How Much Concrete Do You Need for a Shed Slab? A Step-by-Step Guide

Nothing derails a weekend project faster than running short on concrete halfway through a pour. Or worse, ordering too much and staring at a leftover cubic metre that the truck driver is very unhappy about taking back. Getting the quantity right before you call the supplier is just basic prep — and the maths is simpler than most people expect.

This guide walks through how to calculate cubic metres for any shed slab, what thickness to use, when to call a truck vs mixing bags yourself, and the mistakes that catch people out.

The basic formula

Concrete volume is just length times width times depth. All in metres:

Volume (m³) = Length (m) × Width (m) × Thickness (m)

Convert your thickness from millimetres to metres first. So 100mm becomes 0.1m, 125mm becomes 0.125m, 150mm becomes 0.15m.

Example: a 6m × 4m shed at 100mm thick:

6 × 4 × 0.1 = 2.4 m³

Use our Concrete Calculator to run these numbers instantly for any slab shape.

Always add 10% for waste

You will never pour a perfectly dimensioned slab. The ground isn't level, the formwork flexes slightly, and some concrete sticks to the chute and tools. Standard practice is to add 10% to your calculated volume before ordering.

2.4 m³ + 10% = 2.64 m³ — round up to 2.7 m³ when ordering.

Suppliers generally don't deliver in perfectly round numbers anyway, but having that buffer means you're not frantically topping up with premix bags at 4pm on a Saturday.

How thick should your shed slab be?

This is where most DIYers underspec and regret it later. Here's a practical guide:

Going thicker than you need adds cost, but going thinner causes cracking under load — especially if the subbase isn't perfect. On sandy or soft soil, increase thickness and make sure you've compacted the base properly first.

What strength concrete do you need?

Concrete strength is measured in MPa (megapascals). For residential shed slabs in Australia, you'll typically specify:

• 20 MPa: Fine for garden sheds and light foot traffic areas
• 25 MPa: Standard for garages and workshop floors — the most common choice
• 32 MPa: For heavier loads or reactive soils — your concretor might recommend this if conditions warrant it

Higher MPa costs more per cubic metre, but the difference between 20 and 25 MPa is typically only $10–$20/m³. Worth the upgrade for anything you're parking on.

Common shed slab sizes and concrete quantities

Premix bags vs readymix truck: which should you use?

This is the question everyone asks, and the answer depends on how much concrete you need.

Premix bags (20kg or 25kg bags from Bunnings):

• A 20kg bag makes roughly 0.01 m³ of concrete
• So 100 bags = 1 m³. That's a lot of mixing.
• At around $7–$9 per bag, 1 m³ costs $700–$900 in bags alone
• Only practical for very small pours under 0.3–0.5 m³

Readymix truck (premixed concrete delivered):

• Costs roughly $250–$350 per m³ delivered (varies by state and supplier)
• Much better quality — professionally mixed, consistent, properly hydrated
• Most suppliers have a minimum order (often 0.2–0.5 m³) plus a short-load fee for small quantities
• For anything over 0.5 m³, a truck is almost always cheaper and faster

For a typical 4m × 3m shed slab (1.2 m³), bags would cost $840–$1,080 plus your time mixing for hours. A truck would cost around $330–$420 delivered. The choice makes itself.

Do you need reo mesh?

For any slab you're parking on or storing heavy things under, yes. Steel reinforcing mesh (SL72 or SL82) sits in the middle of the slab and prevents cracking from loads and ground movement.

F72 mesh sheets are 6m × 2.4m and cost around $70–$90 each. For a 6m × 4m slab you'd need roughly two full sheets, allowing for 200mm overlap. It's not expensive and it's worth it.

For a simple garden shed where the heaviest thing going in is a lawnmower and some potting mix, you can skip the mesh on stable ground. But if you're in Melbourne on reactive clay soil, use it regardless.

The prep work that matters more than the concrete

A slab is only as good as what's under it. These steps get skipped and cause regret:

• Excavate enough: Remove topsoil and any organic material. You want to pour onto subsoil or compacted fill, not on lawn.
• Compact the base: Hire a plate compactor for a day. Soft fill under a slab = cracking guaranteed.
• Lay a vapour barrier: 200 micron polythene sheeting under the slab stops moisture wicking up. Essential for any shed you're putting anything valuable in.
• Level your formwork: Get a decent spirit level. Wonky formwork means a wonky slab, and you'll be explaining it to yourself every time you roll something in.

Estimating the total cost of a shed slab

Using a 6m × 4m slab at 100mm thick as an example:

The DIY saving is real, but pouring concrete is physical work and there's a small window to get it right once it starts setting. If you've never done it before, doing a smaller project first (like a garden path) is good practice.