Why hours and duration are different quantities
Every renovation estimate starts as a list of tasks with hours against them. Adding those hours up gives you effort, which is a real and useful number — it is what you pay for, and it is what the DIY versus contractor calculator prices. It is not duration. Duration is how long the calendar takes to get through the effort, and three things stand between the two.
The crew rate. Hours divide by the number of people who can work productively at once, times the hours each genuinely puts in. Six productive hours from an eight-hour day is a realistic figure for site work once travel, setup, breaks and cleanup are removed, and DIY days are often shorter still.
The shape of the week. Ten working days is two weeks for a trade working Monday to Friday, and five weeks for a homeowner working Saturdays and Sundays. This is the factor most DIY schedules get wrong, and it is a multiplier of three and a half.
The waiting. Renovations contain long periods when nobody can do anything: paint between coats, adhesive and grout curing, screed drying, flooring acclimating, a special-order worktop in production, an inspector booked for Thursday. These are calendar days that pass at the same rate whatever your crew size, and no amount of labour compresses them.
The result is a schedule where doubling the crew does not halve the duration, and where the honest answer to "when will it be done" depends more on sequencing than on speed.
The three conversions, in order
Hours to working days. Divide by people times productive hours per person. Two people at six hours produce twelve hours of work per day, so 120 hours takes ten working days. The model assumes the work divides evenly, which is true for painting and tiling and false for anything where one person waits on another. On a single room, two or three people is usually the practical ceiling.
Working days to calendar days. Multiply by 7 ÷ (working days per week). Five days a week gives a factor of 1.4; two days a week gives 3.5. This is a straightforward stretch and it is where weekend projects get their reputation for lasting all summer: eight days of work becomes four calendar weeks.
Waiting days are added, not stretched. A grout cure or a paint recoat runs through Saturday and Sunday exactly as it runs through Tuesday, so waiting is already in calendar units and dividing it by the working week would be wrong. The only reduction that applies is overlap — the share of the waiting during which you can usefully work somewhere else. On a whole-house project that can be substantial; on a single bathroom it is close to zero, because the one room you need is the one that is drying.
The buffer multiplies the whole thing. Ten to twenty-five per cent is the working range for renovation. It is not padding: it is the recognition that some deliveries arrive late, some inspections fail, some hidden condition appears, and a programme with no float converts every one of those into a late finish. The same logic and the same range apply to the money side in the contingency calculator.
Worked example: a 120-hour bathroom with two people
Your task list totals 120 labour hours. Two of you will work, at a realistic 6 productive hours each per day, 5 days a week. You have identified 4 waiting days — grout and silicone cure, a paint recoat, and a two-day wait for the glass screen — and you reckon you can work through about 25% of that time on other parts of the house. You allow a 15% buffer.
- Crew output. 2 × 6 = 12 hours per day.
- Working days. 120 ÷ 12 = 10 working days.
- Spread across the week. 10 × 7 ÷ 5 = 14 calendar days of working time.
- Waiting that stops you. 4 × (1 − 0.25) = 3 calendar days.
- Before buffer. 14 + 3 = 17 days.
- Buffer. 17 × 0.15 = 2.55 days.
- Calendar days to finish. 17 + 2.55 = 19.55 days, or 2.79 weeks.
- Waiting share. 3 ÷ 17 = 17.6% of the pre-buffer schedule.
Now test the intuition that a bigger crew fixes it. With three people the labour falls to 120 ÷ 18 = 6.67 working days, which is 9.33 calendar days; add the same 3 waiting days and the same 15% buffer and you finish in 14.18 days. Half again as much labour bought you 5.4 days, not 6.5, because the waiting did not move. Push to four people and you get (120 ÷ 24) × 1.4 = 7 calendar days of work, plus 3 waiting and 15% buffer, which is 11.50 days — another 2.68 days for another whole person, against the 5.37 days the third person bought. The returns fall because the fixed part of the schedule is fixed.
Reading the waiting share
Below about 20%, the programme is labour-limited. More hands or longer days genuinely shorten it, and the schedule responds roughly in proportion to crew size. Painting, drywall finishing and demolition-heavy work sit here.
