Use this retaining wall block calculator to estimate straight-wall courses and a block purchase count. Enter the installed block dimensions and total block-wall height, including buried courses but excluding caps. It estimates quantity, not structural safety.
How it works
Divide wall length by installed block face length and round up to a whole block for each course. Divide total block-wall height by block height and round up to whole courses. Multiply those two rounded counts, add the selected extra percentage, then round the order up. This is a conservative straight-course layout estimate, not an area-only calculation.
Worked example
A 20 foot wall with 3 feet of total block height (including buried block, excluding caps), using example 12 x 4 inch faces: 20 blocks per course, 9 courses, 180 blocks. Add 5% to get 189.
Tips
- Walls over a few feet high, or holding a slope or a structure, may need engineering and a permit. Check local rules.
- Plan a compacted gravel base and drainage behind the wall. Use the gravel calculator.
- Block sizes differ by product. Use the actual dimensions.
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What you’ll need
A plain checklist. Needs vary by project, so follow the instructions on your materials.
- Compacted gravel base – retaining walls sit on a firm base.
- Drainage gravel and drain pipe – water behind a wall adds pressure.
- Level and string line – each course must be level.
- Rubber mallet – to seat blocks.
- Block adhesive and cap blocks – per the block maker.
- Masonry saw or splitter – for cuts.
- Landscape fabric – keeps soil out of the drainage stone.
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Estimates only. Confirm quantities and product details with your supplier before you order.
Guide: How much gravel for a retaining wall?
Guide: How many concrete blocks do I need
Landscape block estimator: use total height, not just visible height
The height above the ground is not always the height you need to buy. Buried block is still part of the wall. Allan Block’s estimating worksheet explicitly says to include buried block in the wall height and use the approved plans for final height. Its burial guidance is a minimum of 6 inches or 1 inch per foot of wall height. Do not treat that as a design for another product or for your site: slopes and project conditions can change the approved embedment.1
For a quantity example, a 2-foot visible wall plus 8 inches of buried block makes 32 inches of total block height. With an 8-inch-high block, that is four courses, not three. The calculator does not choose embedment for you. Enter the total shown on the plan, and leave cap height out of the block count.
How the block-count formula works
- Blocks per course = round up (wall length in feet x 12 / installed face length in inches).
- Courses = round up (total block height in feet x 12 / block height in inches).
- Base count = blocks per course x courses.
- Purchase count = round up (base count x (1 + extra percentage / 100)).
Allan Block publishes length divided by width, multiplied by height divided by block height, as its estimating method. It recommends an extra 5 percent and separately notes that step-ups and stairs need extra blocks. Our calculator rounds each course and row count up first so it does not hide a partial course inside a final area-based rounding. That can produce a higher count than a manufacturer’s area-based chart. Check the planned bond, cut reuse and exact product layout with the dealer.1
Free straight-wall block count chart
Example block face: 12 inches long x 4 inches high. Heights below are total block height, not exposed height. Counts include a 5 percent example allowance; caps and special units are excluded. These are our calculations, not an Allan Block product chart.
| Wall length | 1 ft total height | 2 ft total height | 3 ft total height |
|---|---|---|---|
| 10 ft | 32 | 63 | 95 |
| 15 ft | 48 | 95 | 142 |
| 20 ft | 63 | 126 | 189 |
| 25 ft | 79 | 158 | 237 |
| 30 ft | 95 | 189 | 284 |
Two more checked examples
20-foot wall, 32 inches total height, example 18 x 8-inch block: 240 / 18 = 13.333 blocks per course, rounded up to 14. Height gives 32 / 8 = 4 courses. Base count is 14 x 4 = 56. At 5 percent extra, 56 x 1.05 = 58.8, rounded up to 59 blocks. The installed layout may use end cuts instead of 14 intact full blocks per course; our purchase estimate does not assume those cuts can be reused.
15-foot wall, 30 inches total height, example 12 x 4-inch block: 180 / 12 = 15 per course. Height gives 30 / 4 = 7.5, rounded up to 8 courses. Base count is 15 x 8 = 120, and 5 percent extra makes 126. Eight complete 4-inch courses are 32 inches tall, so the counted wall is taller than the requested 30 inches. If the exact finished height matters, choose the course layout before ordering rather than assuming half a course is buildable.
