A shopping list for a wiring loop: conduit, wire by the reel, and the boxes that join them.
Electrical and plumbing. 5 questions answered below, each with the arithmetic behind it and the limits of what the tool decides. Open the Conduit and Wiring Estimator tool.
Conduit is the one electrical material that is bought almost entirely by length, and its length is decided by the route rather than by the number of rooms. The estimator takes the runs you describe, adds the boxes and couplings those runs need, and returns an order in the units a supplier sells. Cable inside the conduit is measured on the same run, because it is pulled through the same path. That single decision, measuring conduit and cable from one route, is what keeps the two orders consistent. Where a run carries three circuits the conduit is larger, and the estimator lets you say so per run instead of applying one size across the building.
A flat with twenty two light and socket points on a measured route averaging nine metres gives a hundred and ninety eight metres of conduit, and the same length of cable per single circuit run. Allow ten percent for terminations and the order is about two hundred and eighteen metres of conduit with the same in cable, plus twenty two boxes, the couplings for the joints and saddles at the spacing you set. Add one run of four circuits on a longer path and that run is sized up, with its cable counted three times over, which is what the per run inputs are for.
Measure the runs. Conduit is bought by length, so the only figure that matters is the path each run takes from the board through the building to its last point, plus the drops at each end. An average of nine metres over twenty odd points is a typical flat; a larger house with long corridors and two floors is nearer twelve. The estimator multiplies your measured runs by their count and adds the boxes and couplings the joints need.
From the circuit design, not from the estimate. Cable size follows the current the circuit carries, the protective device, the grouping and the volt drop, and every one of those is a design decision made against the wiring regulations where you are building. The estimator asks you what size you are pulling and adds the lengths by size, so the cable order matches the design. Using the tool to choose a size would be using a measuring tape as a rule book.
Yes, both are counted. Boxes follow the points, with a box at every accessory and at every joint, and switch plates follow the switches. Couplings follow the joints in the run, which is a function of the length and the stock lengths you buy. That is why the accessory count is not a rate per square metre: a room with four switches and no sockets needs four boxes and four plates, whatever its floor area.
Surface, concealed and buried, each on its own line, because they are different products with different costs and different installation work. A surface run in a garage and a buried run to a gate light do not belong in one total. The estimator keeps them apart and applies your measured length to each, so a change of method shows in the order instead of disappearing into an average.
The cable runs inside the conduit on the same path, so its length follows the conduit's length with the number of cores you specify for the circuit. Where a run carries three circuits, the cable is counted three times and the conduit sized for the group. A draw wire or spare capacity is a separate item you add deliberately, not an allowance folded into the cable, because a spare capacity that nobody records tends to be forgotten at second fix.