Measure length and true height
Measure the wall length along its face, following any curves. For height, include the buried course: block walls start below grade. Keystone's installation manual recommends embedding at least 1 in for every 8 in of wall height, or 6 in, whichever is greater. Allan Block's best-practice guide cites 1 in per foot of height, with a typical 6 in minimum on commercial jobs. For a wall that steps down along a slope, calculate each section separately.
How blocks are counted
Blocks per course = wall length (in) ÷ block face length, rounded up. Courses = total height ÷ block height, rounded up. Multiply them and add waste. Caps run along the top, so cap count = length ÷ cap length. The defaults (30 ft long, 24 in tall, 12 × 4 in face) give 30 blocks per course × 6 courses, which is 189 blocks with 5% waste.
Blocks are specified by their face dimensions: the visible length and height. The depth (front to back) doesn't change the count, but it does change the base width.
Base and drainage stone
Keystone's manual calls for a leveling pad of at least 6 in of compacted angular granular fill (road base or ½–¾ in crushed stone), at least as wide as the block depth plus 12 in. It says not to use rounded pea gravel or sand for the pad. The calculator assumes a 24 in × 6 in trench, which suits a block about 12 in deep, and a 12 in-wide zone of drainage stone behind the wall for its full height. Change these if your block or design differs.
| Wall | Base gravel (24 × 6 in) | Drainage stone (12 in zone) |
|---|---|---|
| 20 ft × 18 in | 0.74 yd³ (20 ft³) | 1.11 yd³ (30 ft³) |
| 30 ft × 24 in | 1.11 yd³ (30 ft³) | 2.22 yd³ (60 ft³) |
| 50 ft × 30 in | 1.85 yd³ (50 ft³) | 4.63 yd³ (125 ft³) |
| 40 ft × 36 in | 1.48 yd³ (40 ft³) | 4.44 yd³ (120 ft³) |
| Wall | Base gravel (61 × 15 cm) | Drainage stone (30 cm zone) |
|---|---|---|
| 6 m × 45 cm | 0.557 m³ (557 L) | 0.823 m³ (823 L) |
| 9 m × 60 cm | 0.836 m³ (836 L) | 1.65 m³ |
| 15 m × 75 cm | 1.39 m³ | 3.43 m³ |
| 12 m × 90 cm | 1.11 m³ | 3.29 m³ |
Use the gravel calculator if you need these volumes in tons.
Block counts for common walls
| Wall | Block face | Blocks (no waste) | Caps |
|---|---|---|---|
| 20 ft × 18 in | 12 × 4 in | 100 | 20 |
| 30 ft × 24 in | 12 × 4 in | 180 | 30 |
| 30 ft × 24 in | 16 × 6 in | 92 | 23 |
| 50 ft × 30 in | 18 × 6 in | 170 | 34 |
| Wall | Block face | Blocks (no waste) | Caps |
|---|---|---|---|
| 6 m × 45 cm | 30 × 10 cm | 100 | 20 |
| 9 m × 60 cm | 30 × 10 cm | 180 | 30 |
| 9 m × 60 cm | 40 × 15 cm | 92 | 23 |
| 15 m × 75 cm | 45 × 15 cm | 170 | 34 |
Worked examples
Garden terrace, 24 ft × 18 in, 12 × 4 in blocksGarden terrace, 7.32 m × 45.7 cm, 30.5 × 10.2 cm blocks
Wall length 24 ft · Wall height 18 in · Block face length 12 in · Block height 4 in · Add cap row Yes · Cap length 12 in · Waste / overage 5 %Wall length 7.32 m · Wall height 45.7 cm · Block face length 30.5 cm · Block height 10.2 cm · Add cap row Yes · Cap length 30.5 cm · Waste / overage 5 %
- Wall blocks: 126
- Courses (rows): 5
- Cap blocks: 26
- Base gravel (24 in × 6 in trench): 0.89 yd³ (24 ft³)Base gravel (61 × 15 cm trench): 0.68 m³ (680 L)
- Drainage stone (12 in behind wall): 1.33 yd³ (36 ft³)Drainage stone (30 cm zone behind wall): 1.02 m³
Backyard wall, 40 ft × 30 in, 16 × 6 in blocksBackyard wall, 12.2 m × 76.2 cm, 40.6 × 15.2 cm blocks
Wall length 40 ft · Wall height 30 in · Block face length 16 in · Block height 6 in · Add cap row Yes · Cap length 16 in · Waste / overage 5 %Wall length 12.19 m · Wall height 76.2 cm · Block face length 40.6 cm · Block height 15.2 cm · Add cap row Yes · Cap length 40.6 cm · Waste / overage 5 %
- Wall blocks: 158
- Courses (rows): 5
