Retaining Wall Calculator

Enter the wall and your block's face size for a block count, plus the cited gravel quantities for the leveling pad under it and the drainage layer behind it. Read the height limit first: above about 4 feet — or lower, with fill or a slope behind the wall — this stops being a DIY estimate and becomes an engineering job.

When this stops being a DIY project

A retaining wall is not a stack of blocks; it is a structure holding back a mass of soil that gets heavier every time it rains. Seattle's building department states the threshold for segmental block walls directly: engineering is usually required if the wall exceeds approximately 4 feet in height, fill material is placed behind it, and surcharge loads or difficult ground conditions exist.

Read those conditions as cumulative risk rather than a checklist you must fail completely. A 3-foot wall with a bank rising behind it, a driveway at the top, or groundwater in the slope can need designing just as much as a taller one on flat, dry ground. A surcharge, in the cited definition, is anything adding pressure to the retained soil — "sloping soils, groundwater, adjacent structures, stored materials".

Thresholds and permit rules are local, and some jurisdictions measure height from the bottom of the footing rather than from grade, which quietly catches walls that look under the limit. Seattle is quoted here because it says it clearly, not because it governs your site. Check with your own building department before ordering — and note that codes still apply to work that needs no permit.

How this is calculated

Wall face = length × exposed height, and one block's face = its length × its height.
Blocks = wall face ÷ block face, rounded up. Both numbers come from you, so the arithmetic is checkable.
Leveling pad = wall length × pad width × 6 in, at the cited thickness, with the pad a foot wider than the block is deep.
Drainage aggregate = wall length × height × 12 in, the cited minimum depth of clean stone behind the blocks.

Data used, with sources

Value	Used as	Source
6 in	gravel leveling pad under the bottom course — at least 6 in thick, and extending at least 6 in in front of and behind the lowest block	National Concrete Masonry Association (NCMA)
12 in	clean drainage aggregate behind the wall — at least 12 in deep, measured horizontally back from the blocks (1 in minus crushed stone; not a substitute for a designed drain)	National Concrete Masonry Association (NCMA)
Engineering threshold — about 4 ft of wall height	a limit on what this calculator is for, never a preset — height at which engineering is usually required — approximately 4 ft, and lower if fill is placed behind the wall or any surcharge exists (NOT a depth: a limit on what this calculator is for)	Seattle Department of Construction and Inspections (SDCI)

Why we ask for your block's size

Every other calculator here starts from a cited figure — the brick calculator can count a wall from published tables because brick sizes are standardized. This one cannot, and saying so is more useful than pretending otherwise. Segmental retaining wall units are proprietary products — Keystone, Versa-Lok, Allan Block and dozens of regional makes — and the standard that governs them, ASTM C1373, specifies how they are manufactured rather than how big they are. There is no industry "blocks per square foot" to cite, because there is no industry block.

So the dimensions are yours to supply, exactly as bag sizes are on the other pages, and the count becomes arithmetic you can verify rather than a number you have to trust. One consequence worth planning for: block is usually sold by the pallet, and part-pallet quantities are often priced badly, so ask what a pallet holds before you decide how much rounding up costs you.

The gravel is not optional, and it is not a drain

The two gravel quantities on this page do different jobs. The leveling pad — at least 6 inches of well-compacted gravel, extending at least 6 inches in front of and behind the bottom block — spreads the weight of the whole stacked column and gives you a flat, drainable surface to start from. A wall built on unprepared soil telegraphs every settlement into the courses above. A compacted pad also consumes more loose stone than its finished thickness — the crushed stone calculator shows that allowance as its own line.

The drainage aggregate behind the wall, at least 12 inches of clean 1-inch-minus stone, relieves water pressure at the face and keeps the retained soil from washing through the joints. The source is careful, and so are we: that gravel "alone is not meant to be a primary drainage path". Water still needs somewhere to go, which means a perforated pipe and an outlet that daylights — designed for the site, not sized by this page.

