Retaining Wall Calculator
Turn wall and block sizes into block counts, backfill gravel and cost.
Wall and block sizes in metric or imperial units, twenty currencies at live rates.
Updated October 23, 2026
What the page works out
A block retaining wall is bought in three separate orders: the wall blocks, the cap row if the wall gets one, and the gravel that sits behind it. This page turns four measurements — the wall’s height and length, and the height and length of one block — into those three quantities, and then into money.
The block count is not wall area divided by block face. It is whole rows multiplied by whole columns, each rounded up on its own, because you cannot lay two thirds of a block at the end of a row and you cannot buy two thirds of one either. That is also why the printed number is safe to order from: it already covers the last, partly filled row and column.
The gravel is the part that is easy to forget and awkward to re-order. The page carries it through the volume of the backfill layer, the weight of that volume at the density it uses for loose dry gravel, and the price of that weight — because gravel is usually invoiced by the tonne or the ton, not by the cubic metre you dug out.
Most of the quantities can also be read backwards. If the yard quotes a total for the wall blocks, type that total instead of the unit price and the page works out the per-block price the quote implies; give it a delivery weight of gravel and it works out the volume that weight fills. The two block counts are the exception: they are rounded up from your measurements and are not solved from a total.
84 + 15
Wall blocks and cap blocks for the wall worked through below
2,693 kg
Gravel behind that wall: 2.001 m³ at 1,346 kg/m³
20 currencies
For the block price, the cap price and the gravel price
Quick start
Answer the cap-row question first
A cap row is the decorative row along the top of the wall, usually a slightly wider block that overhangs the face. It changes the block order, not just the finish, so decide before you type any measurements: Yes counts a cap row in addition to the wall blocks, No counts the wall blocks alone.
Enter the wall’s height and length
The height is measured from the base of the bottom block to the top of the highest wall block. On a wall that steps down a slope, measure the tallest section, because the whole wall has to be covered to that height.
The length is the finished run of the wall. The height box offers centimetres, metres, inches, feet, feet and inches, or metres and centimetres; the length box adds yards. One measurement can be in a different unit from the other — a wall height in metres with a wall length in feet is fine.
Enter the height and length of one block
These two boxes start empty on purpose, because there is no standard block: a garden wall can be built from a 20 cm high block or a much smaller masonry unit. Use the face size the block presents when it is laid — the height and the length along the wall, not the depth into the wall.
The block menus offer centimetres and inches, which covers both the metric sizes and the imperial ones. Both numbers have to be filled in before any block count appears.
Set the gravel layer behind the wall
The thickness starts at 30 cm (12 in), which is the minimum the page is built around: a thinner layer does not carry water away from the blocks and gets a warning rather than being quietly accepted. The backfill length and height have no starting value and are not copied from the wall, so enter them yourself — in most cases they are the wall length and the wall height, or a little more if the gravel extends past the ends.
Add the prices you have, or skip them
Each material can be priced separately and each is optional: the wall block price, the cap block price when there is a cap row, and the gravel price per unit of weight. Pick the currency, and for the gravel pick the weight unit the yard actually quotes in — kilograms, pounds, tonnes or short tons are all there.
Read the order on a real wall
A terrace wall 1.15 m high and 5.8 m long, built from 20 cm by 40 cm blocks, with a cap row, the default 30 cm gravel layer, blocks at 4.50 each, caps at 6.00 each and gravel at 0.08 per kilogram:
Rows and columns
Blocks to buy
Gravel volume and weight
Money
The totals on the page read 215.47 and 683.47, rounded to cents.
One wall, four results
84 wall blocks and 15 cap blocks are what you order, 2.001 m³ is the gravel to have delivered, about 2,693 kg is what the delivery note will say, and 683.47 is the material bill. If the delivery weight disagrees with the supplier’s figure, the gravel density is the first thing to check, not the volume.
The cap row and the block order
The cap row is the one answer on the page that changes which blocks you buy, so it is worth seeing what it does to the arithmetic rather than just to the look of the wall.
With No, the wall blocks are the whole wall: rows multiplied by columns. With Yes, the page counts one column fewer of wall blocks per row, as if the cap row overhung the end of the wall and the wall behind it stopped one block short, and then adds one cap block for every column.
A 1 m high, 3 m long wall of 20 cm by 40 cm blocks is five rows of seven and a half blocks. The half block becomes an eighth column, and then the two answers part company:
Without a cap row
With a cap row
So the same wall is 40 pieces one way and 43 the other, and the difference is not only the three extra pieces: the cap blocks are usually the more expensive unit, and the two answers are priced in separate boxes on the page.
