Construction Draw Schedule in Excel: S-Curves and Interest Carry

Construction Draw Schedule in Excel: S-Curves and Interest Carry

A construction loan is not a sum of money; it is a pattern of money. Funds draw monthly as costs are incurred, interest accrues only on what has been drawn, and that interest — the carry — is itself a project cost, capitalized into the budget through the interest reserve. Which means the draw schedule is not paperwork downstream of the pro forma: it is the calculation that produces one of the pro forma's budget lines, and getting its shape wrong misprices the project before ground breaks.

This guide builds the schedule in Excel — the monthly draw engine, the S-curve and how to model it, the carry calculation, and the reserve sizing — with every figure verified by month-by-month computation. It also corrects, with arithmetic, a piece of folk wisdom about S-curves that most explainers repeat and the numbers do not support. All figures are illustrative examples.

The running example: $6,000,000 of construction costs funded by a construction loan at an illustrative 8.5%, drawn over an 18-month build.

The Layout: a Month-by-Month Engine

One column per month of the construction period, with rows in this order:

1. Costs incurred this month — the spend curve (the subject of the next section)
2. Funding source split — equity first, then debt: the standard lender convention is that the borrower's equity funds before loan dollars draw, so the debt rows stay at zero until cumulative spend exhausts the equity commitment
3. Loan draw this month and cumulative loan balance
4. Interest accrued this month = prior cumulative balance × rate ÷ 12 — and, if interest is financed (the usual case), added to the balance: interest accruing on interest, which is why late-project months are so expensive
5. Interest reserve remaining — the budget line, depleting as accruals post

The engine's outputs — total carry, peak balance, reserve depletion date — feed straight back into the development budget, which is the integration point with the full development pro forma.

One refinement real schedules carry: retainage — the percentage (commonly around 5–10%) of each draw the lender or owner holds back until substantial completion, released at the end. In the engine, retainage is one more row pair: the holdback reduces each month's funded draw (and therefore the accruing balance) and returns as a large late-schedule draw at release. The effect is a mild, systematic back-loading of the balance — modest relief on carry during the build, a bump at completion — and, more practically, a cash-flow fact the contractor's schedule of values and your funding plan both need to agree on. Models that ignore retainage overstate the mid-project balance and misdate the final draw.

The S-Curve: What It Is and How to Build It

Construction spending is not flat. Projects start slow (mobilization, sitework), accelerate through the middle (structure, envelope, MEP running concurrently), and taper at the end (finishes, punch list) — a cumulative spend line shaped like an S.

Two Excel implementations:

The formula version. A normalized smoothstep gives a clean symmetric S: with t as the month's fraction of the schedule (month ÷ 18), cumulative spend fraction = 3*t^2 − 2*t^3. Each month's cost is the cumulative fraction's increment times the budget. One formula, no lookup tables, and a shape parameter away from skewed variants (t^2 for back-loaded, t^0.5 for front-loaded).

The schedule-of-values version. For real projects, replace the formula with the actual trade-level schedule: each trade's budget spread over its construction window, summed by month. More work, more truth — and the natural evolution of the same layout as the project moves from pro forma to execution.

The Carry Math — and the S-Curve Myth

Now the part most guides get wrong. The folk claim is that modeling a straight-line draw "underestimates the carry" and the S-curve corrects it. Run the numbers on our example — $6,000,000 over 18 months at 8.5%, computed month by month:

The symmetric S-curve produces exactly the same carry as the straight line — because carry depends on the average outstanding balance, and a symmetric S spends less than straight-line early and more late in amounts that cancel precisely. What actually moves the carry is the curve's skew: back-loading the spend (typical of projects where the expensive trades stack late) keeps the balance low longer and cuts carry by a third in our example; front-loading (heavy early sitework, big deposits) raises it by a third. The honest modeling instruction, then, is not "use an S-curve" — it is match the curve's skew to the project's actual trade sequencing, because ±$128,000 of budget rides on it in this small example alone.

