How to Estimate a Retaining Wall (Blocks, Gravel & Cost)

Updated 2026-07-28 · Yardly

Retaining walls earn their keep: they hold a slope, flatten a yard, and add planting beds. Estimating one is mostly about face area and a gravel base — get those right and the block count follows.

Start with face area

Face area = wall length × wall height. A 20 ft wall, 3 ft tall = 60 sq ft of face. Block coverage depends on block size; a standard 8×4 in face covers about 0.22 sq ft, so 60 ÷ 0.22 ≈ 273 blocks, +10% ≈ 300 blocks. The retaining wall calculator derives course counts from your block height so you don’t math it by hand.

Don’t forget the gravel

A wall lives or dies by its base and drainage:

• Base: 4–6 in of compacted gravel under the first course
• Drainage: ~12 in of clean gravel behind the wall

That 20×3 ft wall needs roughly 1–1.5 cu yd of base gravel plus 2–3 cu yd of backfill. The retaining wall project estimator combines blocks, base, and backfill into one bill of materials. The National Concrete Masonry Association publishes drainage guidance the estimator’s defaults reflect.

Cost without the guesswork

Enter a price per block and per yard of gravel and the estimator returns a total — blocks, base, backfill, and caps. Add a waste margin so a short count doesn’t stop the build mid-run.

Ordering tips

• Buy 5–10% extra blocks for cuts and future repairs.
• Round gravel up; a short base is hard to fix later.
• Check local frost depth — taller walls may need a deeper base.

Run the numbers with the retaining wall calculator or the full project estimator.

Frequently asked questions