A monthly billing projection with the general contractor's twist — billed vs. received, retention withheld and released, and payment lag. Project a curve shape or enter your own billing. Runs entirely offline; everything persists in this browser via localStorage.
Use limits: This models revenue timing, not cost. True cash flow nets your billing against payroll, supplier, and subcontractor payment terms — a fuller model than this page attempts. Use it to see the billed-vs-received gap and what retention does to the tail, not as a treasury forecast. The contract's payment and retention provisions govern the actual dates and percentages.
Cash flow — billed vs. received net of retention
Total billed (contract value)
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Peak financing gap (billed − received)
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Peak retention held
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Cash complete (fully received)
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How the Numbers Are Derived
The curve shapes
Projected billing spreads the contract value over the duration by a cumulative fraction f(t), t = m / T. The smoothstep S-curve is f(t) = 3t² − 2t³ — slow start, steep middle, slow finish (f(0)=0, f(0.5)=0.5, f(1)=1), so the monthly amounts are bell-shaped. Front-loaded uses f = 2t − t², back-loaded f = t², linear f = t. Each month's billing is (f(m/T) − f((m−1)/T)) × value, and the monthly amounts sum exactly to the contract value.
Retention, lag, and the received line
Each progress payment pays (1 − retention) × billing, shifted later by the payment lag. The withheld retention accumulates and is released as a lump sum N months after the final billing month — the late jump you see in the received line. The peak financing gap is the largest value of cumulative billed − cumulative received: the money you've earned but not yet collected, which is the exposure this simple model shows. Peak retention held is retention × total billed, reached just before release.
Field notes
This is revenue timing only. Your real cash position also depends on when you pay labor (weekly), suppliers (net 30), and subs (often pay-when-paid). Retention you hold on your subs partially offsets the retention the owner holds on you — a real model nets the two.
Front-loading the schedule of values is common, visible, and a negotiation with the owner's rep. Mobilization line items exist for a reason; padding early activities that aren't really done is a different thing and gets caught.
Stored-materials billing changes both the curve and the paperwork — you can bill for material on site ahead of installation, but expect to document it and insure it.
The retention tail is real money sitting for months after you finish. On thin margins, the release date is a line on the cash forecast, not an afterthought.
References
The contract's payment and retention provisions (progress payment schedule, retention percentage, and release terms). Governing.
AGC / CFMA guidance on construction cash flow and the schedule of values.