Retaining Wall Calculator

A retaining wall calculator returns the blocks a wall needs, with waste included. It divides the wall height by the block face height to get courses, and divides the wall length by the block face length to get blocks per course. It then multiplies courses by blocks per course and adds a waste allowance for cut blocks.

Last reviewed September 2026Method & sources Report an error

Not to scale.

Courses (rows) High

Includes a 10% waste allowance.

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Retaining wall blocks66 blocks4 courses of 15.

Breakdown
Courses (rows) High	4
Blocks per Course	15
Blocks to Buy (with waste)	66
Estimated Cost

How to Calculate Retaining Wall

A retaining wall is built up in courses (horizontal rows) of blocks. The number of courses is the wall height divided by one block's face height, rounded up. The blocks needed per course is the wall length divided by one block's face length, rounded up. Multiplying courses × blocks per course gives the raw block count. A waste allowance covers cut blocks at corners or curves. Unlike a standard concrete masonry unit, retaining wall block has no single industry-standard size. Manufacturers like Versa-Lok, Allan Block, and Keystone each publish their own face dimensions, which is why block length and height are adjustable inputs here rather than fixed constants. Check your specific product's spec sheet for its exact face dimensions before ordering.

Example: 20 ft long, 2 ft tall wall, using a block with a 16 in. × 6 in. face

A 20 ft long, 2 ft tall wall, using a block with a 16 in. × 6 in. face, 10% waste: 1. Wall length in inches: 20 × 12 = 240 in. Wall height in inches: 2 × 12 = 24 in. 2. Courses: 24 ÷ 6 = 4 courses 3. Blocks per course: 240 ÷ 16 = 15 blocks 4. Raw block count: 4 × 15 = 60 blocks 5. With 10% waste: 60 × 1.10 = 66 blocks. 66 blocks of this size builds a 4-course, 20 ft long wall. A different manufacturer's block with a taller 8 in. face would only need 3 courses (24 ÷ 8) for the same height, changing the total block count.

Retaining Wall: Compare Scenarios

Scenario	Courses (rows) High	Blocks per Course	Blocks to Buy (with waste)
Small (10 ft, 1.5 ft tall garden wall)	3	8	27
Medium (20 ft, 2 ft tall wall)	4	15	66
Large (50 ft, 4 ft tall wall)	8	38	335

What Is Retaining Wall?

A retaining wall calculator converts a wall's length and height, plus your chosen block's face dimensions, into the number of blocks needed to build it. It's built around adjustable block dimensions specifically because retaining wall block, unlike a standard concrete masonry unit, is a proprietary, manufacturer-specific product with no shared industry sizing.

Retaining Wall: Pros and Cons

Pros

Adjustable block dimensions match how retaining wall block is actually sold. As a manufacturer-specific product, not a standardized unit
Separates courses and blocks-per-course as visible intermediate results, useful for sanity-checking the wall's actual coursing before ordering
Applies waste for corner and curve cuts on top of the raw block count

Cons

Doesn't size the wall's engineering requirements. Retaining wall height, batter (backward lean), base embedment depth, and drainage all depend on soil conditions and local code. Walls above a certain height (commonly around 3-4 ft, but jurisdiction-dependent) often require an engineer's design
Doesn't include base leveling material, drainage gravel behind the wall, or cap units. Budget these as separate materials specific to your block system's installation instructions
Assumes a straight wall. A curved wall typically needs more blocks per course than the straight-line length would suggest, since curved coursing has more cut waste

Retaining Wall: What a Pro Checks

Check your local building department before planning a wall taller than about 3-4 ft of exposed height. Many jurisdictions require an engineered design above that, and this calculator only estimates block quantity, not structural design.
Budget a drainage zone of clean gravel behind the wall separately. It's needed essentially always to relieve the hydrostatic pressure that would otherwise push the wall over, and this calculator doesn't size that volume.
Check your specific block manufacturer's spec sheet for its exact face dimensions before ordering. Retaining wall block has no industry-standard size the way a standard concrete masonry unit does.

Frequently Asked Questions

How many blocks do I need for a retaining wall?

Divide the wall height by one block's face height to get the number of courses, divide the wall length by one block's face length to get blocks per course. Then multiply the two together and add a waste allowance for cuts. A 20 ft × 2 ft wall using a 16x6 in. block needs about 66 blocks including 10% waste.

What size are retaining wall blocks?

There's no single standard size. Each manufacturer (Versa-Lok, Allan Block, Keystone, and others) produces its own proprietary block dimensions. These are commonly somewhere around 12-18 in. long and 4-8 in. tall, but always check your specific product's spec sheet rather than assuming a standard size.

How tall can I build a retaining wall without an engineer?

This varies by local jurisdiction, but many building codes require an engineered design above roughly 3-4 ft of exposed wall height, especially where the wall retains a slope near a structure or property line. Check your local building department before planning a tall wall. This calculator only estimates block quantity, not structural design.

Do I need gravel behind a retaining wall?

Yes, essentially always. A drainage zone of clean gravel behind the wall relieves hydrostatic pressure that would otherwise build up and push the wall over. A drain pipe at the base is needed for anything beyond a low garden wall. This calculator doesn't size that gravel volume. Follow your specific block manufacturer's installation guide for the recommended drainage zone depth.

Does a curved wall need more blocks than a straight one?

Generally yes, for the same length. Curved coursing means more blocks get cut or use manufacturer-specific curved/tapered units, which typically increases waste beyond what a straight wall's default allowance covers. Increase the waste percentage for a wall with significant curves.

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