If you’re wondering how much concrete do I need for a shed base, you’re asking the right question before starting your project. Getting the exact amount of concrete right saves money on materials and ensures you have enough to complete the job without delays. A properly calculated concrete slab provides the stable base your shed needs to remain level for years to come.
How to Calculate Concrete for a Shed Base
Calculating how much concrete you need for a concrete shed base involves a straightforward formula: length × width × depth. The key is converting all your measurements to the same unit before you multiply.
For a concrete slab, you’ll need to know your shed dimensions and decide on the right depth for your project. Most shed bases use a 4 inch (100mm) thick slab, though larger sheds or those storing heavy equipment may need 6 inches (150mm).
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The Basic Calculation Formula
To calculate the volume of concrete required, use this formula:
Volume = Length × Width × Thickness
When working in feet, divide the thickness in inches by 12 to convert to feet. For metric calculations in metres, divide millimetres by 1000.
For example, a shed measuring 12 × 10 feet (3.7 × 3 metres) with a 4 inch (100mm) thick slab needs: 12 × 10 × 0.33 = 39.6 cubic feet, or approximately 1.47 cubic yards (1.12 cubic metres).
Converting to Bags of Cement
Once you know your volume, calculate how many bags of premix you’ll need. A standard 60-pound (27kg) bag covers roughly 0.45 cubic feet (0.013 cubic metres). An 80-pound (36kg) bag covers approximately 0.6 cubic feet (0.017 cubic metres).
The easy way to estimate bags: multiply your cubic feet total by 2.2 for 60-pound bags, or by 1.7 for 80-pound bags.
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Concrete Quantities for Common Shed Sizes
Planning your project becomes simpler with a quick reference for standard shed sizes. The table below shows concrete needed for a 4 inch (100mm) thick concrete base with 10% added for waste.
| Shed Size (ft) | Shed Size (m) | Square Feet | Cubic Yards | Cubic Metres | 80lb Bags |
|---|---|---|---|---|---|
| 6 × 4 | 1.8 × 1.2 | 24 | 0.33 | 0.25 | 15 |
| 8 × 6 | 2.4 × 1.8 | 48 | 0.65 | 0.50 | 30 |
| 10 × 8 | 3 × 2.4 | 80 | 1.09 | 0.83 | 49 |
| 10 × 10 | 3 × 3 | 100 | 1.36 | 1.04 | 61 |
| 12 × 10 | 3.7 × 3 | 120 | 1.63 | 1.25 | 74 |
| 12 × 12 | 3.7 × 3.7 | 144 | 1.96 | 1.50 | 88 |
| 16 × 12 | 4.9 × 3.7 | 192 | 2.61 | 2.00 | 118 |
These figures assume the slab dimensions match the shed size exactly. Many builders prefer to extend the concrete base 2-3 inches (50-75mm) beyond the shed perimeter, which adds slightly to the material needed.
Choosing the Right Thickness for Your Slab
The thickness of your concrete slab depends on what you’ll store and your ground conditions. Getting the right thickness is crucial for creating a long lasting foundation that won’t crack under load.
Standard 4 Inch (100mm) Slab
A 4 inch thick slab suits most garden structures and storage buildings. This depth handles the weight of a typical structure plus stored items like garden tools, bicycles, and seasonal equipment. It’s the most cost-effective option for bases under 150 square feet (14 square metres).
Heavy-Duty 6 Inch (150mm) Slab
Increase to 6 inches if you plan to store heavy equipment, use the building as a workshop with machinery, or construct on ground prone to movement. The extra depth provides greater stability and reduces cracking over time.
Site Preparation Before Pouring
Proper groundwork is essential for a stable concrete shed base. Rushing this stage often leads to problems with the finished slab.
Clearing and Excavating
Start by marking out your site using string lines and bricks or stakes to define the perimeter. Dig out the area to accommodate your sub-base layer plus the concrete thickness. For a 4 inch slab with a 4 inch gravel base, you’ll need to dig down 8 inches (200mm) from your finished floor level.
Remove any organic material, roots, and soft spots. The ground beneath needs to be firm and compact before you lay the sub-base.
Creating the Sub-Base
Lay a 3-4 inch (75-100mm) layer of compacted gravel, ballast, or crushed stone across the excavated area. This sub-base provides drainage, prevents moisture from wicking into the concrete, and creates a stable base for pouring.
Use a plate compactor or hand tamper to compact the ballast thoroughly. Check the surface is flat using a spirit level and straight edge. Then lay a damp-proof membrane over the sub-base for additional moisture protection.
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Building the Formwork Frame
Formwork creates the mould for your concrete slab. Use 25mm (1 inch) thick timber boards cut to match your slab depth. Position the frame around the perimeter, ensuring it’s square by checking the diagonals measure the same length.
Secure the frame with wooden stakes every 600mm (2 feet). The top edge should sit at the finished slab height. Use a spirit level to check it’s level, or add a slight fall of 1-2% for drainage if the base will be exposed.
Tips for Strong Formwork
Coat the inside of the timber with form release oil to make removal easier after the concrete cures. Ensure all joints are tight to prevent the wet mix from leaking out. Brace corners firmly as wet concrete exerts significant pressure on the frame.