So why does the S-curve still matter enormously, if a symmetric one leaves total carry unchanged? Three reasons the straight line cannot deliver:

1. Draw timing and lender administration. The monthly draw requests, inspections, and funding the schedule generates are shaped like the S, not the line.
2. Equity depletion and the debt start date. Under equity-first funding, the spend curve's early shape determines when the loan starts drawing at all — which moves the accrual start and the peak balance even when total carry nets out.
3. Delay pricing — the big one, next.

The Delay Asymmetry: When Slippage Happens Matters More Than That It Happens

Because the balance grows over the project, a month of delay is not one price — it is a price that depends on when it lands. Our example, computed:

• Three months of slippage late in the project (balance near the full $6,000,000): additional carry of roughly $127,500
• Three months of slippage early (balance around 20% drawn): roughly $25,500

A five-to-one asymmetry on the same three months of calendar. This is the single most useful output of a real draw schedule, and a straight-line model structurally cannot produce it — the line's constant balance prices every month of delay identically, and wrongly. The practical consequences write themselves: schedule risk in the finishes-and-inspections phase deserves several times the contingency attention of the same risk in sitework; a delay claim's cost depends on the month it names; and the interest reserve's stress case is specifically a late slip, not a generic one.

Sizing the Interest Reserve

The reserve is the budget line that pre-funds the carry, and the engine sizes it properly: base-case computed carry, plus a stress margin for the delay case that matters — a late slip at high balance, plus any rate float if the construction loan is variable. On our example, a base carry near $382,500 with a three-month late-slip stress of ~$127,500 argues for a reserve well above the base number; the exact margin is a risk decision, but a reserve set equal to the base-case carry is a reserve designed to run dry at the worst moment. When it does run dry, the shortfall is an equity call mid-project — the least welcome capital request in real estate — which is why the reserve-depletion row deserves conditional formatting and the stress case deserves a column of its own.

The reserve interacts with the loan sizing too: capitalized interest is part of total development cost, the loan sizes to cost, and the loan's size feeds the carry — a circularity Excel resolves with iterative calculation enabled or a one-step manual convergence. The construction loan calculator guide walks that loop; the draw engine here is what feeds it.

Tracking Against Actuals

Once construction starts, the same layout becomes the control tool: add an actuals row-pair under the plan (cost incurred, drawn to date), and the variances do the monitoring — spend running ahead of plan warns of overrun, behind plan warns of delay, and the carry recomputes live off the actual balance rather than the projection. The schedule that priced the project becomes the schedule that manages it, which is the argument for building it properly in the pro forma rather than bolting it on at the first draw request.

Frequently Asked Questions

What is a construction draw schedule? The month-by-month plan of construction spending and loan funding: costs incurred, equity and debt sources, the running loan balance, and the interest accruing on it. It generates the lender's draw requests and computes the project's interest carry.

What is the S-curve in construction? The characteristic shape of cumulative construction spending — slow start, fast middle, tapering finish. In Excel it is one formula (3t²−2t³ for the symmetric case) or, better, the sum of trade-level schedules whose skew matches the real sequencing.

How do I calculate construction loan interest in Excel? Month by month: prior loan balance × annual rate ÷ 12, added to the balance if interest is financed. Total carry is the sum across the schedule — and it depends on the draw pattern's skew, not merely its total.

How big should an interest reserve be? Computed base-case carry plus a stress margin — specifically for late-schedule slippage, where each month costs several times an early month (5:1 in our example), plus rate float on variable-rate loans. A reserve equal to the base case has no room for exactly the scenario reserves exist for.

Does the draw schedule matter if I'm building with cash? The lender rows disappear; the shape does not. The spend curve still times your capital's deployment, prices delay in opportunity-cost terms, and becomes the actuals-tracking tool — the engine is the same with the interest rows at zero.

The Engine, Built Into the Model

The Development Sheet carries this draw engine inside the full development pro forma: the monthly spend curve with equity-first funding, the running balance and computed carry, the interest reserve as a calculated budget line with its depletion tracked, and the schedule stress wired into the cost-overrun sensitivity — fully unlocked, formula-transparent, versioned, with a documented methodology PDF. The complete model catalog is in the store.

This article is for educational purposes only and does not constitute investment, legal, or tax advice. All figures are illustrative examples, not market data, cost estimates, or rate quotes. Consult qualified professionals before making investment decisions.