The S-Curve Explained

Visualizing Cumulative Cost Over Time

Free previewIntermediate
⏱️ About 12 min
The S-Curve Explained — illustration

A single cost number tells you where you are today. An S-curve tells you where you are heading.

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The big idea: The cost S-curve plots cumulative cost over time, creating an S shape because projects spend slowly at first, rapidly in the middle, and slowly again at the end.
🎯 By the end, you'll be able to
  • Define the S-curve as cumulative cost over time
  • Calculate cumulative cost from per-period spend
  • Explain why PV, EV, and AC each form their own S-curve
📎 Helpful to know first

Completion of Module 4: Forecasting (EAC/ETC).

What Is an S-Curve?

A cost S-curve is simply cumulative cost plotted against time. The curve is shallow at the start when work is ramping up, steep in the middle when execution is at peak intensity, and flat near the end as the project winds down. This creates the characteristic S shape.

\[ C_n = \sum_{i=1}^{n} c_i \]
⚠️ Why the S Shape Matters

The S shape reflects how real projects behave. You cannot instantly mobilize a full team on day one, and you cannot finish 100% of remaining work overnight. The curve captures this natural rhythm of execution.

Cumulative vs. Per-Period Cost

Per-period spend fluctuates, but cumulative cost always increases. If a project spends $60k in period 1, $150k in period 2, and $90k in period 3, the cumulative cost at each point is $60k, $210k, and $300k respectively.

⚠️ Three Curves, One Story

Planned Value (PV), Earned Value (EV), and Actual Cost (AC) each produce their own S-curve. Plotting all three on the same graph lets you see schedule and cost health instantly: if EV sits below PV, you are behind schedule; if AC sits above EV, you are over budget.

🎮 S-Curve Explorer LIVE
Predict first: Predict first: if per-period spend is [$60k, $150k, $90k], what will the cumulative total be at period 3?
Adjust the per-period spend sliders to see how the cumulative S-curve changes shape.
📝 Worked example: A project has per-period costs of $60k, $150k, and $90k over three periods. What is the cumulative cost at each period, and what is the total project budget?
  1. 1. Period 1 cumulative: $60k
  2. 2. Period 2 cumulative: $60k + $150k = $210k
  3. 3. Period 3 cumulative: $210k + $90k = $300k
  4. 4. Total budget (final cumulative): $300k
✓ Cumulative costs: $60k, $210k, $300k. Total budget: $300k.
✏️ Practice: If per-period spend is $40k, $80k, and $60k, what is the cumulative cost at period 2?
💡 Hint
Add period 1 and period 2 spend together.
Answer
120000

Check your understanding

1. Using the canonical example (per-period spend of $60k, $150k, $90k), what is the cumulative cost at period 2?
Cumulative at period 2 = $60k + $150k = $210k.
2. Why does the cost S-curve flatten near the end of a project?
The curve flattens because the bulk of spending occurs during middle execution phases; near the end, little work remains and spend drops.
🔎 Related lessons
Next: Trend and Forecast Lines · soon
✅ Key takeaways
  • An S-curve plots cumulative cost over time, producing a characteristic S shape.
  • Cumulative cost is the running sum of per-period spend.
  • PV, EV, and AC each form S-curves; comparing them reveals project health.
➡️ Now that you can read an S-curve, the next lesson shows how to extend it into the future using trend and forecast lines.
Ready for the next step? Back to the course outline →