Retaining Wall Block Calculator

Why Accurate Block Estimates Save Money and Headaches

Retaining wall blocks are among the heaviest and most expensive landscaping materials you will ever haul home. A single pallet of medium blocks can weigh 2,000 pounds and cost $200 or more. That makes over-ordering painful and under-ordering even worse — a second delivery fee, a gap in your project schedule, and a weekend wasted waiting. Use this free retaining wall block calculator to nail your order the first time.

The other trap homeowners fall into is ordering only blocks. A retaining wall is a drainage system first and a decorative feature second. In addition to wall blocks and cap blocks, every retaining wall needs compacted gravel under the base course, drainage gravel behind the wall, landscape fabric to keep soil out of that drainage layer, and perforated pipe to carry the water away. This calculator gives you all of it in one place. For the planted bed behind the wall, the raised garden bed soil calculator gives you cubic feet, mix ratios, and bag counts once you know the bed dimensions.

How to Use This Calculator

1. Enter wall length and height. Measure the total horizontal run of the wall in feet and the exposed height — the distance from finished grade at the base to the top of the cap blocks — in inches.
2. Select a block size. Choose the size that matches what your local supplier carries. Medium (16×6×10) is the most common big-box option. Larger blocks build faster; smaller blocks make curves easier.
3. Review the full materials list. The results card shows wall blocks, cap blocks, courses, base gravel in cubic yards, backfill gravel in cubic yards, landscape fabric in square feet, and drainage pipe in linear feet — everything you need to order before you break ground.

How to Calculate Retaining Wall Blocks Manually

If you want to check the math yourself or price a job without a calculator, follow these steps:

1. Convert wall height to inches. A 2-foot wall is 24 inches tall.
2. Divide by block height to get number of courses. A 24-inch wall with 6-inch blocks = 24 ÷ 6 = 4 courses. Round up to the nearest whole number.
3. Convert wall length to inches. A 20-foot wall is 240 inches long.
4. Divide by block length to get blocks per course. 240 ÷ 16-inch block = 15 blocks per course. Round up.
5. Multiply courses × blocks per course. 4 courses × 15 blocks = 60 wall blocks.
6. Add 10% for cuts at corners and ends. 60 × 1.10 = 66 wall blocks total.
7. Calculate cap blocks. Divide wall length in inches by cap block length. 240 ÷ 16 = 15 caps, plus 10% waste = 17 cap blocks.
8. Calculate base gravel volume. Multiply 6 inches of depth × (block depth + 12 inches for drainage clearance) × wall length. Convert to cubic yards by dividing by 46,656 (cubic inches per yard). For the example: 6 × (10 + 12) × 240 = 31,680 cu in ÷ 46,656 ≈ 0.68 cubic yards.

Common Retaining Wall Block Sizes

Common Retaining Wall Mistakes

Skipping the gravel base. This is the most common reason retaining walls lean and settle within a few years. The first course must rest on at least 6 inches of compacted angular gravel, not native soil. Soil compresses unevenly under load. Gravel compacts once and stays there. Compact the base in 2-inch lifts; do not dump 6 inches and try to compact it all at once.

No drainage pipe behind the wall. After a hard rain, saturated soil can exert hundreds of pounds per square foot of lateral pressure against your wall. A 4-inch perforated pipe at the back of the base course, sloped toward daylight at least 1/8 inch per foot and surrounded by drainage gravel, costs very little and prevents the most common structural failure mode. Do not skip it.

Building over 4 feet without an engineer. Most jurisdictions classify a retaining wall over 3 to 4 feet of exposed height as a structure requiring a building permit and an engineered design. The forces involved are significant, and a failure can injure someone or damage adjacent property. Check with your local building department before you start any wall taller than your knees.

Forgetting landscape fabric. Fine soil particles migrate into drainage gravel over time, filling the voids and turning your drainage layer into a solid mass. A layer of geotextile fabric between the soil and the drainage gravel prevents this. Wrap the fabric over the top of the drainage gravel and tuck it behind the blocks — do not let the fabric block the drainage pipe outlet. Once the wall is finished and the bed is planted, add 2–3 inches of mulch to protect the soil surface and reduce watering.

Frequently Asked Questions

How many blocks do I need for a retaining wall?

Divide your wall length in inches by block length to get blocks per course, then divide wall height in inches by block height to get the number of courses. Multiply those two numbers and add 10% for waste. A 20-foot wall, 24 inches tall, using 16×6-inch blocks needs about 66 wall blocks and 17 cap blocks. Use the calculator above for an instant result with full materials.

How tall can a retaining wall be without engineering?

Most jurisdictions allow 3 to 4 feet of exposed height for a DIY retaining wall without a permit or engineered plans. Above that threshold, soil loads and hydrostatic pressure require a stamped design from a licensed structural or civil engineer. Regulations vary significantly by location, so confirm the limit with your local building department before you order materials.

Do retaining walls need drainage behind them?

Yes — drainage is not optional. Water-saturated soil can exert enormous lateral pressure against a wall. Without a perforated drain pipe at the base and a 6-inch curtain of drainage gravel behind the blocks, walls push forward and eventually fail. Install a 4-inch perforated pipe, sloped at least 1/8 inch per foot toward a daylight outlet, and wrap the drainage layer in geotextile fabric.

What goes under a retaining wall base?

A minimum 6-inch layer of compacted crushed angular gravel — ¾-inch minus or #57 stone are common grades. Compact it in 2-inch lifts using a plate compactor. Rounded pea gravel or river rock does not interlock when compacted and will shift under load. Bury the entire first course of blocks so only the top edge is exposed above finished grade. If you're also setting fence posts along the wall line, the fence post concrete calculator handles the footing math per post.

How much gravel do I need behind a retaining wall?

Plan for a 6-inch-wide curtain of drainage gravel running the full height and length of the wall, directly behind the blocks. For a 20-foot wall 24 inches tall, that works out to roughly 0.7 cubic yards. The calculator computes both base gravel and backfill gravel automatically. Use the Gravel Calculator to price the fill separately if your supplier sells by the ton.