The hole is one volume, the heap beside it is a bigger one, and what you cart away is bigger still. This gives all three, plus the bedding and backfill, so you order the right lorries and the right stone.
Notes
Why spoil is bigger than the hole
Undisturbed ground is compacted. Dig it and it breaks into lumps with air between them. Clay bulks about 30 percent, sand and gravel around 15, rock 50 or more. Order skips on the bulked figure, never on the dig volume.
Battered sides
Anything deeper than about a metre normally needs the sides sloped back or supported. Sloping widens the top and adds volume fast — a 1 metre wide trench 2 metres deep at 45 degrees moves twice the material of a vertical one.
Backfill is less than you dug
Bedding stone and the pipe itself take up part of the trench, so you backfill less than you excavated — and the spoil you kept has to be compacted back in layers, which is why some of it never fits.
What this does not do
It does not tell you whether the sides need support. Trench collapse is the classic fatal accident on small sites, and depth alone does not decide it — ground conditions do.