S-curve financial model: planned and actual project costs

An S curve financial model compares cumulative planned and actual project costs on the same timeline. Calculate each curve from monthly amounts, then read the gap alongside completed work and the latest estimate of remaining cost. Spending below plan can reflect delayed work rather than a saving.

Start with a comparable monthly export

Take the approved project budget and the project ledger export. Keep project ID, work package, cost category, accounting month, amount, and budget version. Add an actual-cost cutoff and identify any accruals separately so a later invoice does not duplicate an accrued amount.

Use incurred costs for this example. A cash payment schedule answers a different question; connect it to the cash flow forecast when treasury needs payment timing. Keep the same cost boundary on both curves: labor, materials, subcontractors, and any consistently allocated project overhead.

Aggregate transactions to one row per month for the selected project. Include closed months with no spending as numeric zero. Leave unreported future actuals empty. Retain the detailed export so each monthly total can be checked against its source rows.

Freeze the original baseline. If an approved change adds work, retain that baseline and create a separately labeled revised budget. Otherwise a rising actual line can appear to return to plan simply because someone rewrote history. The broader approval and ownership process belongs in project cost management.

Calculate the two cumulative curves

Cumulative planned cost = Sum of monthly planned costs through the reporting month

Cumulative actual cost = Sum of monthly actual costs through the same month

Spending gap = Cumulative actual cost minus cumulative planned cost

A US facilities team budgets a warehouse refit from January through June 2026. April 30, 2026 is the last closed date. These are illustrative project figures, with all amounts in USD.

Put month in column A, monthly plan in B, monthly actual in C, cumulative plan in D, cumulative actual in E, and spending gap in F. The resulting worksheet is:

Enter these formulas in row 2 and fill down through row 7:

D2 =SUM($B$2:B2)
E2 =IF(C2="",NA(),SUM($C$2:C2))
F2 =IF(C2="",NA(),E2-D2)

The formulas return #N/A for future actual points, which should remain unplotted. Do not enter the words "Not reported" in numeric cells; the table uses those words to explain empty inputs.

At April, actual cost is $340,000 against $380,000 planned. The gap is $340,000 minus $380,000, or negative $40,000. Dividing by $380,000 gives negative 10.5%, rounded to one decimal place. This is a spending comparison, not an earned-value cost variance.

Plot the lines and check the proof

Select the month column and both cumulative columns. Insert a line chart with dates on the horizontal axis and cumulative USD on the vertical axis. Use one scale, name the budget version, and mark the April cutoff. The planned line continues through June; the actual line stops at April.

Before interpreting the chart, make these checks:

• =SUM(B2:B7)-D7 returns zero: the full monthly budget equals the final planned cumulative value of $500,000.
• =SUM(C2:C5)-E5 returns zero: closed-month actuals equal April cumulative actual cost of $340,000.
• April ledger rows, including supported accruals, reconcile to that same $340,000.
• Project scope and cost categories match across both series.

Do not smooth away abrupt changes. A large equipment delivery can produce a legitimate step. A construction financial model connects project economics more broadly; this curve isolates when the approved cost is scheduled and incurred.

Separate a timing shift from an overrun

Actual cost below plan cannot establish cost efficiency without progress information. The Department of Energy's earned value guidance explicitly warns that comparing spending with planned cost without physical progress can mislead. Ask which planned work has actually been completed and which costs remain unrecorded.

Now add the project manager's latest estimate to complete:

Expected cost exceeds budget by 6.0%, calculated as $30,000 divided by $500,000. The lower actual curve at April coexists with an expected overrun. Do not add outstanding purchase orders again if the remaining-cost estimate already includes them.

NASA's earned value tutorial separates earned value against planned value from earned value against actual cost. Those comparisons need measured work completion. Keep them separate from the simpler spending gap shown here.

Where it goes wrong

• Different cutoffs. A partially closed actual month compared with a complete budget month exaggerates underspending.
• Cash against incurred cost. Supplier payment terms create a gap even when work and cost follow plan.
• Future blanks turned into zero. The actual curve appears to flatten through the remaining project and implies reported results that do not exist.
• Baseline changes without labels. Rephasing the original budget removes the evidence of a timing shift.
• Commitments counted twice. Adding purchase orders to an estimate that already includes them overstates expected final cost.

Compare your project workbook in Covirage

Bring your monthly project budget, actual-cost export, and prepared remaining-cost estimates in a file or Google Sheet. Covirage maps the columns, and its tools compute and check the cumulative figures and differences; the external AI model explains the results with every figure traced to its rows. Create a free Covirage account and bring a sample of your own project records.

See project cost control for the supporting use case.