Glossary
S-Curve
A cumulative cost or progress curve that typically forms an S-shape over the project lifecycle, starting slowly, accelerating through execution, then tapering at handover.
An S-curve is a graph of cumulative value against time — most commonly showing planned expenditure, actual expenditure, and earned value on a single chart. The characteristic S-shape reflects the natural rhythm of a project: slow at the start when mobilisation, design, and procurement are underway; steep through the main construction or delivery phase when the bulk of work and spend occurs; then tapering as the project completes snagging, commissioning, and handover activities. Any significant deviation from the planned S-curve is a signal worth investigating.
S-curves are used in EVM reporting to give a visual summary of project performance that is easy for non-specialists to read. The gap between the planned value (PV) curve and the earned value (EV) curve shows schedule performance — if EV is below PV, the project is behind plan. The gap between the earned value (EV) curve and the actual cost (AC) curve shows cost performance — if AC is above EV, the project is spending more than the work done justifies. An S-curve is also used to show the probabilistic range of outcomes from a Monte Carlo simulation, where the fanned-out envelope of curves at the right-hand end shows the range of possible completion dates.
S-curves are easy to manipulate if the underlying data is not robust. If earned value is overstated (progress claimed but not physically achieved), the EV curve will sit artificially above the AC curve and will not reflect the true performance of the project. The classic symptom is an EV curve that tracks closely below the PV line for months and then suddenly drops sharply — an indication that someone has been making the numbers look good rather than reporting honestly. Independently verifying earned value claims through physical inspection or defined milestone completion is the best check against this.
Frequently asked
- What is an S-curve in project management?
- An S-curve is a cumulative graph that shows how a project variable (typically cost or resources) accumulates over time. It takes its characteristic S shape because projects are slow to mobilise, then accelerate through peak delivery, then taper as work completes. In project controls, three S-curves are typically plotted together: the Planned Value (PV — the baseline plan), the Earned Value (EV — the budgeted cost of work completed), and the Actual Cost (AC — money actually spent). The gaps between these three curves tell the performance story of the project.
- Why does a project S-curve have an S shape?
- The S shape reflects the natural lifecycle of project expenditure: slow ramp-up during mobilisation and early design, rapid increase during peak construction or delivery, then a tail-off as snagging, commissioning, and close-out proceed. The inflection point — where the cumulative spend rate is at its maximum — typically falls somewhere between 40% and 70% through the project timeline depending on the project type. Projects with very long mobilisation phases (e.g. large infrastructure) have a flatter lower portion of the S; those that go straight into construction have a steeper initial curve.
- What does it mean when actual cost runs above the planned S-curve?
- When the Actual Cost (AC) S-curve sits above the Planned Value (PV) S-curve, the project is spending faster than planned. This can mean two very different things: if Earned Value (EV) is tracking close to AC, the team is ahead of schedule and spending is justified by faster-than-planned progress. But if EV is below PV while AC is above it, the project is both behind schedule and over budget — a cost overrun without the compensating benefit of faster completion. Always read AC and EV together, not AC in isolation.
- How is an S-curve used in earned value management?
- In EVM, the S-curve is the standard output format for displaying the three core metrics: PV (the baseline plan), EV (physical progress valued at planned rates), and AC (actual spend). Schedule Variance = EV − PV (shown as the horizontal gap between EV and PV at a given date), and Cost Variance = EV − AC (the vertical gap between EV and AC). A project performing to plan has all three curves tracking closely together. Divergence between the curves is the earliest visual signal of performance problems — often visible on the S-curve before it shows up in other reporting.
Related terms
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