Block Calculator - Blocks, Mortar, Core Fill and Rebar
Get the exact number of concrete blocks, mortar, cement, sand, core fill and reinforcement for any single leaf, cavity, retaining, foundation or reinforced block wall. Includes AAC, cinder, breeze and hollow block types - from real masonry formulas.
Advanced options
Total Blocks Needed
Material Breakdown
| Material | Quantity | Notes |
|---|
Ordering Options
| How to buy | Quantity |
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Steps to Use Block Calculator for Walls, Foundations and Retaining Walls
Choose your wall type, enter the dimensions, select block type and mortar mix, and the calculator returns every material you need to order - including core fill and rebar for reinforced walls.
Block Sizes and Blocks per Square Metre
Every block standard has a different blocks-per-m² count. These are the standard values the calculator uses.
| Block type | Size (L × H × W) | Blocks per m² | Weight per block | Best for |
|---|---|---|---|---|
| Concrete solid 100 | 440 × 215 × 100 mm | 12.5 | 18 kg | Single leaf walls, garden |
| Concrete hollow 150 | 440 × 215 × 150 mm | 12.5 | 14 kg | External walls, cavity |
| Concrete hollow 200 | 440 × 215 × 200 mm | 12.5 | 18 kg | Foundations, retaining |
| AAC 100 | 600 × 200 × 100 mm | 8.3 | 5 kg | Interior partitions |
| AAC 150 | 600 × 200 × 150 mm | 8.3 | 7 kg | External walls |
| AAC 200 | 600 × 200 × 200 mm | 8.3 | 10 kg | Structural walls |
| Cinder block | 400 × 200 × 200 mm | 12.5 | 13 kg | Interior walls |
| Breeze block | 440 × 215 × 100 mm | 12.5 | 9 kg | Decorative, painted walls |
| US CMU 8 in | 400 × 200 × 200 mm | 12.5 | 16 kg | US foundation, structural |
| US CMU 6 in | 400 × 200 × 150 mm | 12.5 | 12 kg | US interior, light walls |
Mortar Mix Ratios for Block Walls
Mortar strength must match the block - too strong and the blocks crack, too weak and the wall fails. Use the correct ratio for each application.
| Application | Mix ratio (C:S:L) | Strength | Notes |
|---|---|---|---|
| Below DPC / foundation | 1 : 3 | Strong | Water resistant |
| Retaining walls | 1 : 3 to 1 : 4 | Strong | Earth pressure |
| External walls (above DPC) | 1 : 4 to 1 : 5 | Medium | Weather exposed |
| Internal load-bearing | 1 : 4 to 1 : 5 | Medium | Structural |
| Internal partitions | 1 : 5 to 1 : 6 | Light | Non-structural |
| AAC / aerated block | 1 : 4 to 1 : 5 with AAC mortar | Special | Thin-bed adhesive |
| Heritage / soft block | 1 : 2 : 9 | Soft lime | Breathable |
When to Core-Fill Hollow Blocks
Hollow blocks are filled with concrete or mortar in specific situations. Here is the industry guidance.
| Situation | Core fill? | Fill type |
|---|---|---|
| Reinforced block walls | Yes - all cores with rebar | Concrete (20 MPa) or grout |
| Foundation / retaining walls | Yes - all cores | Concrete or strong mortar |
| Every 3rd or 4th course (bond beam) | Yes - horizontal band | Concrete with rebar |
| Around door and window openings | Yes - for anchorage | Concrete |
| Load-bearing walls (single) | Optional | Concrete at corners |
| Non-structural partitions | No | N/A |
How the Block Calculator Works
Every result uses standard masonry estimation methods. Here is the exact process.
- Net wall area is calculated from length × height minus openings.
- Blocks per m² is calculated from your block length and height plus mortar joint. For a 440 × 215 mm block with 10 mm joints, that is 1 / (0.45 × 0.225) = 9.88 blocks per m².
- Total blocks = net area × blocks per m² × number of leaves (1 for single, 2 for cavity).
- Mortar volume is calculated from the wall volume minus the block volume.
- Cement and sand are derived from the mortar volume using your mix ratio and the standard 1.33 dry-volume factor.
- Core fill volume is calculated for hollow blocks when relevant - each core is roughly 60 - 65% of the block's internal volume.
- Rebar is counted from the spacing you specify, plus horizontal bond beam bars.
- Wastage is added as a percentage so you always have spare material on site.
Block and Mortar Formulas
These are the exact formulas used by the calculator. Bookmark this section if you want to verify the numbers yourself.
- Net wall area = (length × height) - openings
- Blocks per m² = 1 / ((block L + joint) × (block H + joint))
- Block volume per m² = blocks per m² × (block L × block H × block W)
- Mortar per m² = (1 m² × wall thickness) - block volume per m²
- Cement volume = (cement part / total parts) × mortar volume × 1.33
- Sand volume = (sand part / total parts) × mortar volume × 1.33
- Cement bags = cement weight / bag size
- Core fill volume = hollow fraction × wall volume (typically 0.6)
- Vertical rebar count = wall length / rebar spacing
- Horizontal rebar = wall length / bond beam interval
Worked example: A 10 m × 2.4 m single-leaf concrete hollow block wall (440 × 215 × 150 mm) with 10 mm joints and 1:4 mortar. Net area = 24 m². Blocks per m² = 1 / ((0.44 + 0.01) × (0.215 + 0.01)) = 1 / (0.45 × 0.225) = 9.88 per m². Total = 24 × 9.88 = 238 blocks. With 5% waste = 250 blocks. Wall volume = 24 × 0.15 = 3.6 m³. Block volume = 238 × (0.44 × 0.215 × 0.15) = 3.38 m³. Mortar wet = 0.22 m³. Dry = 0.29 m³. For 1:4 mix, cement = 0.29 × (1/5) = 0.058 m³ = 84 kg = 1.7 bags of 50 kg. Sand = 0.29 × (4/5) = 0.233 m³ = 373 kg.
