The wall does not care how many terraces you build
This is the result worth taking away. Total retaining face area is the length along the contour multiplied by the total fall of the hill, and nothing else. Thirty feet of garden across a slope that drops 9 ft is 270 square feet of face — one 9 ft wall, three 3 ft walls, or six 18 in walls. The arithmetic is exact: N benches each with a riser of fall over N gives N times blen times fall over N, and the N cancels.
People terrace expecting to save wall and are surprised by the quote. What terracing actually saves is earthwork. Cut and fill run as blen times run times fall divided by eight N, so they fall away as the bench count rises. One bench across the default slope is 75 cubic yards of cut and 75 of fill. Three benches is 25 and 25. Six is 12.5 and 12.5. That is the trade, and it is a good one — but it is a dirt trade, not a wall trade.
Balanced by construction, short by shrink
Each bench here is levelled at the mid height of the ground beneath it, which puts the cut in the uphill half and the fill in the downhill half in equal measure. The cross section is two triangles of the same size, so the cut area is the bench width times the riser divided by eight, per foot of contour, and the fill area is identical.
Identical in bank measure, that is. Fill has to be compacted, and compaction drives out voids the soil had in the ground, so a bank yard of cut does not build a bank yard of fill. At 12 percent shrink, the 25 cubic yards of fill in the default layout needs 28.4 bank yards to build it, against 25 available from the cut. The site is 3.4 yards short despite balancing perfectly on paper — two thirds of a truck load, which nobody would order and everybody has to find.
What happens if the cut is not usable
| Cut usable as fill | Import | Export | Total trucked |
|---|---|---|---|
| 100 percent | 3.4 cu yd | none | 3.4 cu yd bank |
| 50 percent | 15.9 cu yd | 12.5 cu yd | 28.4 cu yd bank |
| 0 percent | 28.4 cu yd | 25.0 cu yd | 53.4 cu yd bank |
The bottom row is the one that catches people. If the material coming out of the hill is topsoil, organics or soft wet clay, it cannot be placed as structural fill behind or under anything, and a job that balanced on paper becomes two separate trucking operations in opposite directions. That is not an unusual outcome on a garden slope, where the top foot or two is exactly the material you would not want to build on. Test holes settle it before the machine arrives.
Everything this does not answer
It gives geometry and volumes. It does not tell you that a wall of any particular height is fine without engineering, because that is not something a page can say — retaining structures are an engineered design, and so is any question about whether a cut or fill slope will stand. It cannot say whether fill placed in the outer half of a bench will support what you put on it. And it says nothing about drainage, which is what actually destroys terraces: a stepped hillside gives water places to pond and places to get behind a face, and where that water ends up can land on a neighbour.
Use it to decide how many benches make sense and what each one costs in dirt and face, then take those numbers to somebody who can look at your soil.
Questions people ask
Does terracing a slope reduce how much retaining wall I need?
No, and this is the most common misunderstanding about terracing. The total face area is the length along the contour times the total fall, whatever the bench count. Thirty feet of contour across 9 ft of fall is 270 square feet of face as one wall, three walls or six. Terracing divides that area into shorter pieces, which changes what the pieces are and how they are built, but it does not reduce the area. What it does reduce, sharply, is the volume of earth moved.
How many terraces should I cut into a slope?
That is a design decision, and the page gives you the trade rather than an answer. More benches means less earthwork — cut and fill go down in inverse proportion to the count — narrower usable treads, and more, shorter faces. Fewer benches means wider usable space, taller faces and a great deal more dirt. On the default 60 ft by 9 ft slope, one bench is 75 cubic yards of cut and a 9 ft face; six benches is 12.5 cubic yards and six 18 in faces on 10 ft treads. Which is right depends on what you intend to do on the benches.
Why does a balanced terrace still need imported fill?
Because cut and fill are measured in different states. The cut comes out as bank material and the fill goes in compacted, and compaction drives out voids the soil had in the ground, so a bank yard builds less than a bank yard of fill. At 12 percent shrink, the default layout needs 28.4 bank yards to build 25 yards of fill against 25 available. The shortfall is small in absolute terms and it is always in the same direction, which is why experienced graders order a little extra rather than assuming a balance.
How tall can a terrace wall be before it needs engineering?
There is no number this page can give you, and any page that gives you one is guessing about your soil, your surcharge, your drainage and your local requirements. Retaining structures are an engineered design, and whether a given slope stands without one is the same kind of question. What the calculator does is tell you what face area a given bench layout produces, so you can take a real geometry to somebody qualified rather than a sketch. The maximum riser field is your own choice of face height, not a limit anyone here is stating.
What about drainage across the benches?
The page adds a cross fall across each tread so water leaves rather than standing — at 2 percent across a 20 ft bench that is 4.8 in of drop from back to front, which has to be found somewhere and usually comes out of the riser. Beyond that it is a design question. Level benches on a hillside give water places to pond and places to get behind a face, and water that used to run over the surface now has to be collected and taken somewhere. Where that somewhere is can create a problem for a neighbour, which is a local matter rather than an arithmetic one.