Retaining Wall Calculator: Blocks, Caps and Gravel
A retaining wall succeeds or fails on the ordering. Too few blocks and the job stops mid-course with a half-built wall sitting in the rain; too little gravel and the wall starts to lean within a few winters. This retaining wall calculator builds the full shopping list from two measurements — wall length and wall height: blocks counted course by course, cap blocks for the top row, and gravel worked out twice, once for the base trench under the wall and once for the drainage backfill behind it.
Enter your block size — 16 × 6 in is the common landscape block — set the backfill width and a waste allowance, and the calculator returns courses, blocks per course, the net block count, the number to buy with waste added, and gravel in cubic yards and tons for the base, the backfill and both combined. Imperial is the default; switch to metric and wall dimensions are in metres while block sizes switch to millimetres.
How a retaining wall goes together
A wall starts below ground. It sits in a shallow trench holding about 6 in of compacted gravel, 2 ft wide, levelled with care, because everything above inherits the accuracy of that base. The gravel spreads the load, gives the first course a firm, flat bed and lets water escape instead of pooling under the wall. This calculator includes that base gravel in every estimate, converted at 1.4 tons per cubic yard.
The first course of blocks is then buried below ground level. Burying it locks the wall in, hides the base trench and stops the bottom course from sliding forward under pressure. Because that course sits underground and does no visible work, allow one extra course of blocks when you order for taller walls — the height you enter in the calculator is the height above ground, and a buried extra course is cheap insurance.
Above ground, the blocks stack in staggered courses, each one set back slightly from the one below so the wall leans gently into the slope it holds back. Cap blocks finish the top row, bedded in construction adhesive for a clean edge that stays put. Behind the wall goes a column of clean gravel about 1 ft wide, separated from the soil by landscape fabric so mud cannot wash in and clog it. That gravel column is the wall’s drainage, and the next section explains why it matters more than any other part of the build.
Retaining Wall Calculator
Enter the wall length and height, your block size and a waste allowance. Blocks, caps and gravel update as you type, or press Calculate.
Counts are rounded up to whole blocks, course by course. Backfill gravel = wall length × wall height × backfill width. Base gravel = wall length × 2 ft wide × 6 in deep. Gravel is converted at 1.4 tons per cubic yard.
The fixed assumptions behind the numbers: the base trench holds 6 in of gravel, 2 ft wide, along the full length of the wall; the backfill is a gravel column behind the wall as wide as the figure you enter — 1 ft is standard; and gravel converts at 1.4 tons per cubic yard. Block counts are rounded up to whole blocks at every course, and the buy figure adds your waste percentage on top of the net count.
Why the gravel backfill matters
More retaining walls are pushed over by water than by soil. Rain soaks the ground behind the wall, and if that water has nowhere to go, pressure builds against the back of the blocks until courses bulge, lean or blow out altogether — usually during the wettest week of the year. A 1 ft column of clean, free-draining gravel gives water a fast route down and away instead of a slow push against the wall, and landscape fabric keeps that route open for decades rather than a few seasons. Skimping on backfill to save a little money is the most expensive shortcut in wall building: the gravel is not fill, it is the insurance.
A note on height and safety
Keep do-it-yourself walls modest. Walls over about 4 ft usually need engineering — a deeper base, a drainage pipe, and sometimes reinforcement tied back into the soil — and many areas require a permit above that height. If your wall holds back a driveway, a slope with a building on it, or anything vehicles cross, get professional advice before you build, whatever the height. A failed garden wall is an inconvenience; a failed wall under a driveway is a serious problem.
What to calculate next
The wall is usually one part of a bigger garden project. These tools cover the jobs that tend to come with it:
- Gravel Calculator — gravel and stone for paths, drives and beds, by area and depth.
- Cubic Yard Calculator — soil, mulch and other bulk materials in cubic yards.
- Paver Calculator — pavers and base material, if the wall edges a new patio or path.
Frequently asked questions
How many blocks do I need for a 20 ft × 3 ft wall?
With standard 16 × 6 in blocks, a wall 20 ft long and 3 ft high takes 6 courses of 15 blocks, which is 90 blocks net. Add 10% for cuts and breakages and you order 99 blocks, plus 15 cap blocks for the top row.
Do I need gravel behind a retaining wall?
Yes. A column of clean gravel about 1 ft wide behind the wall is what drains water away. Without it, water pressure builds behind the blocks and pushes the wall over. Lay landscape fabric between the gravel and the soil so the drainage stays clear.
How deep should the base be?
About 6 in of compacted gravel in a trench roughly 2 ft wide, levelled before the first course goes down. That first course is buried below ground level, which is why ordering one extra course for taller walls is a sensible habit.
How tall can a DIY retaining wall be?
Up to about 4 ft for simple interlocking block walls on a proper gravel base. Above that, walls usually need engineering and may need a permit, so check your local rules and get advice before building higher.
Estimates only — for walls over 4 ft or near structures, get professional advice.