Using an Allan Block calculator or product chart
This page is an independent generic estimator, not an official Allan Block tool. For a named AB product, use the dealer’s exact installed dimensions and the manufacturer’s collection chart. The Allan Block residential chart distinguishes full-height, half-height, half-width and Fieldstone units, with separate capstone quantities. An 18 x 8-inch example is not a promise that every block in those collections measures exactly that size.2
Manufacturer charts and this calculator can disagree without either formula being a typo. Allan Block’s residential chart lists 41 blocks for a 20-foot wall with three 8-inch courses in its main collection table. A hypothetical exact 18-inch face, rounded up to 14 units on each of three courses, gives 42 before any extra. The approaches differ: a collection chart is product-specific; our method budgets each whole course separately. Do not force one result to match by changing dimensions. Confirm actual dimensions and layout with the dealer.2
Caps, corners and wall materials are separate
Do not count capstones as another course of regular wall block. Estimate a straight cap row from wall length divided by the installed cap length and round up, then allow for cuts and corners according to the product. Allan Block lists capstones separately and says its capstone height adds to finished wall height. Its corner guidance also calls for special corner blocks by course, including where walls step down.2
Base rock, core fill, backfill, drain pipe and geogrid are not covered by this block count. The Allan Block worksheet uses approved plans for these materials and directs buyers to their supplier for geogrid specifications. Our retaining-wall gravel guide explains volume estimating, and the gravel calculator converts measured layers into yards or tons. Neither decides whether the wall design is safe.1
Plan the full wall, not only the block count
Our quote-comparison checklist: give each supplier or contractor the same approved layout and product. This calculator estimates regular blocks for a straight wall; it does not design the wall or decide whether it needs engineering or a permit.
- State the planned dimensions. Record each section’s length, total block height including buried courses, exposed height and separate cap height. Keep stepped sections distinct instead of quoting one average height.
- Identify all the units. List the exact block collection, installed dimensions, regular blocks, caps, corners and other special units. Ask the dealer to check quantities against the planned bond and cut layout.
- Separate the rest of the material list. Ask for base rock, core fill, backfill, drain pipe, outlets and any reinforcement specified by the plan. Keep their quantities and prices separate from the regular-block total.
- Define the included work. Ask who handles design review, permit work, excavation, spoil disposal, access, drainage connections and cleanup. Confirm exclusions and how unexpected ground conditions will be priced before comparing totals.
- Check supply and delivery terms. Ask about stock, pallets, delivery and unloading, tax, returns and extra-unit ordering. A block-only quote and a fully installed wall quote are different scopes.
Use our project planning and quote checklist to record quantities, included work and changes. These are our comparison prompts, not a wall design or claims about any supplier’s terms.
Frequently asked questions
Can I use this for curved or stepped walls?
Not as a complete estimate. Split a stepped wall into sections with their planned heights, but account separately for corners, ends, step units and cut loss. Curves change installed face coverage and bonding. Use the manufacturer’s layout method or design software for those jobs.
Is wall area divided by block face area enough?
It can be a first estimate, but it hides rounding by course. A 15-foot wall that needs 7.5 courses cannot generally be assembled from seven and a half complete rows. This calculator rounds both counts before multiplying. An engineered or manufacturer takeoff can account for cut reuse and varied units more precisely.
Does this tell me if I need a permit or engineer?
No. Height is only one factor; slopes, soil, water and nearby loads matter too. The Allan Block residential page says most municipalities require permits for walls over 4 feet, but that is not your local rule. Check with the local building official and the block manufacturer before construction. An estimating table extending to 6 feet is not permission to build that height without design review.2
Sources
- Allan Block: Retaining Wall Estimating Worksheet (manufacturer formula, buried height, 5 percent extra and material planning; read October 6, 2026)
- Allan Block: Retaining Wall Material Estimator (collection-specific charts, caps and corners, permit caveat; read October 6, 2026)
Budgeting the build? See our retaining wall cost guide for per-square-foot prices by material.