Common mistakes

Judging the wall by its exposed height. Permit thresholds are often measured from the bottom of the footing, and the buried course counts.
Treating the gravel as the drainage. It relieves pressure; it does not carry water away. That needs a pipe and an outlet.
Ignoring what is above the wall. A slope, a driveway, or a shed behind it is a surcharge, and it changes the engineering answer even on a short wall.
Skipping the buried first course. It is what stops the base kicking out. Your block maker publishes the depth.
Building tall in one go instead of terracing. Two short walls with a bench between them are often within DIY reach where one tall wall is not — though spaced too closely, the upper wall surcharges the lower one.

Frequently asked

How many blocks do I need for a retaining wall?

Wall length × wall height gives the face area; divide by the face area of one block. That second number is the one no calculator can look up for you: segmental retaining wall block is proprietary, and a 12 × 8 inch face and an 18 × 6 inch face give very different counts for the same wall. Read the dimensions off the product or its spec sheet and enter them — the arithmetic above is then something you can check by hand.

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

Roughly 4 feet, and less than that in common situations. Seattle's building department puts it plainly for segmental block walls: "Engineering is usually required if the wall exceeds approximately 4 feet in height, fill material is to be placed behind the wall, and surcharge loads and/or difficult ground conditions exist." Note that those conditions are additive rather than alternative — a shorter wall with a slope above it, a driveway behind it, or groundwater in the bank can need engineering too. Thresholds and permit rules vary by jurisdiction; Seattle is quoted here because it states the rule clearly, not because it governs your site. Ask your own building department before you buy block.

What is a surcharge?

Anything that presses down on the soil the wall is holding back, adding to the load the wall must resist. The cited definition covers "sloping soils, groundwater, adjacent structures, stored materials" — so a bank that keeps rising behind the wall, a parking area, a shed, a pool, or a neighbouring foundation all count. It is the single most common reason a wall that looks modest still needs designing, and the reason a wall that stood for a decade can fail after someone regrades above it.

How much gravel goes under and behind the wall?

Under: a leveling pad at least 6 inches thick, extending at least 6 inches in front of and behind the bottom block — so a foot wider than the block is deep. Behind: at least 12 inches of clean 1-inch-minus crushed stone, for the full height. The calculator sizes both, and the drainage figure is worth reading carefully, because the source is explicit that this gravel "alone is not meant to be a primary drainage path". It relieves water pressure at the face; a designed drain with somewhere to go is a separate thing.

Why is the buried first course not in the count?

Because no universal figure exists to cite. Walls are built with the bottom course below finished grade so the base cannot kick out, but how deep depends on the wall height, the slope in front of it, and the frost and soil conditions — the NCMA guide says the pad and drainage depend on "site conditions, water sources, embedment depth" and "have to be defined by the wall designer". Your block manufacturer publishes an embedment rule for their product; follow it. It will add both blocks and excavation to what this page shows.

Do I need a permit?

Often, and the rules are local. The threshold that triggers one is commonly the same 4-foot height, sometimes measured from the bottom of the footing rather than from grade — which catches walls that look shorter than they are. Worth knowing: codes still apply where no permit is needed. Seattle states it directly, that all such work must comply with applicable codes "even where no permit or approval is required". A wall built without a permit is not a wall exempt from the rules.

Sources

Segmental Retaining Wall Installation Guide — National Concrete Masonry Association (NCMA), accessed Aug 11, 2026.
Director’s Rule / Tip 508: Grading and Retaining Wall Requirements — Seattle Department of Construction and Inspections (SDCI), accessed Aug 11, 2026.

Quantities on this page are planning estimates, not engineering advice or a purchase guarantee — product sizes vary by producer, so confirm coverage with your supplier before ordering. A retaining wall is a structure. Above roughly 4 feet, or with any fill, slope, or surcharge behind it, the cited guidance says it needs engineering — and most jurisdictions require a permit. This page is a shopping estimate, not a design, and nothing on it substitutes for a qualified engineer or your local building department.