Forecasts a construction cash-flow S-curve: value earned each period and the cumulative curve from £0 to the full contract value.
Pick a loading shape (front / symmetric / back) and the peak-earning period and any retention deduction are flagged automatically.
Tender and programme tool. Confirm against the agreed valuation schedule.
For programming guidance only. Always confirm against the agreed valuation schedule.
About this cash-flow S-curve calculator
This free cash-flow S-curve calculator turns a contract value and a programme duration into a month-by-month forecast of earned value and the cumulative cash-flow curve. Enter the contract sum, the number of months and a loading shape, and it returns the value earned each period, the running cumulative total, the cumulative percentage complete and the peak-earning month, the cash-flow “hump”. It is aimed at UK contractors, quantity surveyors and commercial managers building a tender-stage cash-flow forecast or sense-checking an interim valuation profile. Add a retention percentage and it shows the cash held back and the net of retention alongside the curve. All figures exclude VAT.
How the S-curve is calculated
Construction value rarely accrues in a straight line. It starts slowly, accelerates through the main works and tapers off at handover, tracing a sigmoid or S-curve. This tool models the cumulative earned fraction with the Hermite “smoothstep” curve C(x) = 3x² − 2x³ over normalised programme time x = t ÷ T, which satisfies the boundary conditions C(0)=0 and C(1)=1 with zero slope at each end. A documented exponent skew on x (front 0.7, symmetric 1, back 1.4) shifts value earlier or later without changing the start and end points. The value earned in a period is the increment of the cumulative curve, Value × (C(xₜ) − C(xₜ₋₁)); retention is a flat deduction that does not reshape the curve. Method per Kenley & Wilson, Construction Project Cash Flow Modelling (2003), and the Hudson/DHSS S-curve formula.
Frequently asked questions
What is a cash-flow S-curve?
It is a graph of cumulative value earned against time on a project. Because work ramps up, peaks and tapers, the cumulative line forms an elongated “S” running from 0% at the start to 100% at completion. It is used to forecast valuations, plan working capital and compare actual progress against the planned profile.
What is the difference between front, symmetric and back loading?
Loading describes when value is earned. Front-loaded curves earn more early, typical of heavy enabling or groundworks, and peak sooner. Back-loaded curves earn more late, as on fit-out-heavy jobs, and peak nearer handover. Symmetric loading is the balanced default with the peak at mid-programme.
How does retention affect the curve?
Retention is a percentage held back from each valuation as security. It does not change the shape of the earned-value curve. The calculator applies it as a flat deduction on the contract value and shows the cash held and the net of retention so you can see both the gross profile and the money actually received.