The maths, explained
An invoice value and its immediate cash receipt are not always the same. This model applies your entered retention percentage to the current invoice and adds the newly withheld amount to prior outstanding retention. It separates the money due now from money still withheld. Other deductions, payment timing, release conditions and taxes need separate treatment in the actual payment record.
New retention = invoice × retention rate; current cash receipt = invoice − new retention; total outstanding = prior + new retention
A worked example
The worked example below uses the calculator’s initial inputs and the same calculation function as the live interface. Replace example amounts with your own records; changing currency does not convert the numbers. Physical reference dimensions are stored in metric units, and the interface can display their imperial equivalents. Rounded display values may differ slightly from the full precision retained in the calculation.
| Your inputs | Editable example — not a market quote |
|---|---|
| Progress invoice amount (GBP) | 50,000 |
| Retention withheld (%) | 10 |
| Prior outstanding retention (GBP) | 15,000 |
| Cash due on current invoice | £45,000.00 |
| New retention withheld | £5,000.00 |
| Total retention outstanding | £20,000.00 |
Assumptions used
All cost inputs must cover the same period and use one currency. Employer charges and overhead remain business-specific inputs. Defaults are educational scenarios, not market averages. This model does not determine tax obligations or contractual rights. Ratios with a zero denominator are marked as undefined rather than presented as a misleading zero percent.
On your side of the Atlantic
Use metres, square metres and supplier quotations in pounds. UK VAT depends on the work and eligibility: 20% is a common standard-rate reference, not an assumption applied by this tool.
Common mistakes
Do not count outstanding retention as cash already in the bank. Apply the percentage to the contractual base and check whether previous retention has been released. This tool does not calculate statutory deadlines or interest.
Before ordering or tendering, compare at least two scenarios: your base takeoff and a realistic alternative. Keep assumptions with the result so differences are explainable. Save an estimate only on a suitable device. A shared link contains the entered values, so avoid including confidential information. An exported estimate is a snapshot and does not update when a supplier changes a price.
Questions from the jobsite
What does this calculator measure?
What is the first mistake to avoid?
Are the example prices supplier quotes?
Which tax assumptions apply in United Kingdom?
How should I handle waste and contingency?
Can I use the result in a tender or estimate?
Planning estimate only. Not a substitute for a professional quote or local building-code compliance. Verify quantities, tax treatment and contract requirements.
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