Switching the answer clears the cap numbers
The cap block count, the cap block price and the cap block cost only belong to the Yes answer. Answer No and those three boxes leave the form with their values, so switching back to Yes asks for the cap price again. If you are comparing the two answers on paper, note the cap figures down before you switch.
When the cap rule does not fit the block system
The page applies one rule to every wall: the cap row is one block per column and the wall behind it is one block shorter per row. A mitred corner return, a flush cap that sits inside the wall line, or a cap that overhangs both ends will need an adjustment — check the manufacturer’s laying diagram and add or subtract the difference on the order, not in the walls you have already measured.
How the numbers are built
Most results here are a single product or a single quotient of numbers you can see, which is what lets the page work from either end: a quoted total gives back the unit price, a delivery weight gives back the volume. The block counts are the deliberate exception, because rounding up has no honest inverse.
Rows from the wall height and the block height
A wall 1.15 m high with 20 cm blocks is 5.75 rows, which is bought as 6. The ceiling is why the last row is never short.
Columns from the wall length and the block length
The same rounding applies along the wall. Both counts are read from the two sizes you measured — the page will not suggest a block size that would fit a whole number of columns, so measure the block you actually intend to buy.
Wall blocks without a cap row
Rows times columns, both already rounded up. With a cap row the first factor stays and the second loses one, exactly as in the section above.
Volume of the gravel layer
Thickness, length and height of the backfill — not of the wall, although in most walls those are nearly the same numbers. Give the volume a quoted figure and the page will find one of the three lengths if the other two are known.
Weight at the density the page uses
One density is used throughout: 1,346 kg/m³, which is about 84.03 lb/ft³. That is what turns the volume you have to dig into the weight you have to pay for.
Cost of the wall blocks
The same product gives the cap block cost, with the cap count and the cap price. Give the page the total instead and it recovers the per-block price the total implies.
Cost of the gravel
The price is per unit of weight, with the weight unit chosen next to the currency, so a quote per ton and a quote per kilogram both work without converting anything by hand.
Total expenses
With no cap row the cap term is simply absent. Type a budget and the page takes it apart instead: with everything else filled in, it reports the wall block cost that budget allows.
Units are converted before anything is multiplied
A wall height in feet, a block height in inches and a gravel thickness in centimetres are all converted to the same base unit before the arithmetic, and the same happens to volumes and weights. The same wall entered in metric and in imperial units gives the same counts — what changes is only how the numbers are written on screen.
Why a total is a safer anchor than a unit price
A quoted total is a number someone committed to on paper, while a unit price is often a rounded conversation. Typing the total and reading the per-block price back is a quick way to see whether the quote is consistent with the block count — and if the block count is wrong, the per-block price will look wrong by exactly the same factor.
Two walls, worked through
Both examples use the same page in a different unit system and with a different answer to the cap-row question, so they double as a check that the answer really does change the order.
A 12 ft wall with no cap row, gravel sold by the short ton
Situation: A garden wall in feet and inches, built from 6 in by 12 in blocks, with the gravel layer priced per US short ton. The yard’s gravel quote arrives as a total, so the per-ton price is what we want to recover.
Inputs
Result
Five rows (30 in ÷ 6 in) by twelve columns (144 in ÷ 12 in), so 60 wall blocks and no cap blocks. The gravel layer is exactly 30 ft³:
The page shows 2,520.84 lb, which is 1.26 US short tons — and at 45 per short ton that is 56.72 of gravel. With the blocks at 3.20 apiece the wall blocks come to 192.00 and the total is 248.72.
Now clear the gravel price, type the 60 the yard quoted as the gravel total, and read the price box: it comes back as 47.6 per US short ton. The quote is about six percent above the rate you assumed.
A 6 m wall, priced with and without the cap row
Situation: The same 1 m high, 6 m long wall of 20 cm by 40 cm blocks, priced at 4.50 for a wall block and 6.00 for a cap block. The question is not how many blocks but what the cap row adds to the block bill.
Inputs
Result
Five rows and fifteen columns. Without a cap row that is 75 wall blocks at 4.50, which is 337.50. With a cap row the wall blocks drop to five rows of fourteen — 70 blocks, 315.00 — and fifteen cap blocks are added at 6.00:
So the cap row adds 67.50 to the block bill, about a fifth again, before the gravel is counted at all.
Neither example needs a separate gravel calculation: the same three backfill boxes answer both, and once the volume is settled the weight follows from one density. Change the wall length and the volume changes with it, which is why the backfill length is measured along the wall rather than copied from it.
Ordering tips
The arithmetic is repeatable; the measurements that feed it are not. These are the points that decide whether the delivery matches the wall.