- Cap blocks: 32
Height limits, permits and engineering
Small garden walls are a reasonable DIY project. Taller walls hold back a lot of soil and can fail dangerously. Many jurisdictions require a permit and an engineered design once a wall retains more than about 4 ft of soil (the IRC uses 48 in of unbalanced fill as its threshold for designed retaining walls), and sooner if there's a slope, a driveway or a building above the wall. Manufacturers publish maximum heights for their blocks without reinforcement. Taller walls use geogrid layers, which add excavation and backfill. Check with your local building department before you buy.
Building tips
- Start at the lowest point and step the base up in block-height increments (Keystone uses 8 in steps for its units).
- Level the first course carefully. Mistakes there get bigger with every course above.
- Put a perforated drain pipe at the back of the base, with an outlet to daylight, and backfill the drainage zone with clean stone.
- Glue cap blocks with masonry adhesive made for the purpose.
- Call 811 before digging the trench.
Cap units
Most block walls finish with a row of cap units. With "Add cap row" set to Yes, the calculator counts caps as the wall length divided by the cap length, plus your waste allowance. Caps are counted separately from the wall blocks because they're usually a different product. On curved walls, caps may need to be cut or alternated (narrow and wide ends) to follow the curve, so add a few extra. Manufacturers generally say to secure caps with a concrete adhesive made for masonry. Check the installation guide for your block system.
Checked against
Each case below is re-run against the calculator by our automated test suite.
| Case | Our result | Checked against | Result |
|---|---|---|---|
| 30 ft wall, 24 in tall, 12 × 4 in blocks, no waste | 180 | Hand calculation | Match |
| Case | Our result | Checked against | Result |
|---|---|---|---|
| 9.14 m wall, 61 cm tall, 30.5 × 10.2 cm blocks, no waste | 180 | Hand calculation | Match |
Frequently asked questions
How many retaining wall blocks do I need?
Multiply blocks per course (length ÷ block length) by the number of courses (height ÷ block height), then add 5–10% for cuts.
Do I include the buried course in the height?
Yes. Enter the total height, from the bottom of the first course to the top of the last course (not counting caps).
How much gravel behind a retaining wall?
A drainage zone about 12 in wide behind the wall for its full height is common. The calculator estimates it from your wall length and height.
How many blocks for a fire pit?
A ring uses fewer blocks than its outside circumference suggests, because tapered blocks fit tighter on the inside. Use the manufacturer's fire pit kit count, or enter the inner circumference as the length.
Sources & assumptions
The results are planning estimates, and the cost figures only use prices you enter. Product yields, densities and coverage rates vary, so check the label or ask your supplier, and follow local codes. Figures marked "approximate" or "rule of thumb" are general practice, not exact standards.
- Keystone Retaining Walls: Installation (embedment and leveling pad)
- Allan Block: Best Practices for SRW Construction
- International Residential Code R404.4 (retaining walls)
- 811: Call before you dig
See how we calculate and test our formulas and the changelog. Last updated 2026-10-08.
Reviewed by the MaterialWhiz Editorial Team. This estimate uses the assumptions shown in the calculator. Review the result against your supplier's product coverage and site conditions. Last reviewed: October 10, 2026. See how we calculate · Report a correction.