Retaining Wall Block Calculator

How to use the retaining wall block calculator

1. Choose a block size. Presets cover small 4 × 12 in garden block, standard 6 × 16 × 12 in segmental units and large 8 × 18 in units. Use “Custom size” for anything else.
2. Enter the length along the face. Add the legs of a cornered wall together.
3. Enter the exposed height. Measure from finished grade in front of the wall to the top of the last block, leaving out the caps. When you change it, the buried depth updates to the larger of 6 in or one-tenth of the exposed height. You can override that.
4. Check the cap length. Set it to 0 for no caps.
5. Turn on surcharge if the wall holds back a slope, a driveway, a parking pad or a building. It changes the code checks.

A note under the results tells you whether the wall is past the IRC limits.

Retaining wall block formula

The buried course is the one people forget. Manufacturer guides start the wall below grade. Belgard says to bury at least 6 in of block. VERSA-LOK says walls usually have one-tenth of the exposed height buried, so a wall showing 10 ft has at least 1 ft below grade.

Under the first course goes a leveling pad of compactable ¾ in minus base. Belgard’s guide calls for at least 6 in after compaction, extending at least 6 in in front of and behind the block. A 12 in deep block therefore sits on a 24 in wide pad. We convert that volume to tons at 1.85 tons per compacted cubic yard, WSDOT’s figure for compacted crushed surfacing base.

Behind the wall, Belgard calls for 12 in of free-draining ¾ in aggregate. The calculator counts that zone for the full block height and converts it at 1.40 tons per cubic yard, MoDOT’s factor for loose gravel. Hollow-core blocks also need core fill. Add it from the spec sheet.

Worked examples

30 ft garden terrace, 24 in exposed, 6 × 16 in block

360 in ÷ 16 = 22.5, so 23 blocks per course. Buried depth is 6 in, so 30 ÷ 6 = 5 courses. 23 × 5 = 115, and with 5% extra that is 121 blocks plus 24 caps. The base is 30 ft × 2 ft × 0.5 ft = 30 cu ft, about 2.1 tons. The drainage zone is 30 × 1 × 2.5 = 75 cu ft, about 3.9 tons. At 36 in from the bottom of the pad, it is under the 4 ft permit line.

16 ft bed edge, 12 in exposed, 4 × 12 in garden block

16 blocks per course. 18 in total height ÷ 4 = 4.5, rounded up to 5 courses. That is 80 blocks, or 84 with extra, plus 17 caps. Base is about 0.9 ton and drainage stone about 1.4 tons.

40 ft wall, 40 in exposed, 8 × 18 in block

27 blocks per course. 46 in ÷ 8 = 5.75, so 6 courses (48 in of block). That is 162 blocks, or 171 with extra, plus 28 caps, 2.7 tons of base and 8.3 tons of drainage stone. Over 3 ft exposed, VERSA-LOK recommends a perforated drain pipe at the base, so add 40 ft of pipe. With the 6 in pad, the wall is 54 in from the bottom of its footing, over the 4 ft exemption, so expect a permit.

When a retaining wall needs a permit or an engineer

Two sections of the International Residential Code (IRC) matter here. Many towns adopt them with changes, so these are a starting point for a call to your building department, not the final word.

• Permit (R105.2): the IRC exempts “retaining walls that are not over 4 feet (1219 mm) in height measured from the bottom of the footing to the top of the wall, unless supporting a surcharge.” For a block wall, the footing is the leveling pad. That means the buried course and the base count toward the 4 ft.
• Engineering (R404.4): walls not laterally supported at the top that retain more than 48 in of unbalanced fill, or walls over 24 in high that also resist other lateral loads, must be designed to accepted engineering practice so they do not overturn.

A surcharge is any extra load pushing on the wall: a slope above it, a driveway, a parked car, a shed or a house foundation. One city’s guidance says retaining walls that support a surcharge need an engineered design and a building permit regardless of height.

Manufacturers say the same in their own terms. VERSA-LOK notes that its standard units, stacked as a gravity wall with level backfill, good soils and no extra load, are generally stable to about 4 ft. Above that, walls use layers of geogrid soil reinforcement designed for the site.

Building tips that change the order

• Step the base on a slope. When the grade along the front changes, step the leveling pad in block-height increments, starting at the lowest point. VERSA-LOK steps its 6 in units in 6 in increments, and Belgard says 6 to 8 in. Run the calculator for each stepped section and add the results together.
• Filter fabric. VERSA-LOK notes that some projects, often along shorelines, need a geosynthetic filter fabric behind the drainage aggregate. It stops soil and sand fines from migrating into the stone and the wall joints. Backfill the soil behind the stone in lifts no thicker than 6 in, compacting each one.

For the gravel itself, the gravel calculator converts any area and depth to tons. To build a patio above the new terrace, the paver calculator counts pavers, base and sand together. The French drain calculator handles drainage trenches.

Sources & assumptions

How this calculator gets its numbers. Full method: our methodology.

• Blocks per course = wall length ÷ block length, rounded up. Courses = (exposed height + buried depth) ÷ block height, rounded up. The extra allowance (default 5%) is applied to blocks and caps. A straight wall is assumed, and setback per course is ignored.
• Buried depth defaults to the larger of 6 in (Belgard) or one-tenth of the exposed height (VERSA-LOK).
• Base: leveling pad 6 in deep after compaction and as wide as the block depth + 12 in (6 in in front and behind), at 1.85 tons per compacted cubic yard (WSDOT).
• Drainage stone: a zone 12 in deep behind the wall for the full block height, at 1.40 tons per loose cubic yard (MoDOT). Fill for hollow block cores is not included.
• Code notes (IRC R105.2 and R404.4) are a screening aid, not approval. Block presets other than the 6 × 16 × 12 in unit are typical sizes, not a specific product.

Sources

1. Belgard, SRW Quick Reference Install Guide. 6 in compacted ¾ in minus pad, 6 in past the block front and back; bury 6 in; 12 in drainage aggregate behind.
2. VERSA-LOK, VERSA-LOK Standard Design & Installation Guidelines. 6 × 16 × 12 in unit; bury one-tenth of exposed height; drain pipe over 3 ft; gravity walls generally stable to 4 ft.
3. Grant County, Washington, 2021 IRC Section R105.2 Work Exempt from Permit. “Retaining walls that are not over 4 feet (1219 mm) in height measured from the bottom of the footing to the top of the wall, unless supporting a surcharge.”
4. Georgia Department of Community Affairs, IRC 2018 Top 20 code sections. R404.4 design thresholds: 48 in unbalanced fill, or 24 in with other lateral loads.
5. City of Port Townsend, Washington, Retaining Walls information sheet. Walls supporting a surcharge need a design and permit at any height.
6. Washington State Department of Transportation, Design Manual M 22-01, Chapter 620, Exhibit 620-1 (Unit Dry Weight – Gravel Base and Surfacing). Compacted base 1.85 T/cy (leveling pad).
7. Missouri Department of Transportation, Engineering Policy Guide 450, Estimate Factors (archived PDF). Loose stone 1.40 T/cy (drainage stone).

Last reviewed October 25, 2026. Product figures vary by brand; the label on what you buy wins.