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Interest Reserve Calculator

Construction and rehab lenders set aside an interest reserve at closing to cover monthly interest during the build. This calculator sizes that reserve based on loan amount, rate, build term, and the average outstanding balance as draws flow in.

Construction loan amount

Interest rate

Build term (months)

Average funds drawn

linear draw ≈ 50%

Interest reserve needed

$18,408

held back from proceeds

Average monthly interest

$2,045

Average outstanding balance

$233,750

If fully drawn day 1

$33,469

upper bound

Reserve savings vs full draw

$15,061

How the math works

Construction loans charge interest only on drawn funds. Since draws happen gradually over the build, the average outstanding balance is well below the full loan amount — typically 45–60% for a standard linear draw schedule.

Lenders usually set the reserve aside from proceeds at closing so borrowers don't have to write monthly interest checks during the build. A reserve that runs out before substantial completion becomes a cash call on the borrower.

Editorial noteMaintained by EveryCalc - Reviewed June 2026

EveryCalc calculators are designed for fast, practical estimates with transparent inputs and no required account. We use plain formulas, visible assumptions, and related tools so visitors can check the result from more than one angle.

Results are informational only. For financial, tax, legal, medical, construction, or other high-impact decisions, verify the output against primary sources or a qualified professional.

Learn more about our review process on the EveryCalc methodology page.

How this calculator works

What this page estimates

This Interest Reserve Calculator is built to give a quick, browser-based estimate for interest reserve. Construction and rehab lenders set aside an interest reserve at closing to cover monthly interest during the build. This calculator sizes that reserve based on loan amount, rate, build term, and the average outstanding balance as draws flow in. The inputs stay on the page during normal use, and the result should be treated as an estimate for planning, comparison, or education rather than professional advice.

Calculation approach

The calculator applies the standard relationship implied by the inputs, then formats the answer so it can be checked and reused. For finance tools, the most important step is using consistent units, rates, time periods, and assumptions before comparing the result with another calculator or outside quote.

Example workflow

For example, start with a realistic value you already know, change one input at a time, and watch how the answer moves. That makes it easier to tell whether the result is being driven by the main amount, the rate, the time period, or a unit conversion.

Practical checks

Use current, real-world numbers when the result affects money, health, tax, or legal decisions.
Run a low, base, and high case when the inputs are estimates.
Check the related calculators below when the next decision depends on a different assumption.

How to interpret the interest reserve result

Best use

Use the result as a planning number for comparing payments, rates, returns, tax reserves, or cash-flow choices before you request a quote or make a commitment.

Cross-check

Compare the answer with the contract, lender estimate, tax form, brokerage statement, payroll record, or invoice that will control the real-world outcome.

Watch for

Do not rely on a single optimistic rate, return, or fee assumption. Money pages work best when you run low, base, and high cases and keep professional advice separate from the estimate.

This page belongs to the Finance calculator library, so the answer should be read in the context of the decision you are modeling rather than as a universal rule.

Before relying on this interest reserve estimate

Most calculator mistakes come from the inputs, not the arithmetic. Use this short audit before you reuse the answer in a spreadsheet, quote, application, or important conversation.

Confirm source numbers

Match balances, rates, fees, taxes, income, and payment dates against the lender quote, payroll record, tax form, statement, invoice, or contract.

Separate cash flow from total cost

A lower monthly payment can still cost more over time if fees, interest, taxes, or a longer term are hidden in the structure.

Run conservative cases

Test at least one higher-cost or lower-return case before using the output for a purchase, refinance, investment, loan, or tax decision.

Rerun this page when the rate, price, term, fee, tax rule, income, expense, or expected holding period changes.

How to Use

Enter the total construction/rehab loan amount.
Enter the note rate. Construction loans price higher than take-out mortgages, often prime + 1% to 4%.
Enter the build term in months. Most residential builds run 6–12 months.
Enter the expected average drawn balance. Linear-draw projects average around 50%; heavy-front projects can be 60–70%.

Frequently Asked Questions

Why do lenders require an interest reserve?

The borrower isn't generating income from the property during construction, so they'd have to write monthly interest checks out of pocket. A reserve protects against default and smooths cash flow during the build.

How is the reserve size calculated?

Lenders estimate the average outstanding balance over the build (usually 50–60% of the loan amount) and multiply by rate and term. A 9-month build at 10.5% on $425k with a 55% average outstanding ≈ $18,400 reserve.

What if the reserve runs out?

If draws accelerate or the build drags past term, the reserve can run dry. The lender will either require a capital call (borrower writes checks) or modify the loan. Budget a 15–20% cushion on reserve size.

Does the reserve count toward loan-to-cost?

Usually yes — it's funded by the loan itself, so it consumes LTC capacity. A $425k loan with $18k reserve delivers $407k of actual construction capital.