A trade says the room will take eighteen working days. Two weeks later you are still sleeping in the spare room and nobody has done anything wrong — because eighteen working days is not eighteen days. It is three and a half calendar weeks before anyone has spilled anything, and closer to four once a delivery slips. This takes the working days each stage actually needs, stretches them by the days a week that will really be worked and by a buffer, then adds the drying and curing time that no amount of working faster can compress, and draws the whole thing as a bar chart you can plan around.
The job, stage by stage
Working days are what a trade quotes. Calendar days are what you live through.
Ripping out, disposal, making good the substrate.
Electrical and plumbing in the rough, before closing up.
The working days on the trowel, not the drying that follows.
Calendar days. Nobody works and nothing can be rushed.
Including grouting and the day before it can be walked on.
Fittings, skirting, filling, priming and top coats.
Last in, because everything else has to be finished first.
Fine tuning
Ten for one well-understood trade, twenty for several, thirty if you live on site.
Result
Eighteen working days is not eighteen days
The gap between what a trade quotes and what you experience is made of three things, and only one of them is anybody's fault. The first is weekends: at five working days a week, two days out of every seven have nobody on site, which is 29% of the calendar doing nothing. The second is the slippage every project absorbs — a delivery that lands late, a pipe where the drawing said there was not one, a trade finishing on Thursday and starting the next job on Friday. The third is the part people forget entirely: plaster, screed, adhesive, grout and paint all need time to cure, and curing is measured in calendar days whether or not anyone is standing there watching it.
Put those together and the arithmetic stops being mysterious. Twenty-four working days of genuine work, buffered by twenty percent, becomes about twenty-nine days of work. Spread over five days a week that occupies six calendar weeks — forty-two days — before a single day of drying is added. Seven days of drying on top and the room is out of action for seven weeks. Nothing went wrong. That is just what the calendar does.
Drying time is the stage nobody schedules
Every other stage on a renovation can, in principle, be compressed. Two plasterers instead of one halves the working days. Working Saturdays pulls the calendar in. Drying cannot be compressed by anything except heat and airflow, and both of those have limits before they cause cracking. A skim coat needs about a week in normal conditions before it is ready for a mist coat; a sand-and-cement screed needs a day per millimetre of thickness for the first few centimetres and far longer after that; tile adhesive is walkable in a day and fully cured in a fortnight.
This is why drying is entered here as calendar days and shown differently on the chart. It gets no buffer, because a buffer on drying time is meaningless — you cannot add twenty percent to a chemical reaction — and it gets no weekend adjustment, because it is already calendar time. It simply occupies the schedule between the stage that creates it and the stage that cannot start until it is finished.
Scheduling the painter for the day after the plasterer finishes is the classic. The plaster looks dry because the surface has gone pale, but the moisture is still in the depth of it, and paint over damp plaster either flakes off or traps the water in and causes a bloom that shows through every subsequent coat.
The same applies to flooring over a screed, tiling over a tanking membrane that has not cured, and silicone over grout that has not set. In every case the fix is to wait, and waiting costs the same whether you planned for it or not — except that unplanned waiting also costs the trade's remobilisation.
Sequence matters more than speed
A renovation is a chain, and the schedule is set by the chain rather than by the sum of its links. Flooring goes last not because it is the least important but because everything else would damage it. Tiling precedes second fix because the skirting sits on top of the tile. First fix precedes plastering because the cables and pipes have to be inside the wall before it is closed. Reversing any of those does not save time; it creates rework, which costs more time than the original sequence did.
Everything old comes out and the substrate is made sound. This is the stage where surprises live: what is behind the plaster is unknown until the plaster is off.
Cables, pipes, back boxes, insulation and any structural work go in. Nothing is closed up yet, and any inspection that is needed happens here.
The working days are short and the calendar days are long. The board or skim goes on in a few days and then needs a week of the calendar doing nothing at all.
Tiling, tanking, grouting and silicone. Each has its own cure, and the room is generally unusable for part of it.
Fittings, skirting, architrave, filling, priming and top coats. The visible finish, and the stage where snagging begins.
Last, and deliberately so. Everything above it has to be finished, dry and clean before a single board is laid.
How much buffer is honest
A buffer is not pessimism and it is not an admission that the estimate is wrong. It is the recognised difference between a plan built from ideal conditions and a plan that survives contact with a building that was wired in 1974. The question is only how much, and that depends on how many independent things can go wrong at once.
Note what the buffer does not do: it does not change the drying time, and it does not change the number of weekends. It is applied to working days before the conversion, so it buys real working capacity rather than being diluted across days nobody was on site anyway. That ordering is deliberate and it is the only way a buffer means anything.
Working six days a week is not a twenty percent saving
Going from five working days to six removes weekends from the calendar but adds working capacity, so the effect is larger than it looks — and the cost is larger too, because Saturday is usually premium time and because a six-day programme burns out the people on it by the third week.
| Days per week | Weeks of work | Calendar days of work | With 7 drying days |
|---|---|---|---|
| 5 days | 6 weeks | 42 days | 49 days |
| 6 days | 5 weeks | 35 days | 42 days |
| 7 days | 5 weeks | 35 days | 42 days |
The rounding is not laziness. Trades are booked by the week in practice: a plasterer who finishes on Wednesday does not send a team back for a day and a half elsewhere. Whole-week figures are what you can actually plan a delivery, a skip and a hotel around.
