Glossary · Cashflow S-Curve
S-Curve
An S-curve is a graphical representation of cumulative project spend over time, named for its characteristic S-shape: slow at the start, rapid in the middle, slow at the end.
Used in
Why spend follows an S-curve
In the early weeks of a construction project, spend is dominated by mobilisation, preliminaries, and ground works — cumulative spend builds slowly. Through the middle of the project, the bulk of trades work concurrently and spend accelerates. Towards the end, spend tapers as final fits and finishes complete.
Plotted cumulatively against time, this gives an S-shape — flat-rising at the start, steep through the middle, flattening at the end. The shape is so consistent across construction projects that it's become the default way to plan and track project cashflow.
Plan vs actual vs forecast
A well-managed S-curve has three lines on one chart: the planned curve (locked at project start), the actual curve (updated as valuations are processed), and the forecast curve (revised as the project progresses). Variance between actual and plan is a key project-controls signal — actual ahead of plan can mean front-loading of payments; actual behind plan can mean programme delay or under-valuation.
Common pitfalls
- Treating the planned curve as immovable. The plan is a baseline, not a prediction. Re-baselining at major milestones is healthy; refusing to re-baseline turns variance into noise.
- Not reconciling with the PCSR. The S-curve's forecast cost should match the PCSR's forecast. If they're maintained as separate workbooks, they drift. They're two views of the same data.
- Linear instead of S-shaped plans. Some consultancies plan cashflow as a straight line and accept the variance. The variance is predictable; plan to it.
Related
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