What is being held,
and when you get it back.
Retention is easy to calculate and easy to lose, and firms lose it on the dates rather than the arithmetic. Put in the job and it returns both halves with the date each falls due. Nothing is sent anywhere.
The second date is the one that costs money
The first release you will chase, because you finished last month and the job is fresh. The second falls due a year or more later, when the job is closed, the file is archived and the person who ran it has often left. That is where retention goes missing, and it goes missing to ordinary forgetfulness rather than to anybody refusing to pay.
Which is why this tool returns dates rather than only sums. Put the second date somewhere that will still be looked at in a year, and you have done the single thing that determines whether you collect it.
Release is often conditional, not automatic
One thing this calculator cannot know: on many contracts the second half is not due at the end of the defects period at all. It is due on the issue of a making good certificate, or its equivalent, which somebody has to actually produce.
If that is your contract, the date below is when to start asking for the certificate rather than when the money arrives. The distinction matters, because until the certificate is issued the payment is not late in any contractual sense and never becomes so.
What this does not model
It handles the ordinary case: a percentage of certified value, released in two parts, the second at the end of a defects period running from practical completion. It does not model capped retention, stepped or phased release, retention bonds substituting for cash, or contracts where your release is tied to the main contract's practical completion rather than your own. That last one is common on subcontracts and can push the money months beyond anything in your control.
All of those are worth knowing about before you sign rather than after, and they are covered in retention, and the money that goes missing after practical completion.