual rate to the full loan amount will significantly overestimate early-stage interest and potentially underestimate total interest if the project runs long. The correct approach is to model the draw schedule and calculate interest on the actual outstanding balance each month.
The two calculation methods - and when each applies
The BidFlow Construction Loan Interest Calculator offers two methods that reflect the two real-world scenarios:
Gradual draws (default): Funds are drawn progressively as construction milestones are hit. The calculator models this by applying interest to 50% of the loan principal - the mathematical equivalent of averaging a balance that starts at zero and ends at the full loan amount. This is the appropriate method for a new build where draws are spread across the construction period.
Full balance from start: The entire loan amount (or the amount owed on an existing property being refinanced) is outstanding from day one. This applies when the loan includes payoff of an existing mortgage, or when the full draw is taken at closing. In this case, interest accrues on the full principal for the entire construction period.
| Scenario | Correct Method | Effective Principal for Calculation |
|---|---|---|
| New construction, staged draws | Gradual draws | Loan principal × 50% |
| Refinance of existing property + build | Full balance | New funds + amount owed on property |
| Single large draw at closing | Full balance | Full loan amount from month 1 |
How to calculate construction loan interest - step by step
This example models a $350,000 construction loan for a new single-family build, 10-month construction period, 7.5% annual interest rate, gradual draws.
1Determine the monthly interest rate
FormulaMonthly Rate = Annual Interest Rate ÷ 100 ÷
12
7.5% annual rate ÷ 12 = 0.625% per month (0.00625 as a decimal). This is the rate applied to the outstanding balance each month.
2Determine the effective principal for interest calculation
FormulaEffective Principal (gradual draws) = Loan
Principal × 0.50
$350,000 × 0.50 = $175,000. This reflects the average balance over the draw period - starting near zero and ending at the full loan amount as construction completes.
3Calculate total interest for the construction period
FormulaTotal Interest = Effective Principal × Monthly Rate
× Construction Period (months)
$175,000 × 0.00625 × 10 = $10,937.50 total interest during the construction period. This is the carry cost that needs to be in the project budget before the permanent mortgage begins.
4Calculate average monthly interest payment
FormulaAverage Monthly Payment = Total Interest ÷
Construction Period (months)
$10,937.50 ÷ 10 = $1,093.75 per month on average. Actual monthly payments will be lower early in the project (when the drawn balance is small) and higher near completion (when most of the loan is drawn). The average is useful for budgeting cash flow during construction.
5Model the full balance scenario (refinance comparison)
FormulaPrincipal for Interest = New Construction Funds +
Amount Owed on Existing Property
Total Interest = Principal for Interest × Monthly Rate ×
Construction Period
If this same borrower owns the land with $80,000 remaining on it, and the construction loan refinances that balance: $350,000 + $80,000 = $430,000 effective principal. Full balance method: $430,000 × 0.00625 × 10 = $26,875 total interest - nearly 2.5× higher than the gradual draws scenario. This is why the method selection matters so much for budget accuracy.
Draw schedule: how interest accrues month by month
The gradual draws model assumes a straight-line ramp from zero to full draw. In practice, draws cluster around construction milestones. This table shows a realistic 10-month draw schedule for a $350,000 build and the actual monthly interest at each stage.
| Month | Draw This Month | Cumulative Balance | Monthly Interest (7.5%/yr) |
|---|---|---|---|
| 1 | $35,000 | $35,000 | $219 |
| 2 | $52,500 | $87,500 | $547 |
| 3 | $52,500 | $140,000 | $875 |
| 4 | $35,000 | $175,000 | $1,094 |
| 5 | $35,000 | $210,000 | $1,313 |
| 6 | $35,000 | $245,000 | $1,531 |
| 7 | $17,500 | $262,500 | $1,641 |
| 8 | $17,500 | $280,000 | $1,750 |
| 9 | $17,500 | $297,500 | $1,859 |
| 10 | $52,500 | $350,000 | $2,188 |
| Total | $350,000 | $13,016 |
Notice that the actual interest on this draw schedule ($13,016) is slightly higher than the simplified gradual draws model ($10,938) because draws are front-loaded in months 2–3 for foundation and framing. The BidFlow calculator's 50% effective principal is a close approximation - for precise project budgets, model your actual draw schedule against your lender's anticipated milestone payments.
What delays cost - the timeline risk most builders underestimate
Construction loans accrue interest for every month the project runs. A two-month delay on a $350,000 loan at 7.5% adds roughly $2,188 × 2 = $4,375 in additional interest at the full-draw stage - money that wasn't in the budget and comes directly out of profit or contingency.
| Delay Length | $250K Loan at 7.5% | $500K Loan at 7.5% | $1M Loan at 7.5% |
|---|---|---|---|
| 1 month | $1,563 | $3,125 | $6,250 |
| 2 months | $3,125 | $6,250 | $12,500 |
| 3 months | $4,688 | $9,375 | $18,750 |
| 6 months | $9,375 | $18,750 | $37,500 |
Delay costs are calculated on the full drawn balance, which is at or near its maximum in the final months of a project - the exact stage when weather delays, inspection holds, and subcontractor scheduling issues tend to cluster. Budget for at least one month of delay interest on any project over $200,000. Use the BidFlow Construction Loan Interest Calculator to model extended timelines alongside your base scenario before you finalize the project budget.
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