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Retaining Wall Calculator

Estimate retaining-wall blocks from your wall's face area, then size the compacted base gravel and the drainage backfill behind it — all in one place.

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What you'll need to buy

  • Retaining wall blocksInterlocking concrete wall blocksShop
  • Cap blocks + adhesiveFinishes the top course, glued downShop
  • Base / drainage gravelCrushed stone for base and backfillShop
  • Perforated drain pipeCarries water out from behind the wallShop
  • Non-woven landscape fabricSeparates gravel from backfill soilShop

Links marked Shop open a retailer search (Home Depot) and are affiliate links — we may earn a commission at no extra cost to you.

How the math works

A 20 ft long × 3 ft high wall = 60 sq ft of face. With 16"×6" blocks (0.67 sq ft each) and 5% waste: 60 × 1.05 ÷ 0.67 ≈ 94 blocks. Base gravel ≈ 60 × 0.020 ≈ 1.2 yd³ (≈1.7 tons); drainage backfill ≈ 60 × 0.022 ≈ 1.3 yd³ (≈1.8 tons).

Formula

wall face area = wall length × wall height blocks = (face area × (1 + waste%)) ÷ block face area base gravel (yd³) ≈ face area × 0.020 drainage gravel (yd³) ≈ face area × 0.022

Reference values

Retaining wall block & gravel reference
ItemValue
16" × 6" block0.67 sq ft face
12" × 4" block0.33 sq ft face
18" × 8" block1.0 sq ft face
Base gravel≈ 6" compacted under wall
Drainage backfill≈ 12" gravel behind wall

Step by step

  1. Measure the wall length and the finished height for the face area.
  2. Divide face area by the face area of one block to get the block count.
  3. Add 5–15% waste for cuts, curves, and corners.
  4. Estimate the base and drainage gravel from the face area, and round up.

Waste allowance

Always add a waste factor: more for diagonal, herringbone, or irregular layouts and for trenches in rocky soil. It's cheaper to round up than to pay for a second delivery.

Bags vs. bulk

Bagged material is convenient for small jobs, but loose/bulk delivery is almost always cheaper per unit once you pass roughly 1–3 cubic yards. Compare the per-cubic-yard price before deciding.

Frequently asked questions

How many blocks do I need for a retaining wall?

Divide the wall's face area (length × height) by the face area of one block, then add waste. A 20 ft × 3 ft wall (60 sq ft) using 16"×6" blocks (0.67 sq ft each) needs about 94 blocks with 5% waste.

How much gravel goes behind a retaining wall?

Plan on a compacted gravel base under the wall (about 6" deep) plus a drainage chimney of free-draining gravel behind it (about 12" wide). This calculator estimates both from your wall's face area.

What gravel is best for retaining wall drainage?

Use clean, free-draining crushed stone such as #57 behind the wall, wrapped in landscape fabric, with a perforated drain pipe at the base to carry water away.

Do I need a permit or engineering for a retaining wall?

Walls taller than about 3–4 ft (rules vary locally) usually require a permit and engineered design with geogrid reinforcement. Always check local codes — this tool gives material estimates, not engineering.

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