How to Calculate Concrete Block Quantities for a Garden Wall

Calculating concrete block quantities for a garden wall is straightforward once you know the standard block sizes and a simple formula. Get it right and you'll avoid costly over-ordering or frustrating mid-project shortages. Here's how to do it accurately, NZ-style.

Understand Standard NZ Concrete Block Sizes

In New Zealand, the most common concrete masonry block is the 390 mm × 190 mm × 190 mm (length × height × width) standard block. This is sometimes called a "half-block" or "standard stretcher block." Smaller sizes (e.g. 390 × 90 × 190 mm) are used for thinner walls or decorative applications.

Always confirm the exact dimensions of the block you're buying with your supplier — sizes can vary slightly between manufacturers. Your local PlaceMakers or Mitre 10 branch will have product data sheets available.

Step 1 — Measure Your Wall Dimensions

Before anything else, measure carefully:

• Length of the wall (in metres)
• Height of the wall (in metres)
• Number of wall sections (account for corners, piers, and openings like gates)

Write everything down and double-check. A tape measure error at this stage is the most common cause of quantity mistakes.

Step 2 — Calculate the Wall Area

Use this simple formula:

Wall area (m²) = Length (m) × Height (m)

For example, a wall 8 m long and 1.2 m high:

8 × 1.2 = 9.6 m²

If your wall has multiple sections, calculate each section separately and add them together. Subtract any openings (gate openings, gaps) from the total area.

Step 3 — Calculate Blocks per Square Metre

This is where standard block dimensions come in. With a 390 × 190 mm face and a 10 mm mortar joint, each block occupies:

• Face length: 390 + 10 = 400 mm
• Face height: 190 + 10 = 200 mm
• Face area per block: 0.400 × 0.200 = 0.08 m²

So the number of blocks per square metre is:

1 ÷ 0.08 = 12.5 blocks per m²

This is the widely used industry rule of thumb for standard 390 × 190 mm blocks with a 10 mm mortar joint. If you're using a different block size, recalculate using the same method.

Step 4 — Calculate Total Blocks Required

Total blocks = Wall area (m²) × 12.5

Using our example:

9.6 × 12.5 = 120 blocks

Step 5 — Add a Waste Factor

Always add a waste and breakage allowance. For a straightforward garden wall, add 5–10%. For walls with lots of cuts (corners, curves, piers), allow up to 15%.

Always round up to the nearest full block or pack quantity.

Step 6 — Account for Corners and Piers

External corners typically require special corner blocks or cut blocks. Count the number of corners and check with your supplier whether dedicated corner units are available. Piers (vertical columns) may also require a different block configuration — your supplier can advise.

Step 7 — Estimate Mortar

As a general guide, you'll need approximately one 20 kg bag of mortar mix per 10–12 standard blocks laid, though this varies with joint thickness and block type. Check the manufacturer's coverage rate on the product packaging for accuracy. Bunnings and PlaceMakers stock pre-mixed mortar products with coverage information on the bag.

NZ Building Code and Consent Considerations

Garden walls may or may not require a building consent, depending on their height and location. Under Schedule 1 of the Building Act, some low retaining walls and fences are exempt — but the rules are specific and depend on factors like height, proximity to boundaries, and whether the wall is retaining soil.

Always check with your local council before starting, as exempt work thresholds vary. The MBIE Building Performance website has guidance on when consent is required. Getting this wrong can cause problems when you sell your property.

For walls that retain soil, other structural standards may apply. A structural engineer or Licensed Building Practitioner may need to be involved.

When to Call a Professional

While calculating quantities is a DIY-friendly task, consider getting expert help if:

• Your wall is over 1 m high or retains soil — structural design may be required
• The wall is near a boundary, driveway, or slope where failure could cause damage or injury
• You're unsure about consent requirements
• You want a quantity surveyor to prepare a full material schedule and cost estimate for a larger project

A qualified quantity surveyor can save you money by minimising waste and getting supplier quotes right first time. You can find experienced local tradespeople, including quantity surveyors and builders, on tradedirectory.co.nz.

Quick Reference Formula Summary

1. Wall area = Length × Height (minus openings)
2. Blocks needed = Area × 12.5 (for standard 390 × 190 mm blocks)
3. Add 5–15% for waste
4. Round up to nearest pack
5. Estimate mortar at ~1 bag per 10–12 blocks

Sources & Further Reading

• MBIE Building Performance — When you need a building consent
• BRANZ — Building information and technical guidance
• PlaceMakers — Concrete masonry products
• Mitre 10 — Concrete blocks and mortar
• Licensed Building Practitioners — Find a licensed professional
• Standards NZ — NZ Building Standards