Construction Interest Reserve Calculator for Apartments

Short answer

A construction interest reserve is the budget line that pays the loan's interest until the building earns enough to pay it. Because a construction loan funds in draws, interest builds slowly, then runs on the full balance through lease-up. This calculator models monthly draws on a straight-line or S-curve and adds the lease-up months at full balance.

What an interest reserve is and what this tool does

A ground-up apartment building earns nothing while it is being built, so it cannot pay its own interest. Construction lenders handle this with an interest reserve: a line in the development budget, usually funded by the loan itself, that pays interest each month until the property can.

Get the reserve wrong and the deal runs out of money at the worst time, late in construction or halfway through lease-up. Size it too generously and you borrow and pay for money you never use. This tool builds the reserve month by month from the draw schedule instead of guessing.

Construction interest reserve

Construction loan ($)Rate (%)Construction monthsDraw patternLease-up months at full balance

Interest during construction–

Interest during lease-up–

Interest reserve–

Average balance drawn–

Estimates only, not a loan quote.

How to use it

Loan amount. The total construction loan commitment from your term sheet or lender's sizing, not the total project cost. Equity usually goes in first, so the loan funds the later share of costs.
Interest rate. The all-in note rate. Most construction loans float over an index such as SOFR, so enter today's index plus your quoted spread, then rerun with a higher rate to see how much cushion you need. Our rates page shows the current index prints.
Construction months. The schedule from your general contractor, from loan closing to certificate of occupancy. Add the contingency months your contractor would bet on, not the best case.
Draw curve. How the loan funds over construction. An S-curve is slow at the start (site work, foundations), fastest in the middle (framing, MEP, roofing) and slow again at the end (finishes). A straight line spreads draws evenly. Your contractor's cost-loaded schedule is the best source; the S-curve is a reasonable default if you do not have one yet.
Lease-up months at full balance. How long after completion until the building's income covers its interest. Use your leasing plan or market absorption, and remember the loan is fully funded during this period.

Worked example

Say you have a $20,000,000 construction loan at 8.0%, an 18-month build, draws that follow an S-curve, and 12 months of lease-up during which the full balance is outstanding.

Interest reserve example	Amount
Interest during 18 months of construction	$1,200,000
Interest during 12 months of lease-up	$1,600,000
Interest reserve	$2,800,000
Average share of the loan drawn during construction	50%

Notice where the money goes. The construction months contribute less than you might expect, because for much of the build only part of the loan is out. The lease-up months contribute heavily, because every one of them runs on the full balance. That is why the reserve is far below a full year of interest on the whole loan per year of construction, and why a slow lease-up is what usually breaks a reserve.

Compare the reserve to a simple rule of thumb such as average outstanding balance times rate times months. The draw-curve figure is usually more defensible in a lender's budget review, and it shows exactly which months carry the risk.

What this tool assumes

One fixed rate. The rate does not change over the term. A floating-rate loan can cost more or less; test a higher rate, or price a rate cap with the rate cap calculator.
Interest on drawn balance only. Interest accrues monthly on what has been funded. Unused commitment fees, if your lender charges them, are not included.
Draws follow the curve. Real draws are lumpy, and change orders or delays shift the curve to the right, which adds interest.
No income offset during lease-up. Lease-up interest is carried at full balance. In practice, rent from leased units starts to cover part of the interest; leaving it out builds in cushion.
Interest on the reserve itself. When the reserve is funded by the loan, each interest payment is a draw that also accrues interest. Check whether your lender's budget sizes it this way, because it adds a modest amount on long schedules.
No fees or other carry. Lender fees, taxes, insurance and other carrying costs sit in their own budget lines.

What to do next

Put the reserve into your development budget as its own line and test it against a schedule delay of a few months and a higher rate. If the stressed reserve is meaningfully larger, discuss that cushion with your lender before closing; adding it later usually means new equity.

Then plan the exit. The construction loan is repaid by permanent debt, so run stabilized NOI through the DSCR sensitivity grid to see what a takeout loan will size to. Our construction loans page covers the financing options, and you can send the scenario with "get quotes on these numbers" or email desk@janover.co.

Frequently asked questions

How do you calculate an interest reserve on a construction loan?

Project how much of the loan will be outstanding each month, multiply each month's balance by the monthly rate, and add the months up through construction and lease-up. Because draws start small and grow, construction interest is well below rate times the full loan. Lease-up months run on the full balance. Lenders typically review this schedule as part of the construction budget.

Why does lease-up matter so much for the interest reserve?

By the time construction ends, the loan is fully drawn, so every lease-up month carries interest on the whole balance. Meanwhile the building is only partly leased and may not cover that interest. A lease-up that runs a few months longer than planned can consume more reserve than the entire first half of construction. Base the lease-up period on realistic absorption, not the pro forma.

Is the interest reserve part of the loan amount?

Usually, yes. Most construction lenders include the interest reserve as a line in the total project budget and fund it from loan proceeds, so you are borrowing to pay interest. That also means the reserve counts toward loan-to-cost limits. Some lenders require part of it from equity. Your term sheet and budget will show how your lender treats it.

What happens if the interest reserve runs out?

Once the reserve is spent, the borrower generally has to pay interest from its own funds, and the loan documents typically require the borrower to rebalance the budget by putting in more equity. A depleted reserve before stabilization is a common stress point in construction deals. Size the reserve against a delayed schedule and a higher rate, not only the base case.

Should I use an S-curve or straight-line draws?

Use your contractor's cost-loaded schedule if you have one. Without it, an S-curve is the more realistic default for ground-up apartments, because spending is slow during site work, peaks during vertical construction and slows during finishes. Run both: if the two answers differ a lot, the shape of your draws matters for this deal, and the cost-loaded schedule is worth getting before you lock the budget.