You're standing on your sloped yard planning a retaining wall to level a garden or prevent erosion, and you need to know: how many concrete blocks, how much gravel, and how much soil will this project take? A retaining wall calculator measures your slope and dimensions, then calculates exact material quantities.
What This Calculator Does
This retaining wall calculator determines the wall length, height, and type of blocks you'll use, then calculates the number of blocks needed to build the face, the gravel base and backfill required, and the soil to restore the level area above the wall. It accounts for block size (most common: 8" × 16" or 12" × 18" concrete retaining blocks), adjusts for stacking patterns, and factors in the necessary gravel base for drainage and stability.
How to Use This Calculator
Measure the length of the wall where it will sit. Measure the height from the bottom of the wall (where it sits on the gravel base) to the top of the retaining wall (where it meets the new slope level).
Select your block type and size. Standard retaining wall blocks are 8 inches tall and 16 inches wide (some come as 12" × 18"). Know how many blocks fit per row per linear foot. Typically, 16-inch-wide blocks fit about 7.5 blocks per linear foot of wall length (12" ÷ 16" × 12" = 9 blocks, but offset joints reduce this to about 7-8).
Enter these measurements, and the calculator determines how many rows you need (based on total wall height divided by block height), multiplies rows by blocks per linear foot, then multiplies by wall length. It also calculates gravel (typically 4-6 inches deep as a base) and backfill soil.
The Formula Behind the Math
Wall height in blocks = Wall height (inches) / Block height (inches)
Blocks per row = Wall length (feet) × (12 / Block width in inches)
Total blocks = Blocks per row × Rows, adjusted for offset pattern (multiply by 0.95 to 0.98)
Gravel base volume (cubic yards) = Wall length (feet) × 4 inches depth / 9 sq ft per cy
Backfill soil (cubic yards) = Wall length (feet) × Average height above wall / 9 sq ft per cy
Example:
You're building a retaining wall 20 feet long and 3 feet tall using 8" × 16" concrete blocks.
Height in blocks = 36 inches / 8 inches = 4.5, round up to 5 rows.
Blocks per row: Wall length is 20 feet. Block width is 16 inches. (20 × 12) / 16 = 240 / 16 = 15 blocks per row.
Total blocks = 15 × 5 = 75 blocks (adjusted for offset pattern ≈ 72-75 blocks).
Gravel base (4 inches deep): 20 feet × 4 feet width × 4 inches depth = 26.67 cubic feet ≈ 1 cubic yard.
Backfill soil (assuming the wall levels a slope, average fill of 1.5 feet): 20 × 1.5 × 1.5 feet average slope width = 45 cubic feet ≈ 1.7 cubic yards.
Our calculator does all of this instantly-but now you understand exactly what it's computing.
Small Residential Retaining Walls (Under 3 Feet)
Walls under 3 feet tall typically need only a gravel base and can be built without structural engineers or special reinforcement. Use gravity-stacked blocks (interlocking blocks that grip each other without adhesive) or stack regular blocks with an 8-10 degree inward tilt ("batter") for stability. These walls suit garden terracing, small erosion control, or leveling a gentle slope.
Medium Walls with Geotextile and Drainage (3-4 Feet)
Walls 3-4 feet tall benefit from geotextile fabric (landscape cloth) behind the blocks to prevent soil migration into gravel, and proper drainage pipes to manage water behind the wall. Add the costs of geotextile and 4-inch drainage pipe to your material estimate. The structural demand increases, so batter (inward lean) should be 10-12 degrees, and base gravel depth should be 4-6 inches.
Tall Walls Requiring Rebar and Professional Design (4+ Feet)
Walls over 4 feet tall typically require rebar or other reinforcement, engineer-designed footing, and gravel depths of 6+ inches. Local building codes often require permits and professional design for walls over 4 feet. Budget for rebar, professional installation, and possible drainage systems. These walls support significant soil pressure and need careful construction.
Terraced Walls (Multiple Walls at Different Heights)
A terraced landscape has multiple shorter walls stacked or tiered rather than one tall wall. A 5-foot slope might be two 2.5-foot walls with a level step between them, or three walls of different heights. Calculate each wall separately by its length and height, then add the totals. Terraced walls are often more stable than one tall wall and create interesting landscape platforms.
