Retaining Wall Calculator
Work out how many blocks, cap blocks, and how much gravel base a segmental retaining wall needs, plus the total estimated cost.
🧱 What is a Retaining Wall Calculator?
A retaining wall calculator works out how many segmental blocks, cap blocks, and how much gravel base a dry-stacked retaining wall needs from its length, height, and block size, then estimates the total material cost. Segmental retaining wall block, the interlocking concrete units sold at any landscaping supplier, is priced and counted per unit, so knowing the exact block count before ordering avoids both a stalled build and an expensive overage.
Homeowners terracing a sloped yard, landscapers quoting a client's grading project, and DIYers building a garden bed retaining wall all rely on this figure before placing an order. Retaining walls also involve more than the visible wall blocks, a gravel base under the first course and cap blocks finishing the top are both standard parts of the job that are easy to forget when estimating.
A common misconception is that retaining wall blocks are ordered the same way as bricks or standard masonry, by area alone. In practice, segmental block face dimensions vary by manufacturer and style, so the number of blocks per square foot of wall differs from product to product, which is why this calculator works from your specific block's width and height rather than a fixed coverage rate.
Enter the wall's length and height, your block's width and height, a waste allowance, and optional block and cap prices and a gravel base depth, and this calculator returns the wall blocks needed, the cap blocks needed, the gravel base volume, and the total estimated material cost, with the full working shown.
📐 Formula
📖 How to Use This Calculator
Steps
💡 Example Calculations
Example 1 — A Standard Garden Terrace Wall
20 ft long, 3 ft tall, 18 by 6 inch blocks, 10% waste, $4.50 per block, $6 per cap
Example 2 — A Taller Driveway Retaining Wall
30 ft long, 4 ft tall, 16 by 8 inch blocks, 8% waste, $5.25 per block, $7 per cap
Example 3 — A Small Garden Bed Wall
12 ft long, 2 ft tall, 18 by 6 inch blocks, 10% waste, $4 per block, $5 per cap
❓ Frequently Asked Questions
🔗 Related Calculators
How do I calculate how many blocks a retaining wall needs?
Divide the wall area (length times height) by the face area of one block, then add a waste allowance and round up. For a 20 by 3 foot wall using an 18 by 6 inch block (0.75 sq ft face), that is (20 x 3) / 0.75 = 80 blocks, plus 10% waste = 88 blocks.
How many cap blocks does a retaining wall need?
Standard cap blocks are about 12 inches long, so a rough estimate is one cap block per linear foot of wall, plus your waste allowance for cuts at ends and corners. A 20 foot wall needs about 20 cap blocks before waste, 22 after a 10% allowance.
How deep should the gravel base be under a retaining wall?
A compacted gravel base of about 6 inches is standard for most segmental retaining walls up to a few feet tall, providing drainage and a stable, level foundation for the first course. Taller walls or poor soil conditions may need a deeper base.
How tall can a retaining wall be without engineering?
Many jurisdictions allow gravity retaining walls (no reinforcement) up to about 3 to 4 feet without an engineer's stamp, but this varies by local code and soil type. Taller walls typically need geogrid reinforcement extending back into the soil and often require a permit and engineering review.
What is a segmental retaining wall block?
A segmental retaining wall block is a precast concrete unit designed to interlock with the blocks above and below it, usually through a pin, lip, or textured face, and stacked dry without mortar. This system lets walls flex slightly with soil movement and drains water through the joints between blocks.
How much does a retaining wall cost per square foot?
Segmental block retaining walls typically cost $15 to $30 per square foot of wall face installed, covering blocks, base material, and labor, with taller or reinforced walls costing more. This calculator estimates material cost only, block, cap, and gravel, from the prices you enter.
Why does the block waste allowance apply to cap blocks too?
Cap blocks are cut at corners, ends, and curves just like the main wall blocks, so the same percentage waste allowance is applied to keep enough material on hand for those cuts, avoiding a mid-project trip back to the supplier for a handful of extra caps.
Can I use paver base or crushed stone instead of gravel under a retaining wall?
Yes, a well-draining compacted aggregate, often called paver base, crusher run, or 3/4 inch minus gravel, works well as a retaining wall base. The goal is a material that compacts firmly and drains water away rather than trapping it behind the wall.
Does a curved retaining wall need more blocks than a straight one of the same length?
Yes, slightly, because curved sections generate more angled cuts and offcuts than a straight run, which is why many builders increase the waste allowance to 15% or higher for a wall with curves, versus the standard 10% for a straight wall.
What's the difference between a retaining wall and a garden edging wall?
A true retaining wall holds back a significant soil grade change and must resist lateral earth pressure, often needing a gravel base, drainage, and possibly reinforcement. A low garden edging wall, generally under about a foot, mainly defines a bed edge and carries little structural load, so it needs less engineering.