Free Construction Cash Flow Calculator

Plan project funding in minutes with the free ProQS construction cash flow calculator. Enter a contract sum and a programme length, and it builds a full monthly forecast. It models S-curve spend, retention and payment lag. It also shows the contractor’s own cash position. Everything updates as you type.

Why construction cash flow forecasting matters

Cash, not profit, is what keeps a construction business alive. A contractor can carry a healthy margin on paper and still fail because money arrives too late. Clients face the opposite problem. A funder or board needs to know how much to draw down and when.

The quantity surveyor sits between those two positions. On the client side, a reliable forecast supports funding drawdown schedules and board reporting. On the contractor side, it shows how much working capital a project will absorb. Both rely on the same underlying question. How will value be earned month by month, and when will it actually be paid?

Construction spend rarely follows a straight line. Enabling works and substructure start slowly. Frame, envelope and services packages run in parallel in the middle of the programme. Finishes and commissioning tail off towards completion. Plotted cumulatively, that pattern forms the familiar S-curve.

Cash flow is one strand of wider financial management in construction. Once a project is on site, the forecast becomes a baseline for post-contract cost management and for the monthly cost reports described in our guide to performance reporting.

What the construction cash flow calculator does

The tool is split into four groups of inputs. Each reflects a decision a cost manager would make when preparing a forecast by hand.

Contract inputs

You set the contract sum, the first valuation month and the construction period. Preliminaries are entered as a percentage and spread evenly across the programme. This mirrors how time-related preliminaries are usually valued. An optional annual inflation rate can also be applied. It compounds from the first valuation month, so later work costs more than early work.

S-curve shape

Four presets cover the most common profiles. These are front-loaded, standard, back-loaded and linear. Two sliders then let you refine the curve. The first moves the peak spend point earlier or later in the programme. The second controls how concentrated the spend is around that peak.

A heavy civil engineering scheme with early earthworks might suit a front-loaded profile. A fit-out-intensive project such as a hospital or laboratory is often back-loaded, because the high-value mechanical and electrical installation lands late.

Payment and retention

You set the payment lag between valuation and payment. You also set the retention percentage, an optional retention limit, the share released at completion and the length of the defects period. Half at completion and half at the end of the defects period is the usual JCT pattern. It also matches the default approach in NEC4 secondary Option X16. Our guide to retention in construction contracts covers the contract mechanics in more depth.

Contractor view

Finally, you enter the contractor’s margin and how long it takes to pay its supply chain. The calculator then shows the cumulative gap between money received and money paid out. Wherever that line drops below zero, the contractor is funding the project from its own resources.

A worked example

The calculator opens with a sample project already loaded. Consider a £12.5 million, four-storey office building in the West Midlands. The construction period is 18 months, with the first valuation in January 2027. Preliminaries are 12% of the contract sum. Retention is 3%, with half released at practical completion and half after a 12-month defects period. The contractor’s margin is 6%.

With a standard S-curve and a one-month payment lag, the calculator returns the following.

• The average month is worth around £694,000. The largest payment is £1.18 million. It is received in both October and November 2027, because the two busiest valuations are equal. That is 1.7 times the average month, and the difference matters when agreeing a funding facility.
• Half of the contract sum has been paid by November 2027, ten months after the first valuation.
• Retention builds to £375,000 by completion in June 2028. £187,500 is released at practical completion. The remaining £187,500 is certified at the end of the defects period and paid in July 2029.

Now look at the contractor’s position. The example assumes the contractor is paid one month after each valuation but pays its supply chain after two months. Under those terms, the contractor stays cash positive throughout. It finishes with £750,000 in hand, which is its 6% margin.

Change the terms and the picture moves quickly. The table below shows the contractor’s peak funding need under different combinations. Every figure comes straight from the calculator, with all other inputs left at the example values.

The pattern is clear. What drives the funding need is the gap between being paid and paying out, not either period on its own. Here, each extra month of gap adds roughly £1 million, a little less than one peak month of value. On a project of this size, payment terms alone can decide whether a tender is financially viable.

Reading the outputs

The summary bar at the top gives the headline figures. These are the final account forecast, the peak monthly payment, the month by which half the sum is paid, the maximum retention held and the contractor’s peak funding requirement. Where two months share the same peak payment, the summary shows one of them. The monthly schedule shows both.