Mixing Concrete for Your Shed Base
You have two main options: site-mixed concrete using cement, sand, and ballast, or ready-mixed concrete delivered by truck. For building projects requiring more than 1 cubic metre, ready-mix is usually more practical and ensures consistent quality.
Site Mix Ratios
For site-mixed concrete, you have two options. Using separate materials, mix 1 part cement to 2 parts sharp sand to 4 parts gravel. Using premixed ballast, mix 1 part cement to 5 parts ballast. Both produce a C20 mix suitable for foundations.
The 10/20/30/40 rule offers another guide: 10% cement, 20% water and air, 30% sand, and 40% gravel by volume.
Add water gradually until the mix reaches a workable consistency—stiff enough to hold its shape but wet enough to fill corners. Too much water weakens concrete and causes cracks.
Working Safely with Cement
Wet cement causes chemical burns on prolonged skin contact. Always wear waterproof gloves, long sleeves, and eye protection when mixing and pouring concrete. Wash any cement splashes off your skin immediately.
Pouring and Finishing the Slab
Pour the concrete into your formwork, starting at the far end and working back toward your access point. Use a shovel to spread the mix evenly, working it into corners to eliminate air pockets.
Level the surface using a straight board (screed) drawn across the formwork in a sawing motion. This brings excess concrete to low spots and creates a flat surface.
Creating a Smooth Finish
Once screeded, use a bull float or large trowel to smooth the top and close any small holes. A slightly textured finish provides better grip than a polished one. Wait until the water evaporates before final floating.
Cover the curing slab with plastic sheeting to retain moisture, especially in hot or windy conditions. Concrete that dries too quickly develops cracks.
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Do You Need Reinforcement?
A standard 4 inch concrete slab doesn’t typically require rebar reinforcement. However, adding wire mesh or rebar improves crack resistance in certain situations.
Add reinforcement if the structure will house heavy equipment, if you’re building on clay soil that expands seasonally, or if the slab exceeds 14 square metres (150 sq ft). Position reinforcing mesh in the middle of the slab thickness, supported on chairs or spacers.
Saving Money on Your Project
Ordering the right amount of material prevents waste and saves money on cost. Use a concrete calculator to determine the exact amount needed rather than rough estimates. Over-ordering represents wasted cost and excess material to dispose of.
Consider timing your pour carefully. Ready-mix costs more for small loads and Saturday deliveries. Combine your order with a neighbour’s project to split delivery fees where possible.
For smaller bases under 0.5 cubic metres, mixing your own concrete from bags of cement and ballast may prove economical. Lay the mix in layers and compact each layer before adding more.
Summary
Calculating how much concrete for a shed base comes down to accurate measurements and a simple formula. Measure your building dimensions carefully, decide on the right thickness for your needs, and multiply length × width × thickness to find your volume. Add 10% for waste to ensure you have enough to complete the job.
A well-planned concrete slab creates the solid foundation your shed needs. Take time with site preparation, build sturdy formwork, and allow proper curing time before building on top.
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Frequently Asked Questions
- How do you calculate concrete base for a shed?
- To calculate concrete for a shed base, multiply the length × width × depth of your slab. For example, a 10 × 10 foot (3 × 3 metre) shed with a 4 inch (100mm) thick slab needs: 10 × 10 × 0.33 = 33 cubic feet (0.93 cubic metres). Add 10% extra for waste and spillage to get your final concrete quantities.
- What is the 10/20/30/40 rule when making concrete?
- The 10/20/30/40 rule is a simple mix ratio guide: 10% cement, 20% water and air, 30% sand, and 40% gravel (aggregate) by volume. This produces a standard concrete mix suitable for shed bases and general construction. For site-mixed concrete using premixed ballast, a practical ratio is 1 part cement to 5 parts ballast by volume.
- Do you need rebar for a 4 inch slab for a shed?
- A standard 4 inch (100mm) concrete slab for a lightweight garden shed typically doesn't require rebar. However, you should add rebar or wire mesh reinforcement if: the shed will store heavy equipment, the ground conditions are poor, you're in an area prone to ground movement, or the shed exceeds 150 square feet (14 square metres).
- How much concrete for a 10x10 shed?
- A 10 × 10 foot (3 × 3 metre) shed with a standard 4 inch (100mm) thick slab requires approximately 1.23 cubic yards (0.94 cubic metres) of concrete. This equals roughly 74 bags of 60-pound (27kg) premix concrete or 56 bags of 80-pound (36kg) premix.
- How thick should a concrete shed base be?
- For most garden sheds, a concrete base should be 4 inches (100mm) thick. Increase to 6 inches (150mm) for larger sheds over 14 square metres, workshops with heavy equipment, or areas with poor ground conditions. The slab should extend 2-3 inches (50-75mm) beyond the shed perimeter on all sides.
- Can I pour concrete directly on soil?
- You should never pour concrete directly on bare soil. Always prepare the site by removing topsoil, compacting the ground, and laying a 3-4 inch (75-100mm) layer of compacted gravel or crushed stone as a sub-base. This prevents moisture damage and helps the concrete cure properly without cracking.
- How long before I can put a shed on fresh concrete?
- Wait at least 7 days before placing a lightweight shed on fresh concrete, and 28 days for full cure strength. The concrete reaches about 70% strength after 7 days. Avoid placing heavy loads or building on the slab during this curing period to prevent surface damage and cracks.