Project Management guide · Cost and resources
S-Curve in Project Management: What It Is and How to Use It
An S-curve is a graph of cumulative cost, effort or progress plotted against time. It usually forms an S shape because spending starts slowly, speeds up in the middle of a project and tails off near the end. Comparing planned and actual S-curves shows at a glance whether a project is on track.
Key takeaways
- An S-curve plots cumulative cost or work over time.
- The baseline PV curve ends at BAC.
- Compare EV with PV for schedule and EV with AC for cost.
- Diverging curves are an early warning sign.
Why the curve is S-shaped
- Start: few people, planning and design work, so cumulative cost rises slowly.
- Middle: the full team and most purchases, so the curve is steepest.
- End: testing, handover and closure with fewer people, so it flattens out.
How to build a baseline S-curve
- Create the schedule and assign a budget to each activity or work package.
- Total the planned cost for each period (week or month).
- Calculate the running cumulative total.
- Plot cumulative planned value against time. The last point equals the budget at completion.
- Each reporting period, add cumulative earned value (EV) and actual cost (AC) lines.
Example
| Month | Planned in month | Cumulative PV | Cumulative EV | Cumulative AC |
|---|---|---|---|---|
| 1 | $10,000 | $10,000 | $8,000 | $10,000 |
| 2 | $15,000 | $25,000 | $22,000 | $27,000 |
| 3 | $25,000 | $50,000 | $45,000 | $55,000 |
| 4 | $30,000 | $80,000 | $70,000 | $85,000 |
| 5 | $40,000 | $120,000 | ||
| 6 | $35,000 | $155,000 | ||
| 7 | $25,000 | $180,000 | ||
| 8 | $20,000 | $200,000 |
At month four the AC line ($85,000) sits above the EV line ($70,000): the project is over budget. The EV line sits below the PV line ($80,000): it is behind schedule. The gaps are the cost variance (-$15,000) and schedule variance (-$10,000).
How to read the three curves
| Pattern | Meaning |
|---|---|
| EV below PV | Behind schedule |
| EV above PV | Ahead of schedule |
| AC above EV | Over budget for work done |
| AC below EV | Under budget for work done |
| Curves diverging over time | The problem is growing; act now |
Other uses of S-curves
- Cumulative labour hours for capacity planning.
- Cash-flow and funding requirements for the finance team.
- Cumulative tasks or deliverables completed.
- Target, early-start and late-start curves to show the range of possible spending profiles.
Tips
- Plot the lines on the same axes and scale.
- Extend the curve with a forecast line from today to the EAC.
- Do not judge progress by the AC line alone; spending is not progress.
- Most project management software and spreadsheets can produce S-curves.
Free templates for this topic
Frequently asked questions
What is an S-curve in project management?
A graph of cumulative cost, effort or progress against time, which usually has an S shape.
Why is it called an S-curve?
Because cumulative spending starts slowly, accelerates mid-project and slows at the end, forming an S.
What does an S-curve show?
Planned versus actual progress and cost over time, making cost and schedule variances visible.
How do you create an S-curve?
Total the planned cost per period, calculate the running cumulative total and plot it against time.
What is a baseline S-curve?
The cumulative planned value curve from the approved cost baseline. It ends at BAC.
How is an S-curve used in earned value management?
PV, EV and AC are plotted as three cumulative curves; the gaps show schedule and cost variances.
What does it mean if the actual S-curve is above the plan?
If AC is above EV, the project is over budget for the work done. Compare AC with EV, not PV.
Can an S-curve be made in Excel?
Yes. Put periods in one column and cumulative values in others, then insert a line chart.
Does every project follow an S-curve?
Not always. Projects with large early purchases or a flat staffing profile may produce different shapes.
What is the end point of the baseline S-curve?
The budget at completion, the total planned cost of the work.