Methodology
The arithmetic behind the release dates
The shortest model on this site, and the one most worth checking: a date it gets wrong is a payment chased on the wrong day.
This model asserts nothing about how retention works on your job. It has no default percentage, no assumed limit and no assumed defects period, because all three are clauses rather than conventions — two subcontracts on the same tower disagree about them. Everything below operates on figures you supply.
Outputs
Every figure the calculator prints
In the order the model produces them. Nothing else is involved at any point.
| Figure | How it is calculated |
|---|---|
| Retention accrued | certified to date × retention % ÷ 100 |
| Limit of retention | contract value × limit % ÷ 100 |
| Held right now | the lower of the accrual and the limit, or the accrual alone when no limit is entered |
| Certified value at which the limit is reached | limit ÷ (retention % ÷ 100) |
| First release | held × released-at-taking-over % ÷ 100 |
| Balance | held − first release |
| Still to collect | the balance, plus the first release when it is marked unpaid |
| Date the balance falls due | taking-over date plus the defects period in calendar months |
| Days overdue | whole days from the first release date to today, shown only when that date has passed and the release is marked unpaid |
The whole model, in order
- accrued = certified × rate
- held = min(accrued, contract value × limit)
- first = held × split
- balance = held − first
- balance due = taking-over + defects months
Every money figure is rounded to the whole unit as it is produced, not as it is printed, so the sums on screen add up exactly as shown. The balance is calculated by subtracting the rounded first release from the rounded total held rather than by rounding a second percentage, which is what stops the two halves from differing by one unit from the whole.
What is deliberately not here
No default is offered for any of the four terms
The retention percentage, the limit of retention, the split at taking-over and the length of the defects period are all fields. It would be easy to pre-fill them with the figures that appear most often, and it would be wrong: the reader would then be checking our version of a contract against their memory of theirs, rather than reading their own contract.
There is no country selector for the same reason. What changes the arithmetic is the contract, not the jurisdiction — and a control that changes no output while implying it knows local practice is worse than no control at all. The currency selector is a label on the reader's own figures and nothing converts between them.
The starting values in the fields are a working shape so the page is not a wall of empty boxes. The taking-over date is the exception and is deliberately left blank: it is the one input with no plausible stand-in, and it is the one the dates turn on.
Dates
Three decisions the calendar arithmetic makes
Small choices, each of which moves a payment date by a day or more if it goes the other way.
- Everything is computed in UTC from the parts of the date, never through a local-time constructor. A release date that changes when the reader travels, or twice a year when clocks move, is a release date chased on the wrong day.
- Adding the defects period clamps to the end of the month. Taking over on 31 August with a six-month period falls due on 28 or 29 February, not on 2 or 3 March — rolling forward would put the final certificate in the wrong month.
- Today is read on the reader's device after the page loads, not while it renders. Every page on this site is rendered once at build time, so a date read during rendering would be frozen at the deploy and would report the day of the last release as today for as long as it stood.
Where it stops
What this does not do
It reads your contract back to you. It does not read your contract for you.
- It assumes retention is withheld in cash. Where it has been replaced by a retention bond or a bank guarantee, the amounts here are not money being held and the dates mean something different.
- It prices no cost of money. The figure is what is owed, not what it has cost you to be without it — that would need a financing rate, and there is no defensible one to supply on your behalf.
- It treats the certified value as final. Where an amount is under dispute, or where a variation has been carried out and not yet certified, the accrual moves with it.
- It reads the release dates as the dates the money falls due, not the dates it arrives. The final certificate usually has to be applied for, and a defect still open at the end of the period can hold the balance past the date printed here.
- It knows nothing about your other projects. A subcontractor's real exposure is the sum of this figure across every employer at once, and this page answers for one contract at a time.
Retention Calculator
The model is the easy half
Knowing what is owed and when is arithmetic. Getting a certificate issued for it is a different job, and it is the one we can help with.
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