Tips and Things to Watch Out For
Check building codes and permits before starting. Many jurisdictions require permits for retaining walls over 3-4 feet, in certain soil types, or in areas with flood risks. A $300 permit now beats a $5,000 rebuild or legal issue later.
Measure your wall length and height carefully. A wall that's actually 22 feet long instead of 20 feet needs 10% more blocks and material. Use a 50-foot tape and walk the entire planned wall line. Measure height at multiple points if the ground is uneven.
Understand soil conditions before designing. Different soil types (clay, sand, silt) have different drainage and stability characteristics. Sandy soil drains quickly but shifts more; clay drains poorly and pushes harder against the wall. If you're uncertain, have a soil test done ($100-300) before building a large wall.
Don't skip the gravel base or drainage. Proper gravel base and drainage prevent water buildup behind the wall, which increases pressure and can cause collapse. A 4-6 inch gravel base is non-negotiable for residential walls.
Account for geotextile fabric in calculations. Behind the gravel, you'll install landscape cloth to prevent soil from washing into gravel and clogging it. This isn't a huge cost but needs to be included in the material order.
Always order 10-15% more material than your calculation to account for waste, cuts, and breakage. Concrete blocks can crack or break during stacking. Gravel settles and compacts, so ordering extra ensures you have enough after settling. It's better to have leftover gravel than to run short mid-build.
Plan for batter (inward tilt) during construction. Every 8 inches of height, the wall should lean back (batter) about 1 inch. This tilts the wall toward the uphill side, making it more stable. It doesn't change block count, but it affects finished dimensions-plan accordingly if the wall is near a property line.
Frequently Asked Questions
How many landscape blocks are in a pallet?
Standard concrete retaining blocks come on pallets with roughly 60-90 blocks depending on block size and manufacturer. A 75-block wall needs about one pallet. Larger walls need multiple pallets. Check with your supplier for exact block count per pallet.
Do I need a gravel base if the ground is already level?
Yes. A 4-6 inch gravel base provides drainage, prevents settling, and creates a level foundation for block stacking. Even on level ground, gravel ensures water drains away from the wall and blocks sit level. Never build directly on soil.
What's the best soil to backfill behind a retaining wall?
Compacted, well-draining soil is ideal. Native soil from your property often works if it's not pure clay. Sand-silt-gravel mixes drain well. Avoid heavy clay alone, which retains water and increases pressure. If your native soil is problematic, consider imported fill or a mix of native soil and gravel.
How do I calculate backfill for an irregular slope?
Measure the slope height at multiple points (maybe every 4-5 feet along the wall length), average them, and use that average height for backfill volume. For an L-shaped wall with two sections at different heights, calculate each section separately and add totals.
Should I use concrete blocks or stone for retaining walls?
Both work. Concrete blocks are uniform, predictable, and come in interlocking designs. Natural stone is aesthetics-driven but requires custom cutting and takes longer to build. The calculator assumes concrete blocks. Stone walls need custom estimates based on piece size and shape.
Can I build a retaining wall myself, or should I hire a contractor?
Walls under 3 feet can be DIY projects with good instructions and careful execution. Walls 3-4 feet are doable DIY but require more precision and physical effort. Walls over 4 feet usually need a professional, especially if permits and engineering are required. Consider your comfort level with the physical work and structural accuracy.
How long does it take to build a retaining wall?
A 3-foot by 20-foot wall with a sound base and good preparation takes a skilled person or small crew 2-3 days. DIY might take a week or more. Tall walls (4+ feet) with rebar and complex drainage take significantly longer. Budget time accordingly.
What if the ground behind the wall is very wet or swampy?
Wet conditions behind a wall increase pressure and cause failure risk. Install a drainage pipe (4-6 inch perforated PVC) at the base of the wall, running to daylight or a drain area. This removes water before it pressurizes behind the wall. Drainage pipe is essential in high-water areas.
Do I need rebar for a short wall?
Walls under 3 feet typically don't need rebar if blocks are interlocking or properly battered. Walls 3-4 feet benefit from rebar for added strength. Walls over 4 feet almost always need rebar, often specified by an engineer. Check local codes and get professional advice if unsure.
Related Calculators
For calculating backfill soil quantities separately, use our soil calculator. The concrete calculator helps if the wall base requires a concrete footer. Rebar calculator estimates reinforcement steel for taller walls. Material cost estimator budgets the total project including labor.