How many retaining wall blocks do I need?
A 20 ft by 3 ft wall takes 100 blocks in the common 12 x 8 in size.
That is 20 per course and 5 courses, plus 20 cap blocks. One of those courses goes underground — see below, because it is the course people forget to buy.
Block counts by wall size
| Wall | Face area | 12 x 8 in | 16 x 6 in | 18 x 6 in | 8 x 4 in | Needs engineering? |
|---|---|---|---|---|---|---|
| 10 x 2 ft | 20 sq ft | 30 | 32 | 28 | 90 | no |
| 20 x 2 ft | 40 sq ft | 60 | 60 | 56 | 180 | no |
| 20 x 3 ft | 60 sq ft | 100 | 90 | 84 | 270 | no |
| 30 x 3 ft | 90 sq ft | 150 | 138 | 120 | 405 | no |
| 40 x 3 ft | 120 sq ft | 200 | 180 | 162 | 540 | no |
| 20 x 4 ft | 80 sq ft | 120 | 120 | 112 | 360 | yes |
| 40 x 4 ft | 160 sq ft | 240 | 240 | 216 | 720 | yes |
Block size changes the count far more than most people expect. The same 20 x 3 ft wall is 100 blocks at 12 x 8 in and 84 at 18 x 6 in — the bigger unit covers more face per piece, so you handle fewer of them, and each one is heavier.
The arithmetic
- Blocks per course = wall length ÷ block length. 20 ft is 240 in, ÷ 12 in = 20.
- Courses = wall height ÷ block height. 3 ft is 36 in, ÷ 8 in = 4.5, rounded up to 5.
- Blocks = 20 × 5 = 100.
- Caps = one more course of 20, if you are capping it.
Round courses up, never down. A course is a whole block tall; you cannot lay half of one, so 4.5 courses is 5 courses and the wall finishes slightly proud of the height you asked for.
The buried course nobody counts
The bottom course sits below grade — the rule of thumb is a tenth of the wall height buried, and never less than one full course. On this 20 x 3 ft wall that is 1 course, so of the 5 courses you buy, about 2.33 ft of wall ends up visible.
This is where block orders go wrong. Somebody measures the 3 ft of wall they want to SEE, buys 100 blocks for it, and is a full course short before the first row is level. Buy for the wall's total height, not its exposed height.
Common questions
- How many blocks in a square foot of wall?
- At 12 x 8 in, each block covers 0.67 sq ft of face, so about 1.5 blocks per square foot. At 16 x 6 in it is 1.5 blocks per sq ft as well; at 8 x 4 in it is 4.5. Face area is the honest way to compare prices between block styles.
- Do I need cap blocks?
- Not structurally, but an uncapped wall shows its hollow cores and the top course is the one most easily knocked out of line. Caps are one course — 20 here — and are usually glued with concrete adhesive rather than stacked dry.
- How many blocks for a 20 ft wall?
- Depends entirely on height: 60 at 2 ft, 100 at 3 ft, 120 at 4 ft, in 12 x 8 blocks. Length sets the blocks per course; height multiplies it.
- Should I add for breakage?
- Add about 5% on a straight wall and 10% if it curves or has corners, where blocks get cut. Unlike lumber, unused blocks are heavy to return and yards often will not take them, so err smaller and go back if needed.
- Does a curved wall need more?
- Yes. Blocks are tapered to allow a curve, but a tight radius means cutting, and the inside of a curve packs more blocks per foot of length than the outside. Measure along the FACE of the wall, following the curve, not across the chord.
- When does the wall need engineering?
- At about 4 ft, measured from the bottom of the footing rather than from where you stand. Above that a wall is a structure that can fail and take a patio or driveway with it, and most jurisdictions require an engineered design, a permit, and geogrid tied back into the soil.
Related answers
Or run any size through the retaining wall calculator.