The first chart shows monthly client payments as bars, with retention releases picked out in a different colour. The cumulative curve is plotted against the right-hand axis. The second chart shows the contractor’s cumulative cash position. Green shading indicates a surplus. Red shading indicates a funding requirement.

Below the charts sits a full monthly schedule. It lists the gross valuation, cumulative value, percentage complete, retention held, net certified sum, client payment and contractor cash position for every month. A single button copies the whole table so you can paste it straight into Excel. Our guide to Excel for quantity surveyors shows how to build on it.

How the calculator models spend

The S-curve uses a beta distribution. This is a well-established statistical shape that always runs from zero to one hundred per cent, so it never produces a negative month or overshoots the contract sum. Many older forecasting formulas can break down at extreme settings. This approach stays stable whatever shape you choose. The curve applies to the works value only. Preliminaries are added on top as a flat monthly sum.

Retention is deducted from each gross valuation until any limit is reached. Each net certificate is then paid after the payment lag you have set. In England, Wales and Scotland, the Housing Grants, Construction and Regeneration Act 1996 requires construction contracts to set a due date and a final date for payment. Northern Ireland has equivalent rules in the Construction Contracts (Northern Ireland) Order 1997. The payment lag input is your way of reflecting those dates in the forecast. Our overview of UK construction legislation explains the wider framework.

A one-month lag suits most standard forms. Under NEC4 with Option Y(UK)2, payment becomes due seven days after the assessment date. The final date for payment is then 21 days later unless the Contract Data says otherwise. JCT contracts reach a similar point a few weeks after each valuation date. Always check the dates in your own contract before relying on the default.

Assumptions to bear in mind

The calculator is a forecasting tool, not a programme. It is ideal for feasibility studies, early budgeting, tender reviews and teaching. Once a resourced programme exists, the forecast should be linked to the actual sequence of trade packages. Its main simplifications are set out below.

• Preliminaries are spread evenly. In reality, set-up costs are often front-loaded and some site staff leave before completion.
• Supply chain costs follow value. The contractor’s outgoings are the gross valuation less its margin, paid after the supply chain lag. In practice, some of the margin covers head office overheads, so the true position may be slightly tighter.
• No retention is held from the supply chain. Many main contractors deduct retention from their subcontractors, which improves their own cash position. The calculator leaves this out, so its contractor view is cautious.
• Payment periods are whole months. The tool cannot model a 17-day or 45-day period exactly, so round to the nearest month.
• Some items are excluded. The model leaves out VAT, interest, loss and expense, and any NEC4 retention free amount. Add variations and compensation events to the contract sum as they are agreed, so that the forecast stays current.

For forecasting on live contracts, compare the curve with actual valuations each month. Our guides to interim valuations and cost value reconciliation show how contractors track the real figures. Earned value management takes the same S-curve logic further by comparing planned value with earned value and actual cost.

A change coming for retention

Retention may not stay in its current form for long. The Commercial Payments Bill, introduced in May 2026, would amend the Construction Act and ban retention clauses in construction contracts after a transition period. The commencement date will be set by regulations, so a full ban is unlikely before about 2030. Until then, the retention inputs reflect current practice. If the ban takes effect, set retention to zero to see how contractor cash flow would change.

Using the tool for your APC

Cash flow forecasting is a core skill for the RICS Assessment of Professional Competence. It is central to the Project financial control and reporting competency. It also links closely to Commercial management of construction. Our guide to commercial management in construction covers the contractor side in more detail.

The calculator is a useful way to test your understanding before the final assessment interview. Try these exercises.

• Change the curve from standard to back-loaded and explain why the peak payment rises and moves later.
• Set a retention limit of 2.5% and work out when deductions stop.
• Find the payment terms at which the contractor first needs external funding.
• Set retention to zero and explain how the Commercial Payments Bill could change contractor cash flow.
• Compare the tool’s output with a cash flow from one of your own projects, then explain any differences.

Being able to discuss these points confidently shows the assessors that you understand the reasoning behind the numbers, not just the method.

Frequently Asked Questions

Try the construction cash flow calculator

The tool is free and runs in your browser. Change the curve, the payment terms or the retention, and see the effect on both client and contractor at once.

Back to the calculator

This is one of a series of interactive tools on ProQS. If there is a calculation you repeat on every project, let us know in the comments. It may well become the next one.