Measure the tallest section
A wall that runs along sloping ground has one height at the high end and another at the low end, and its top is usually level. The row count has to cover the tallest stretch, so measure there rather than in the middle, and if the wall steps in sections, work out each section separately and add the blocks together.
Watch the rounding on small walls
Rounding up happens twice — once for rows, once for columns — and on a small wall that is a real percentage. Adding 10 cm to the height of the terrace wall above moved it from 5.75 to 6.25 rows, which is a seventh row: 14 more wall blocks and 234 kg more gravel. Measure the height, then look at the row count before you order.
Keep spares for cuts and breakage
The block count is whole blocks only — it contains no allowance for cutting the last column, for a chipped block, or for the corner where the wall returns. A few spares of each type are cheaper than a second trip, and left-over wall blocks are the ones you will want in two winters when one cracks.
Order gravel by the weight the page gives you
Suppliers invoice gravel by the tonne or the short ton, and the weight on the page is exactly what those two need. It is built on one density — 1,346 kg/m³ for loose dry gravel — so a wet load or a different aggregate will weigh more or less for the same volume. For a different material at a different density, the gravel calculator works from area and depth with a density menu.
One more habit, from the money side: price the three materials separately before you look at the total. A total that looks steep is usually one line — a cap block price copied from a different size, or a gravel price entered per tonne when the yard quotes per short ton — and the separate lines show which one moved.
Questions
I typed a total for the wall blocks and the price box stayed empty. Why?
Almost certainly because that wall needs no wall blocks at all. With a cap row, a wall that is one block long has a top row of cap blocks and no wall blocks beneath it: the count is zero, so the total cannot be divided by it and the price box has nothing to show.
Check the wall length against the block length. If the wall really is one block long, the cap row is the whole purchase and the cap price is the number to fill in. The page says as much next to the box rather than inventing a price.
How much gravel is that per metre, or per square metre, of wall?
At the default 30 cm thickness, every square metre of wall face is backed by 0.3 m³ of gravel, which at 1,346 kg/m³ is about 404 kg. Per metre of a 1.2 m high wall that is 0.36 m³, or about 485 kg — so a 6 m wall of that height is roughly 2.9 tonnes of gravel before any allowance.
Those rates hold for any wall length because both scale together; only the height and the thickness change them.
Do the block counts change if I measure in inches instead of centimetres?
No. The same wall gives the same row and column counts in either system, because the lengths are converted before the arithmetic: the terrace wall above is 45.2756 in high and 19.0289 ft long, its blocks are 7.87402 in by 15.748 in, and the counts stay at 6 rows, 15 columns, 84 wall blocks and 15 cap blocks.
What does change the answer is a rounded block size. Typing 7.87 in for a 20 cm block is a slightly smaller block than the one you will buy, and on a wall whose height lands close to a whole number of rows, that difference can tip it into an extra row. Type the manufacturer’s stated size in its own unit where you can.
Can I use it for a wall that steps down a slope?
One calculation covers one rectangular wall, so treat a stepped wall as several walls: enter each section’s own height and length, note its blocks and its gravel volume, and add the sections together at the end. The heights in a stepped wall are usually all different, which is exactly why a single average height would under-order the tallest step.
How far the number goes
- This is a materials estimate, not a structural design. Footing depth, the buried base course, geogrid, drainage aggregate grading, compaction and the load behind the wall are all outside the model. A wall that holds soil is a structure: for anything above garden height, on sloping ground, or carrying a driveway or a building, have the design and the drainage checked by a qualified engineer and follow the local requirements that apply to retaining walls.
- Block sizes are nominal, and corners are not modelled. The count assumes one flat run of wall with no returns, pillars, openings or cut blocks, and no waste. A corner, a step or a curve costs extra pieces, so add them from the laying plan before the order goes in.
- The cap-row rule is fixed. The page always counts one column fewer of wall blocks and one cap block per column. A mitred return, a flush cap or a cap that overhangs both ends is a different wall, and the difference belongs on the order.
- One gravel density, whatever the weather. 1,346 kg/m³ describes loose, dry gravel. Wet gravel, a different grading or a different aggregate will weigh more or less for the same volume, and a few percent of weight is a few percent of the delivery and the bill. The volume stays right; the tonnage you order may not.
- The money is materials only. Delivery, pallet deposits, minimum order charges, cutting and tax are not included, and a total shown in another currency follows a published reference rate rather than the rate your bank or card will use.
- Backfill gravel is drainage, not a drainage design. The page warns below a 30 cm layer because that is the minimum it is built around, but where a wall has to deal with a spring, a slope above it or heavy clay, the water needs an outlet. If the wall needs a collector behind it, the French drain calculator sizes the trench, the pipe and the fabric that go with it.
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