A worked example, the way it happens
- Collect the working daysStrip out 3, first fix 4, plastering 5, tiling 4, second fix 6, flooring 2. That is 24 working days of real work, and it is the number every trade would quote you.
- Add the bufferAt 20% that is 4.8 days, giving 28.8 days of work to be fitted into working days.
- Convert to calendarOver five days a week, 28.8 working days is 5.76 weeks, rounded up to six weeks, which is 42 calendar days of working time.
- Add the dryingSeven days of plaster and adhesive curing takes it to 49 calendar days — seven weeks, against the four-and-a-bit weeks the working-day figure implies.
- Plan to the calendar figureBook the skip, the delivery and the time off work against 49 days. If it comes in at 42, everyone is delighted. If you planned for 24 working days and it takes 49, nobody is.
What to measure before you can schedule it
Stage durations are proxies until you know the quantities involved. A room that needs thirty square metres of plastering is not the same job as one that needs nine, and the difference is several days rather than a few hours. The surface areas come first, then the durations follow.
Start with the walls: the wall area calculator gives the figure the plasterer and the decorator will both price from, and the ceiling area calculator covers the one surface people consistently underestimate. For the boarding itself, the drywall sheet calculator turns those areas into a sheet count, which is a far better basis for a duration than a guess. Where insulation is going in behind it, the insulation board calculator does the same job for the boards.
Finishes follow the same logic. The tile calculator and the paint calculator give quantities that map onto days for the tiler and the decorator, and the flooring calculator does it for the last stage. If you want the hours rather than the calendar, the labour hours calculator converts quantities into the working time underneath these stage figures.
Do
- Add the buffer before converting to calendar days, so it buys working capacity.
- Treat every cure — plaster, screed, adhesive, grout, paint — as calendar time.
- Book trades in the sequence that prevents rework, not the order they are available.
- Round up to whole weeks, because that is how trades are actually booked.
- Plan deliveries against the calendar figure and be pleased if it comes in early.
Avoid
- Reading a working-day quote as calendar days and booking time off to match.
- Scheduling the painter against the plasterer's working days rather than its cure.
- Laying flooring before the wet trades and decoration are finished.
- Applying the buffer after the weekend conversion, which quietly halves it.
- Assuming a late trade can be compressed by working longer hours in the last week.
Where the numbers stop being true
This models a single chain of stages running one after another. That is the honest shape of most domestic jobs, where one or two people work through a sequence, and it is not the shape of a larger project where a joiner is in one room while a tiler is in another. Parallel working changes the answer completely and in the favourable direction — but only if the rooms genuinely do not share a trade, a delivery or a dust-producing operation.
It also does not model the critical path in any formal sense. There is no dependency solver here: the order of the stages is fixed by the order they appear, and if your job has an unusual sequence — tanking before a different first fix, say — reorder the reading rather than trusting the chart. Holidays, weather on external work and the possibility that a trade simply does not turn up are all outside the model, and all of them happen.
Finally, the durations are yours to supply. The defaults are a plausible whole-room job, not a survey of what your contractor will say. Replace them with the numbers you have actually been quoted and the conversion — which is the part that is easy to get wrong — still holds exactly.
How the figures are produced
The buffer is applied to working days first, because a buffer applied after the weekend conversion would be spread across days with nobody on site and would therefore buy nothing. The buffered working days are then divided by the days worked per week and rounded up to a whole week, because trades are booked by the week. That week count is multiplied by seven to give calendar days of working time, and the drying days are added afterwards as calendar time with no adjustment at all.
Questions people ask
Why not just add the working days up?
Because the sum of working days is not a duration anyone experiences. Twenty-four working days over five days a week is nearly five calendar weeks before any buffer, and seven weeks with one. Every delivery, booking and arrangement you make is in calendar time, so the useful number is the calendar number.
Should I put a buffer on drying time?
No, and this calculator deliberately does not. Drying is a physical process with its own timescale; adding twenty percent to it would just be padding. If you want contingency around drying, add it as extra calendar days at the end of the job, where it means something.
What if two trades work at the same time?
Then this overstates the duration, and you should use the longer of the two chains rather than the sum. Parallel working is genuinely faster where rooms are independent, and slower where they are not, because two trades in one room spend a surprising amount of time waiting for each other.
Is a twenty percent buffer really necessary?
On a multi-trade job, yes. The buffer is not covering bad work; it is covering the discovery behind the plaster, the delivery that lands on the wrong day and the trade that is two days late from the previous job. Those are normal, not exceptional.
My project is a single room with one trade. What changes?
Use a ten percent buffer and check whether any stage needs a cure at all. A straight decoration job with no wet trades has no drying time, and the whole calculation collapses to the weekend conversion, which is still worth doing.
Does this account for drying time being longer in winter?
Only if you enter a longer figure. Cold, damp air slows every cure, and a skim that takes a week in June can take two in January with the heating off. Enter the drying days that match the season you are actually working in.
Should the flooring really be last?
Almost always. It is the one surface that everything else damages: dropped tools, wet adhesive, paint, and foot traffic during snagging. The exceptions are jobs where the floor is being sanded and sealed in place, which belongs with the decoration rather than after it.