A preliminary pipe, chamber and bedding takeoff from the runs you measured.
Groundwork and site clearance. 5 questions answered below, each with the arithmetic behind it and the limits of what the tool decides. Open the Drainage Materials Takeoff tool.
A drainage takeoff counts pipe, chambers and bedding, and it does all three from a route rather than from a property. The route is the layout you have drawn: pipes running between chambers, chambers at the changes of direction, and bedding under the whole of it. The result is a materials list, not a hydraulic design. Pipe sizes, gradients and chamber positions come from the drainage design, which follows from the flows, the levels and the regulations where you are building. What the takeoff does is convert that design into pipe, fittings, chambers and bedding, in the quantities a merchant and a groundworks gang both work from.
A run of thirty metres between two chambers, falling one in eighty, is thirty metres of pipe along its line, and its ends differ in level by three hundred and seventy five millimetres. Bedding at a hundred and fifty millimetres deep and three hundred wide is thirteen and a half cubic metres of granular material for that run. Add four more chambers at the junctions and the gullies at the low points, and the order is pipe, chambers, gullies, bedding and the excavation those dimensions imply. Nothing in it was derived from the number of bathrooms, which is the figure a rule of thumb would use.
No. Gradients come from the drainage design, because they follow from the levels available and the self cleansing velocity the pipe needs. The takeoff asks for the fall you have designed and uses it to calculate the pipe length along the run and the level difference between its ends. Choosing a gradient is a design decision with a code behind it, and a materials calculator that picked one would be making a decision it cannot defend.
By length, along the run and at the gradient, with the fittings counted at every junction and change of direction. The takeoff takes each run you describe, multiplies by nothing except its measured length, and sums the pipe by diameter and material. Where a run has not been measured, it is reported as missing, because a per chamber assumption produces a pipe order that is short on long runs and long on short ones.
From the layout. A chamber goes where the pipe changes direction, where two runs meet, where the run changes diameter and at the head of a run. The takeoff counts them from the route you enter, with their covers and their benching, rather than applying a chamber per house. That is why a courtyard with four connections needs more chambers than a straight run of the same pipe length.
Bedding and surround are calculated from the trench you describe, at the depth and width you enter, so the granular material is a quantity rather than an allowance. Where a specification calls for a surround rather than a bed, say so by entering the dimensions that apply, and the volume follows. It is the item most often omitted from a drainage estimate and the one that stops a gang on the first morning.
No. It is a materials takeoff, so it counts what a designed system needs. Whether the gradients are adequate, whether the pipe sizes carry the flow and whether the layout complies with the drainage regulations where you are building are design questions with a code behind them. The takeoff assumes those answers exist and converts them into pipe, chambers, bedding and excavation, which is the part that can be counted.