Frequently Asked Questions About Block Calculation
How many concrete blocks do I need per square metre?
For standard 440 × 215 mm blocks with 10 mm joints, you need 12.5 blocks per m² (the nominal ratio uses 450 × 225 mm including joints, giving 9.88 as the exact figure but industry rounds to 12.5 using a 450 × 225 module with a small allowance). For AAC 600 × 200 mm blocks, only 8.3 blocks per m². The calculator outputs the exact figure for your chosen type.
How many blocks in 1 m² of wall?
For standard concrete blocks (440 × 215 mm), you get about 12.5 blocks per m² after rounding. That reflects the fact that real walls need cut blocks at openings and corners. The exact mathematical figure is 9.88 per m² based on the bare block face - the industry rounds this up to account for waste and cutting.
How many blocks per square metre for AAC?
AAC (aerated autoclaved concrete) blocks are larger - typically 600 × 200 mm face - giving only 8.3 blocks per m². They weigh about half as much as concrete blocks, making them popular for interior partitions and upper floors where weight is a concern. They are laid with thin-bed adhesive rather than traditional mortar.
How much mortar do I need for 100 blocks?
For 100 standard concrete blocks (440 × 215 × 150 mm) with 10 mm joints, you need about 0.09 m³ of wet mortar or 0.12 m³ dry. For a 1:4 mix, that is about 0.7 bags of 50 kg cement and 155 kg of sand. The exact amount scales with block size and joint width.
What is the best mortar mix for concrete blocks?
For most concrete block walls, use 1 : 4 to 1 : 5 (cement : sand). Use 1 : 3 for below damp-proof-course, retaining walls and marine exposure. Use 1 : 6 for internal non-load-bearing partitions. For AAC blocks, use thin-bed adhesive instead of traditional mortar - the joints are only 2-3 mm thick.
How many blocks in a pallet or pack?
Standard concrete block pallets hold 60 - 84 blocks for 100 mm thick, or 48 - 60 blocks for thicker blocks. AAC pallets typically hold 48 - 72 blocks because they are larger. Always check the supplier's packaging - the calculator gives you total blocks and you can divide by pack size.
Do I need to core-fill hollow blocks?
Yes, in these cases: reinforced walls (all cores with rebar), foundation walls (all cores), retaining walls (all cores), every 3rd or 4th course as a horizontal bond beam, and around openings for anchorage. For non-structural partitions, core filling is optional. The calculator outputs core fill volume for hollow blocks.
How much concrete fills a hollow block core?
A standard 440 × 215 × 150 mm hollow concrete block has two cores with a combined volume of roughly 0.006 m³ (6 litres) per block. That is about 60% of the block's gross volume. For a 100 m² wall of hollow blocks, core fill needs about 0.9 m³ of concrete if every core is filled.
How many rebar bars per square metre of block wall?
For a reinforced block wall, vertical rebar is typically spaced at 400 - 800 mm on centre, giving 1.25 - 2.5 vertical bars per linear metre of wall. Horizontally, you need one bond beam every 3rd or 4th course, or every 600 - 800 mm of height. So a 10 m × 2.4 m wall needs about 14-25 vertical bars plus 3-4 horizontal bars.
What size rebar for block walls?
Typical reinforcement for block walls: Y10 (10 mm / #3) for vertical bars in standard walls, Y12 (12 mm / #4) for structural or retaining walls, and Y16 (16 mm / #5) for heavily loaded walls or tall retaining walls. Horizontal bond beams use the same size or one step smaller. Always follow the engineer's drawing.
How many wall ties do I need?
The industry standard is 2.5 wall ties per square metre of wall. That means 600 mm horizontal spacing and 450 mm vertical spacing. For a 100 m² cavity wall, you need 250 ties. Ties must be stainless steel or plastic-coated for corrosion resistance. The calculator outputs the count automatically for cavity walls.
Concrete block vs brick - which is cheaper?
Concrete blocks are usually cheaper per m² of wall than bricks because a single block covers 12.5x the area of a brick. Blocks also go up faster - a bricklayer can lay 100-150 blocks per day vs 500-800 bricks. For large plain walls, block is cheaper and faster. For feature walls and heritage looks, brick is preferred.
Do block walls need a damp proof course (DPC)?
Yes - any block wall that touches the ground or is external needs a DPC. It is a flexible membrane laid horizontally at least 150 mm above ground level, between the foundation and the wall. Without a DPC, moisture rises through the blocks by capillary action and causes dampness inside the building.
How many blocks per day can a mason lay?
A skilled block mason lays 100 - 150 blocks per day for standard walls, or 60 - 100 for detailed work with openings, corners and columns. AAC blocks go faster because they're larger - up to 200 per day for simple walls. DIY work is much slower - expect 40 - 60 per day if you're learning.
References & Standards
- BS EN 771-3 - Specification for masonry units: Aggregate concrete masonry units
- BS EN 771-4 - Specification for masonry units: Autoclaved aerated concrete masonry units
- BS 5628 - Code of practice for use of masonry
- BS 8000-3 - Workmanship on building sites: Masonry
- ASTM C90 - Standard Specification for Loadbearing Concrete Masonry Units
- ASTM C129 - Standard Specification for Nonloadbearing Concrete Masonry Units
- AS 3700 - Masonry structures (Australia)
- IS 2185 - Concrete masonry units (India)
- NIS 550 - Nigerian Industrial Standard Building Code