Between 20% and 40%, sequencing starts to matter more than speed. Look for waits that can be started earlier: order long-lead items before demolition rather than after, get the electrical rough-in inspected before the plasterer is booked, let flooring acclimate in the room during the weeks you are working elsewhere. Every wait you move off the critical path is a day off the programme at no cost.
Above 40%, the project is wait-limited and crew size barely matters. Tiling a wet room is the classic example: a day of tiling, a day for the adhesive, a day of grouting, a day of cure, then silicone and another cure. Four people cannot compress it. What works is choosing faster-setting products where they are appropriate, and above all starting the long-lead procurement early.
Treat the buffer as float, not slack you can spend. The buffer exists to absorb variance from the whole programme, and it should sit at the end where anything can draw on it. Distributing it into individual tasks is worse than useless — each task expands to fill its own allowance, and the buffer is gone before the real problem arrives.
Convert the days into money before you accept them. Every calendar day on this schedule is a day of disruption, and on a kitchen or a sole bathroom the cost of that is real. Run the answer through the downtime cost calculator; the result often justifies paying for a faster schedule, or doing the work in stages so that the space is out of use for less of it.
Typical waiting days by task
| Task | Typical wait | Why |
|---|---|---|
| Between coats of paint | 0.2–0.5 day | Recoat interval; multiply by 2 or more in a cold, damp room |
| Tile adhesive before grouting | 1 day | Manufacturer's minimum before the tiles can be loaded |
| Grout before water exposure | 1–3 days | Cementitious grout cure |
| Silicone before use | 1 day | Skin and cure of the sealant |
| Self-levelling compound | 1–7 days | Walk-on within hours, but floor coverings need it dry through |
| Wood flooring acclimation | 0–14 days | Depends entirely on the moisture gap, not on a fixed period |
| Inspection visit | 1–5 days | Booking lead time, and again if it fails |
| Made-to-order worktop | 7–21 days | Template, fabricate, deliver — and it cannot be templated until units are fitted |
Paint intervals here assume reference conditions; compute yours with the paint recoat calculator, and acclimation with the flooring calculator, rather than taking a fixed number of days.
How schedules actually slip
- Long-lead items ordered late. A worktop templated after the units are fitted, then three weeks in fabrication, will hold up the whole kitchen no matter how fast everything else went.
- Estimating tool time instead of task time. Shopping, setup, protection, cleanup and the second trip for the fitting you forgot are all real hours and rarely appear on a task list.
- Assuming full days. Eight hours on site is six hours of work. A schedule built on eight is a third short before anything goes wrong.
- Forgetting the inspection queue. Work that has to be inspected before it can be covered stops until the inspector arrives, and stops again if it fails.
- Sequencing waits in series that could run in parallel. Flooring can acclimate while you are painting; a special order can be in production while you demolish.
- Weekend-only work with a weekday-work mental model. Two days a week stretches every working day by a factor of 3.5, and eight days of work becomes four weeks.
- Spending the buffer early. Float used on the first small overrun is not available for the real one in week five.
What this model simplifies
This is a deterministic single-chain schedule: one pool of hours, one crew, one set of waits. Real programmes are networks, and three simplifications are worth knowing about.
There is no critical path here. A proper schedule identifies which chain of tasks determines the end date and which have float, so that effort is added where it changes the finish date rather than where it is easy to add. If your project has several independent areas, sketch the sequence on paper and run this calculator separately on the longest chain, then treat the rest as work that fits inside it.
Trade dependencies are not modelled. Electrical rough-in before insulation, insulation before board, board before tape, tape before paint. Each handover carries a booking window, and a trade that misses their slot may not come back for a fortnight. In practice this is the largest single source of slippage on contractor-run jobs and no hours-based model captures it.
Productivity is treated as constant. It is not: the first day is slow while you find where everything is, the last day of a repetitive task is fast, and long stretches of overtime reduce hourly output rather than increasing daily output. Enter an honest average rather than your best day.
Used with those limits in mind, the value of this calculation is mostly in the waiting share. That single number tells you whether your project responds to effort or to planning, and it changes what you should do next. Where it is high, the answer is to bring procurement forward and sequence around the cures — and to make sure the material that needs the longest lead, such as flooring that must reach the room's equilibrium moisture content, is on site early rather than exactly on time. Where it is low, the answer is more hands, longer days, or a smaller scope, and the trade-off between those is a budget question for the renovation budget calculator.