Estimate construction-phase interest from staged draws, then see the likely permanent mortgage payment after the build.
| Month | Project done | Loan balance after draw | Interest this month |
|---|
Table of contents
How to use our Construction Loan Calculator
- Enter Total project cost ($), including land only if the construction loan is paying for it.
- Enter Down payment or owner cash used first ($), Construction rate (percent per year), and Build time (months).
- Choose a Draw plan that best matches how the lender may release money during the build.
- Open Advanced options if you want to change Permanent mortgage rate (percent per year), Permanent loan term (years), or When each draw starts costing interest.
- Click Calculate, then sanity-check the Draw schedule table: balances should rise over time, early interest may be $0 if cash covers early work, and the highest payment should usually be near the end of the build.
Definitions
Total project cost: The budget amount used to estimate construction draws. It may include land only if the loan is paying for land too.
Down payment or owner cash used first: Cash or land equity applied before loan funds are drawn. This can delay when interest starts.
Draw plan: The pattern for releasing loan money as construction work is completed.
Construction rate: The yearly interest rate used during the interest-only construction phase.
Interest-only payment: A payment that covers interest for the month but does not pay down principal.
Estimated loan amount: Total project cost minus Down payment or owner cash used first, with a floor of $0.
Permanent mortgage rate: The yearly rate used for the long-term loan after construction is finished.
Permanent loan term: The repayment length, in years, used for the after-build mortgage payment.
Common mistakes and quick fixes
Mistake: Entering the full home value as Total project cost when the land is already owned and not financed.
Fix: Use Total project cost for the costs this project budget is meant to cover, and include land only if the loan pays for it.
Mistake: Putting Down payment or owner cash used first ($) as a percent instead of a dollar amount.
Fix: Enter the actual dollar amount of cash or land equity that reduces the amount the lender has to fund.
Mistake: Using the Permanent mortgage rate for Construction rate.
Fix: Enter the construction-phase yearly rate in Construction rate, and use Permanent mortgage rate only for the after-build payment estimate.
Mistake: Choosing Even monthly draws when the builder expects large early or late draws.
Fix: Change Draw plan to Faster draws early, Slower draws early, or 5 milestone draws so the Draw schedule table better matches the project.
Mistake: Reading Estimated permanent monthly payment as the full housing payment.
Fix: Treat Estimated permanent monthly payment as principal and interest only; add property taxes, insurance, mortgage insurance, and HOA dues separately.
Mistake: Comparing Interest saved by using cash first to a lender quote without checking When each draw starts costing interest.
Fix: Match When each draw starts costing interest to the lender's billing timing as closely as possible before comparing totals.
Limitations & Key Assumptions / Boundary Conditions
- This is an estimate for planning, not a lender approval, payoff quote, or closing disclosure.
- Construction interest is estimated with a simple monthly rate. Some lenders calculate interest daily, so real bills can differ by the number of days in each billing period.
- The Draw plan choices are simplified. A real lender or builder may use inspections, lien waivers, holdbacks, retainage, or a custom draw schedule.
- The calculator assumes Down payment or owner cash used first is applied before loan funds. If your lender handles cash, land equity, or reimbursements differently, the Estimated construction interest can change.
- Estimated permanent monthly payment includes principal and interest only. It excludes property taxes, homeowners insurance, mortgage insurance, HOA dues, escrow changes, and lender fees.
- Permanent loan interest over the full term assumes the permanent loan is kept for the whole selected term with no extra principal payments, refinance, sale, or rate change.
- If When each draw starts costing interest is set to Next month after the draw, the final draw may not add construction interest inside the selected Build time.
- If Down payment or owner cash used first is greater than or equal to Total project cost, the calculator shows a $0 loan amount and $0 construction interest from the entered values.
Methodology
Construction loan amount
The calculator first estimates the amount that must be funded by the loan after owner cash or land equity is applied.
loan_amount = max(0, project_cost - down_payment)
The $0 floor is used because a loan balance cannot be negative.
Draw progress
For Even monthly draws, progress rises evenly from month 1 through the final build month.
completion_ratio_m = m / construction_period_months
For 5 milestone draws, the calculator places simplified milestones at 20%, 40%, 60%, 80%, and 100% of the build time, rounded up to whole months. The draw shares are 20%, 25%, 20%, 20%, and 15%.
completion_ratio_m = sum(milestone_percent where milestone_month <= m)
For Faster draws early, 65% of progress is reached by the halfway month rounded up, then the remaining 35% is spread over the rest of the build.
if m <= h, completion_ratio_m = 0.65 * m / h; otherwise completion_ratio_m = 0.65 + 0.35 * (m - h) / (n - h)
For Slower draws early, 35% of progress is reached by the halfway month rounded down, then the remaining 65% is spread over the rest of the build.
if m <= h, completion_ratio_m = 0.35 * m / h; otherwise completion_ratio_m = 0.35 + 0.65 * (m - h) / (n - h)
Cash-first balance and interest
The main estimate applies Down payment or owner cash used first before loan draws. This is the part that prevents the common mistake of charging interest as if loan funds were used from the first dollar.
balance_m = min(loan_amount, max(0, project_cost * completion_ratio_m - down_payment))
The comparison estimate ignores the cash-first order and lets the loan balance rise with project progress.
comparison_balance_m = loan_amount * completion_ratio_m
The monthly construction rate is the yearly Construction rate divided by 100 and then by 12.
monthly_rate = construction_rate / 100 / 12
If When each draw starts costing interest is Same month as the draw, month m uses balance_m. If it is Next month after the draw, month m uses the prior month's balance, with month 1 starting from $0.
interest_m = interest_balance_m * monthly_rate
total_construction_interest = add all interest_m values
average_interest_only_payment = total_construction_interest / construction_period_months
highest_interest_only_payment = largest interest_m value
cash_first_interest_saved = full_draw_style_interest - total_construction_interest
Permanent mortgage estimate
After construction, the calculator estimates a fixed principal and interest payment using Estimated loan amount, Permanent mortgage rate, and Permanent loan term.
permanent_monthly_rate = permanent_rate / 100 / 12
payment_count = permanent_term_years * 12
if permanent_monthly_rate = 0, permanent_monthly_payment = loan_amount / payment_count
otherwise permanent_monthly_payment = loan_amount * permanent_monthly_rate / (1 - (1 + permanent_monthly_rate)^(-payment_count))
permanent_total_interest = permanent_monthly_payment * payment_count - loan_amount
total_interest_all_phases = total_construction_interest + permanent_total_interest
Mini-example
With a $450,000 Total project cost, $90,000 Down payment or owner cash used first, 8.5% Construction rate, 12-month Build time, Even monthly draws, and Same month timing, the Estimated loan amount is $360,000. The cash-first draw balance is $0 in month 2 because early project progress is still covered by cash, then rises to $360,000 by month 12. The Estimated construction interest is $13,546.88, the Highest monthly interest-only payment is $2,550.00, and the comparison Construction interest if loan draws started right away is $16,575.00. The Interest saved by using cash first is $3,028.13.