A set of forms is a resource with a cycle time
Think of one set of forms the way you would think of one truck on a haul route. It is committed from the moment the pour starts until it has been stripped, cleaned, moved and reset ready for the next one. That whole span is the cycle time, and the only question that matters is whether it fits inside the gap between pours.
If it fits, one set does the whole job and the forms are never the constraint. If it does not, you need as many sets as the cycle divided by the interval, rounded up — two sets if the cycle is anywhere between one and two intervals, three if it is between two and three. That rounding is the whole behaviour of the page and it is why the answer is so insensitive to small changes.
The step function is the useful part
Suppose the cycle is 36 hours and you want to pour every 24. The ratio is 1.5, so you need two sets. Now shave four hours off the hold period: the cycle is 32 hours, the ratio is 1.33, and you still need two sets. Shave four more: 28 hours, ratio 1.17, still two sets. All of that effort has bought you slack in the rotation and no reduction in the forms you have to own or hire. Only when the cycle drops to 24 hours does the answer become one set, and that is a 12 hour saving arriving all at once.
The same works in reverse and it is the more common mistake. A cycle of 23 hours against a 24 hour interval needs one set with an hour to spare. Lose two hours to a late truck and the cycle is 25 hours, which needs two. A schedule sitting a fraction under a boundary is not a tight schedule, it is a schedule with no forms in reserve.
| Cycle time | Interval 24 hr | Sets needed | Slack per set |
|---|---|---|---|
| 20 hr | 0.83 intervals | 1 | 4 hr |
| 24 hr | 1.00 | 1 | none |
| 25 hr | 1.04 | 2 | 23 hr |
| 36 hr | 1.50 | 2 | 12 hr |
| 48 hr | 2.00 | 2 | none |
| 50 hr | 2.08 | 3 | 22 hr |
Where the turnaround time actually goes
The hold period gets all the attention and the turnaround gets none, which is backwards on most jobs. Stripping, scraping, oiling, walking the panels to the next position, plumbing and bracing them and setting the ties is a real day of work, and it is the part of the cycle you have some control over. The hold period is a specification you have been handed.
It is also the part that is worth measuring rather than estimating, once, on the real job with the real crew. Everything on this page is proportional to it, and a turnaround that is two hours longer than assumed across twelve pours is a day of programme.
The hold period is not on this page and will not be
When forms may come off is not a number a calculator gets to supply. It depends on the element and what it is carrying, on the mix, on the temperature history since the pour, and on which criterion the specification uses — an elapsed time, a compressive strength result from cylinders, or a maturity value read from a sensor in the concrete. Those criteria are set by the engineer of record and by the adopted code, and the reading on the day is taken by a competent person on site.
This matters more than the usual disclaimer, because stripping early is one of the ways this work kills people. Formwork and shoring are engineered temporary structures. When a slab is stripped or a shore is disturbed before the element can carry itself, the collapse is sudden and progressive, and people have died under forms and shoring that came down that way. Reshoring and back-propping — how many levels stay supported, and in what order they are released — is part of that same design and appears nowhere on this page.
What this page does is arithmetic on the hold period you were given: cycle, sets, slack and schedule. Change the criterion and the arithmetic runs again. It never proposes the number.
Questions people ask
How many sets of forms does a job need?
The cycle time divided by the interval between pours, rounded up. Cycle time is the pour itself plus the hold period before stripping plus the turnaround to strip, clean, move and reset. A 36 hour cycle against a pour every 24 hours needs two sets; a 20 hour cycle against the same interval needs one. Because it is a ratio rounded up, the answer moves in steps rather than smoothly.
Why did shortening the hold period not reduce the number of sets?
Because the set count is a rounded ratio and small changes usually stay inside the same step. Going from a 36 hour cycle to a 28 hour cycle against a 24 hour interval takes the ratio from 1.5 to 1.17 and leaves the answer at two sets. Only crossing a whole multiple of the interval changes the count, so the saving arrives all at once at 24 hours rather than gradually.
When can formwork be stripped?
That is not a figure this page supplies, and it is a field rather than a constant for that reason. It depends on the element and what it carries, the mix, the temperature history since the pour, and whether the specification sets an elapsed time, a cylinder strength result or a maturity reading. The criterion belongs to the engineer of record and the adopted code, and the reading on the day is taken by a competent person on site.
What should I put in the turnaround time?
A measured figure from the real job if you have one. Stripping, scraping, oiling, moving panels to the next position, plumbing, bracing and setting ties is a genuine day of work on a wall of any size, and it is the part of the cycle you can influence, unlike the hold period. Everything on this page is proportional to it, so an hour of measurement beats an afternoon of adjusting the estimate.
Does the calculator account for reshoring?
No, and that absence is deliberate. How many levels stay supported after the forms come off, in what order supports are released, and what loads the shores carry meanwhile are part of an engineered temporary works design. Formwork and shoring collapses happen without warning and have killed people, so that design belongs to the engineer of record and its execution to a competent person, not to a scheduling calculation.