Retaining Wall Calculator
About Retaining Wall Calculation
The retaining wall calculator estimates the materials required for a block retaining wall based on its dimensions. It determines the number of courses and blocks, the wall height and face area, the number of cap units, and the volumes of backfill and base material. It is useful for preliminary material planning for garden retaining walls, terraces and other areas with changes in ground level.
Guidelines and recommendations
Wall height and number of courses
Total wall height is the sum of the desired visible height and the depth of block embedment. Both dimensions are entered in centimetres. Embedment is the part of the wall below ground level on its exposed side.
Htotal = Hvisible + Hembedded
Number of courses is calculated by dividing the required height of blockwork by the height of one block. If a cap course is included, its height is first subtracted from the total wall height. The height used for the blockwork calculation cannot be less than 0 cm.
Nrows = ⌈max(0, Htotal - Hcap) / hblock⌉
Here Hcap is the cap unit height in centimetres, taken as 0 cm when no cap is used, and hblock is the height of one block. The brackets ⌈ ⌉ mean rounding up to the next whole number. For example, a required blockwork height of 125 cm with 20 cm high blocks gives 7 courses.
Final wall height is calculated from the whole number of block courses, adding the cap unit height when a cap is included.
Hfinal = Nrows × hblock + Hcap
The final height can exceed the specified height because only whole courses are counted. If blocks are embedded, the visible height of the finished wall is the final height minus the embedment depth. For example, a total height of 140 cm with 10 cm of embedment leaves 130 cm above ground.
Blocks and cap units
Blocks per course are calculated by dividing the wall length by the length of one block. Wall length is entered in metres and block length in centimetres, so metres are converted to centimetres by multiplying by 100. The result is rounded up to a whole number of blocks.
Nper row = ⌈100 × L / bblock⌉
Here L is the wall length in metres and bblock is the block length in centimetres. For example, a 10 m wall with 40 cm long blocks requires 25 blocks per course.
Total block count is the number of courses multiplied by the number of blocks in each course.
Nblocks = Nrows × Nper row
Cap units are calculated separately by dividing the wall length by the length of a cap unit, again rounding up. One continuous cap course is assumed along the full wall length.
Ncaps = ⌈100 × L / bcap⌉
Here bcap is the length of one cap unit in centimetres. The calculation assumes a straight wall, blocks of uniform dimensions and no specified joint thickness. Cutting and reuse of offcuts are not taken into account. No allowance factor for breakage or waste is applied.
Block face area
Block face area is the wall length multiplied by the actual combined height of all block courses. It includes both the visible and embedded portions of the blockwork, but excludes cap units.
Aface = L × Nrows × hblock / 100
The result is in m2. Dividing by 100 converts the block height from centimetres to metres. Area is calculated from the overall wall dimensions without subtracting possible gaps between blocks.
Backfill volume calculation
Backfill volume is calculated as the volume of a rectangular material layer behind the wall. The dimensions used are wall length in metres, and layer thickness and backfill height in centimetres.
Vbackfill = L × tbackfill × Hbackfill / 10000
The result is in m3. Division by 10000 converts both centimetre dimensions to metres. By default, backfill height equals the sum of the desired visible wall height and its embedment depth. It can be changed independently of the wall dimensions.
Crushed stone or gravel is commonly used as draining backfill behind garden retaining walls. Layer thicknesses of 20-40 cm are often used, although the appropriate value depends on the structure and site conditions. The formula assumes constant thickness and height along the entire wall. No additional factors for compaction, settlement or material losses are included.
Base material volume calculation
Base volume is determined from the dimensions of a rectangular layer beneath the wall. Base length is taken as equal to wall length, while its width and thickness are entered in centimetres.
Vbase = L × bbase × tbase / 10000
The result is in m3. This calculation gives the geometric volume of a concrete, crushed-stone, gravel or sand base of constant cross-section. The base dimensions are taken from the chosen design, with no automatic increase in width or thickness.
Calculation accuracy and European standards
Assumptions. The calculator uses geometric dimensions without adjustment factors from standards. Quantities of individual units are always rounded up to whole numbers. Areas and volumes are calculated without intermediate rounding and displayed to up to four decimal places.
European standards. EN 771-3, "Specification for masonry units - Part 3: Aggregate concrete masonry units", applies to concrete masonry units. EN 13242, "Aggregates for unbound and hydraulically bound materials for use in civil engineering work and road construction", covers aggregates for certain base and drainage layers. EN 1997-1, "Eurocode 7: Geotechnical design - General rules", sets out general geotechnical design principles. These standards provide a reference for selecting materials and design solutions but do not change the calculator's geometric formulas.
FAQs
Why can the final retaining wall height differ from the requested height?
The height of a block retaining wall depends on a whole number of courses, so it does not always match the desired height exactly. If the calculation calls for part of a course, the number of courses is rounded up. If cap units are used, their height also contributes to the finished structure.
How does block embedment affect the retaining wall calculation?
The embedded portion is added to the desired visible height, increasing the total wall height. This can require another course and more blocks. Embedment need not equal the height of a whole block because only part of the bottom course may be below ground level.
Why is wall area alone not enough to calculate the number of blocks?
An area-based calculation does not account for rounding up the number of blocks in each individual course. For example, if a course length cannot be covered by whole blocks alone, an additional block may be needed for cutting. The retaining wall calculator therefore determines blocks per course first, then the number of courses, and finally the total block count.
How is the amount of backfill material calculated?
Backfill volume is calculated by multiplying wall length by layer thickness and height, and converting the result to cubic metres. The calculation assumes a uniform rectangular layer along the entire wall. If backfill thickness varies, the volumes of separate sections should be calculated and added together for a more accurate estimate.
How is the amount of base material for a retaining wall calculated?
Base material quantity is calculated from its length, width and thickness. The calculator takes the base length as equal to the wall length and calculates volume in m3. If width or thickness varies, divide the base into sections of constant dimensions and add their volumes.