Construction Loan Interest Reserve Calculator
Enter the loan-funded project costs, initial advance, draw pattern, rate, and possible delay to size the interest reserve month by month. The result shows the reserve, the buffered commitment, and the schedule behind the number.
Calculated interest reserve$55,676.75
- Recommended reserve with contingency
- $61,244.43
- Total loan commitment including reserve
- $1,061,244.43
- Average outstanding balance
- $618,630.57
- Effective average balance percentage
- 61.86%
- Peak monthly interest
- $7,858.64
- Difference from 50% shortcut
- $10,676.75
Monthly draw and reserve schedule
| Month | Project draw | Opening balance | Capitalized interest | Closing balance | Cumulative reserve used |
|---|---|---|---|---|---|
| 1 | $200,000.00 | $0.00 | $1,500.00 | $201,500.00 | $1,500.00 |
| 2 | $72,727.27 | $201,500.00 | $2,056.70 | $276,283.98 | $3,556.70 |
| 3 | $72,727.27 | $276,283.98 | $2,617.58 | $351,628.83 | $6,174.29 |
| 4 | $72,727.27 | $351,628.83 | $3,182.67 | $427,538.78 | $9,356.96 |
| 5 | $72,727.27 | $427,538.78 | $3,752.00 | $504,018.05 | $13,108.96 |
| 6 | $72,727.27 | $504,018.05 | $4,325.59 | $581,070.91 | $17,434.54 |
| 7 | $72,727.27 | $581,070.91 | $4,903.49 | $658,701.67 | $22,338.03 |
| 8 | $72,727.27 | $658,701.67 | $5,485.72 | $736,914.66 | $27,823.75 |
| 9 | $72,727.27 | $736,914.66 | $6,072.31 | $815,714.24 | $33,896.06 |
| 10 | $72,727.27 | $815,714.24 | $6,663.31 | $895,104.83 | $40,559.37 |
| 11 | $72,727.27 | $895,104.83 | $7,258.74 | $975,090.84 | $47,818.12 |
| 12 | $72,727.27 | $975,090.84 | $7,858.64 | $1,055,676.75 | $55,676.75 |
Assumes project draws occur at the beginning of each modeled month and interest is capitalized at month-end.
Delay months add no new project-cost draws but keep accruing interest on the accumulated balance.
The contingency increases the recommended reserve but does not itself accrue interest in this model.
How to use this calculator
- Enter the project costs the loan will fund before any interest reserve.
- Enter the amount advanced at closing and choose the planned draw pattern.
- Add the annual rate, construction term, possible delay, and reserve contingency.
- Review the reserve, buffered commitment, shortcut comparison, and monthly schedule.
How the interest reserve is calculated
A construction loan interest reserve covers interest during the build period, when the loan balance changes as draws are funded. This calculator starts with project costs before interest, then builds a monthly schedule from the initial advance, the selected draw curve, and any delay months.
Let P be loan-funded costs before interest, U the initial advance, p planned months, d delay months, n = p + d, and i the annual rate divided by 12. If p = 1, all project costs are drawn in month 1. Otherwise, month 1 draws U, and P - U is allocated over months 2 through p.
For the remaining draw months, the weights are:
- Even: each remaining month receives
1 / k, wherek = p - 1. - Front-loaded: draw
jreceives2(k - j + 1) / (k(k + 1)). - Back-loaded: draw
jreceives2j / (k(k + 1)).
Each month is then calculated as:
interest = (opening balance + project draw) x monthly rate
closing balance = opening balance + project draw + interest
The calculated reserve is the sum of all monthly interest. The recommended reserve multiplies that amount by 1 + contingency. The total loan commitment shown here is project costs + recommended reserve. The 50% shortcut comparison uses P x annual rate x 50% x n / 12, which can miss the effect of draw timing, compounding, and delays.
What moves the reserve most
The funded balance, rate, term, and delay are the major drivers. A front-loaded schedule raises interest because more principal is outstanding earlier. A back-loaded schedule lowers interest during the first part of construction but may still need a larger reserve if the project is delayed after the loan is fully drawn. Capitalized interest also matters because each month of interest becomes part of the next month's balance.
What this leaves out
The schedule assumes draws happen at the beginning of each modeled month, the rate is fixed, and interest is capitalized at month-end. It does not include daily accrual, floating-rate resets, lender fees, unused-commitment fees, inspections, retainage, equity-first funding rules, legal costs, extensions, exit fees, principal repayments, or lender restrictions on reserve advances.
Worked example
Suppose a project has $1,000,000 of loan-funded costs before interest, a $200,000 initial advance, a 9% annual rate, 12 planned months, even draws, no delay, and a 10% contingency. The remaining $800,000 is drawn as $72,727.27 per month from month 2 through month 12, and the monthly rate is 0.75%.
The first month draws $200,000, accrues $1,500 of interest, and closes at $201,500. Repeating the schedule gives a calculated interest reserve of $55,676.75, a recommended reserve with contingency of $61,244.43, and a total loan commitment of $1,061,244.43. The average outstanding balance is $618,630.57, or 61.86% of project costs, and the peak monthly interest is $7,858.64.
Common questions
What is an interest reserve on a construction loan?
An interest reserve is money set aside to pay interest while a project is being built. It is often included in the loan budget so interest can be capitalized instead of paid from separate borrower cash each month.
Why is interest calculated on the drawn balance instead of the commitment?
Construction loans are usually advanced over time, so interest is commonly charged on funded proceeds rather than the full approved commitment. A draw schedule is therefore more useful than assuming the whole loan is outstanding on day one.
How do front-loaded and back-loaded draws change the reserve?
A front-loaded schedule places more draws early, which raises the average outstanding balance and the reserve. A back-loaded schedule delays more principal until later, which usually lowers interest unless delay months keep the final balance outstanding for longer.
Does capitalized interest itself accrue interest?
Yes in this model. Monthly interest is added to the loan balance at month-end, so the next month accrues interest on prior project draws and on prior capitalized interest.
What happens to the reserve when construction is delayed?
Delay months receive no new project-cost draws, but the accumulated balance continues to accrue interest. That can make a delay expensive because the balance is often near its highest point after the planned draw period ends.
Why does this differ from the 50% average-balance shortcut?
The shortcut assumes the average balance is exactly half the project costs and ignores compounding. This calculator calculates each draw month, adds capitalized interest, and includes delay months, so the average balance can be above or below 50%.