Quick answer
Blocks = ⌈wall length × 12 ÷ block length⌉ × ⌈wall height × 12 ÷ block height⌉ × (1 + waste). A 20 ft wall 3 ft high with 16 × 6 inch blocks has 15 per course and 6 courses, so 95 blocks with 5% waste.
Enter the wall length and height, block size and waste. The calculator returns blocks, blocks per course and courses. Use this retaining wall calculator to check your own figures.
Related tools: Sod Calculator: Pieces of sod for a lawn. Lawn Mowing Time Calculator: Time to mow a lawn from its area, mower width and walking speed. Grass Seed Calculator: Pounds of seed for a lawn at your seeding rate.
How to use the Retaining Wall Calculator
- Enter wall length (ft) (above 0).
- Enter wall height (ft) (above 0).
- Enter block length (in) (above 0).
- Enter block height (in) (above 0).
- Enter waste allowance (%) (between 0 and 100). It starts at 5.
- Press Calculate to see the result, and use Reset to start again.
| Input | What to enter | Allowed values |
|---|---|---|
| Wall length | A number, in ft | > 0 |
| Wall height | A number, in ft | > 0 |
| Block length | A number, in in | > 0 |
| Block height | A number, in in | > 0 |
| Waste allowance | A number, in % | ≥ 0, ≤ 100, starts at 5 |
Formula and method
Formula
blocks = ⌈length × 12 ÷ block length⌉ × ⌈height × 12 ÷ block height⌉ × (1 + waste)
IEEE-754 double precision; shown to the decimals indicated.
Face blocks only; caps, base gravel, drainage stone and geogrid are separate. Walls over about 4 ft usually need engineering.
Worked examples
These examples are verified: each one is an automated test case that the live calculator must reproduce exactly.
Example 1
Inputs: Wall length = 20; Wall height = 3; Block length = 16; Block height = 6; Waste allowance = 5.
| Result | Value |
|---|---|
| Blocks needed | 95 |
| Blocks per course | 15 |
| Courses | 6 |
Example 2
Inputs: Wall length = 12; Wall height = 2.5; Block length = 12; Block height = 4; Waste allowance = 10.
| Result | Value |
|---|---|
| Blocks needed | 106 |
| Blocks per course | 12 |
| Courses | 8 |
The first line is the headline result (blocks needed). Below it the retaining Wall Calculator lists blocks per course and courses, which describe the same case from other angles. IEEE-754 double precision; shown to the decimals indicated.
The count
20 ft is 240 inches, which needs 15 blocks of 16 inches per course. 36 inches of height needs 6 courses of 6 inch blocks. 90 blocks plus 5% waste is 94.5, rounded up to 95.
Safety
Tall walls need engineering: many jurisdictions require design or a permit above about 3 to 4 ft. Gravel backfill, drainage pipe and a compacted base are part of every wall but are not counted here.
How the result changes with wall length
Holding every other input at the first worked example, this table changes only wall length. It shows how sensitive the answer is to that one value; each row was produced by running this calculator.
| Wall length | Blocks needed | Blocks per course | Courses |
|---|---|---|---|
| 10 | 51 | 8 | 6 |
| 20 | 95 | 15 | 6 |
| 40 | 189 | 30 | 6 |
| 80 | 378 | 60 | 6 |
What happens with invalid input
These messages are produced by the calculator itself, so you know what to fix.
- Wall length: entering "0" shows "Wall length must be greater than 0." beside that field and no result.
- Block length: entering "0" shows "Block length must be greater than 0." beside that field and no result.
Choosing between the Retaining Wall Calculator and similar calculators
Each row shows what a related calculator on this site asks for and what it returns, so you can pick the one that matches your question.
| Calculator | You enter | It returns |
|---|---|---|
| [Sod Calculator](/construction/sod-calculator/) | area to cover, area of one piece or roll and waste allowance | Pieces needed |
| [Lawn Mowing Time Calculator](/construction/lawn-mowing-calculator/) | lawn area, cutting width and walking speed | Mowing time |
| [Grass Seed Calculator](/construction/grass-seed-calculator/) | area to seed and seeding rate | Seed needed |
| [Polymeric Sand Calculator](/construction/polymeric-sand-calculator/) | paver area and coverage per bag | Bags needed |
| [Gravel Volume Calculator](/construction/gravel-volume-calculator/) | length, width and depth | Material needed |
Background: construction estimates
Material quantities are geometry plus a margin. Calculate the exact volume or area, add an allowance for waste, spillage, cuts and uneven ground (calculator pages commonly mention 5 to 10 percent, a rule of thumb that varies by material and job) and round up to whole units. One cubic yard is exactly 27 cubic feet.
Suppliers often have minimum order sizes and may charge for small loads, so confirm quantities, product yields and local code with your supplier or a qualified estimator before ordering.
Assumptions and limitations
- Tall walls need engineering
Quantities are estimates. Add allowances for waste and site conditions, and follow local codes and a qualified professional's instructions for structural or safety-critical work.
If a required value is empty, not a number, or outside its allowed range, the form marks that field with a message and shows no result instead of a misleading number. The automated checks deliberately try invalid entries for wall length and block length to confirm this.
Frequently asked questions
How much base gravel do I need?
A trench about 6 inches deep, compacted.
Do I need a cap block?
Cap blocks are usually counted separately.
What about curves?
Curved walls need more blocks.
What does the Retaining Wall Calculator calculate?
Blocks for a segmental retaining wall. It returns Blocks needed, Blocks per course and Courses.
What formula does the Retaining Wall Calculator use?
blocks = ⌈length × 12 ÷ block length⌉ × ⌈height × 12 ÷ block height⌉ × (1 + waste). Face blocks only; caps, base gravel, drainage stone and geogrid are separate.
What do I need to enter in the retaining Wall Calculator?
You enter Wall length (ft), Wall height (ft), Block length (in), Block height (in) and Waste allowance (%).
How is the result rounded or limited?
IEEE-754 double precision; shown to the decimals indicated. Tall walls need engineering.
What is an example of a retaining Wall result?
With wall length 20, wall height 3, block length 16, block height 6 and waste allowance 5, the blocks needed is 95.
Can I rely on this calculator for a decision?
Quantities are estimates. Add allowances for waste and site conditions, and follow local codes and a qualified professional's instructions for structural or safety-critical work. For this calculator in particular: Tall walls need engineering.
Related calculators
- Sod Calculator — Pieces of sod for a lawn.
- Lawn Mowing Time Calculator — Time to mow a lawn from its area, mower width and walking speed.
- Grass Seed Calculator — Pounds of seed for a lawn at your seeding rate.
- Polymeric Sand Calculator — Bags of polymeric sand for paver joints.
- Gravel Volume Calculator — Cubic yards (and tons, if you know the material weight) of gravel, sand, soil or mulch to fill an area.
- Wall Area Calculator — Wall area of a rectangular room after openings.
Sources
- OpenStax, Prealgebra 2e (Rice University): fractions, decimals, percents, ratios and rounding
- NIST Special Publication 811, Guide for the Use of the International System of Units (SI)
Content last reviewed 2026-11-04. Formulas and worked examples are checked by automated tests against independently computed values.