If you’re standing in the shop wondering how many bags of concrete mix you’ll need for your weekend project, you’re not alone. It’s a question we get asked all the time here at Express Concrete, your trusted ready mix concrete supplier. Let’s break down exactly how far that 20kg bag will stretch, shall we?
The Short Answer (For Those in a Rush!)
A standard 20kg bag of concrete mix will typically cover about 0.25 square metres at a 50mm depth (that’s about 2.7 square feet at 2 inches deep for those who prefer imperial measurements).
But hang on a sec – there’s more to it than that! The actual coverage depends on a few important factors that might make a massive difference to your project budget.
Understanding Concrete Coverage: It’s All About Volume
When working out concrete coverage, we need to think in three dimensions rather than just the flat area. Concrete fills a volume, not just a surface.
The basic formula is dead simple:
Volume = Length × Width × Depth
Let me walk you through a proper calculation with our 20kg bag of high quality concrete mix as an example.
What’s Actually in That 20kg Bag?
A 20kg bag of standard concrete mix typically yields about 0.01 cubic metres of mixed concrete (roughly 10 litres or 0.35 cubic feet). This is crucial to understand when calculating concrete cover for your project.
Why not more? Well, when you mix concrete, you’re adding water, which doesn’t significantly increase the volume but activates the cement to bind everything together. Clever stuff, that!
Let’s Do Some Real-World Maths Together
Say you’re making a small garden stepping stone path. Each stone is 30cm × 30cm (that’s about 1ft × 1ft), and you want them to be 5cm (2 inches) thick.
Volume of one stepping stone = 0.3m × 0.3m × 0.05m = 0.0045 cubic metres
So, a 20kg bag yielding 0.01 cubic metres would make about 2 stepping stones with a bit left over.
![Image suggestion: Visual diagram showing a bag of concrete being poured to create stepping stones with measurements labelled]
Coverage Table for Different Depths
Here’s a handy table showing approximately what area a 20kg bag will cover at different depths:
| Depth | Area Covered (Square Metres) | Rough Equivalent |
|---|---|---|
| 25mm (1″) | 0.5 | A small coffee table top |
| 50mm (2″) | 0.25 | A doormat |
| 75mm (3″) | 0.17 | A small welcome step |
| 100mm (4″) | 0.12 | A microwave shelf |
| 150mm (6″) | 0.08 | A single paving slab |
Note: These are approximate figures based on standard concrete mix. Your actual results may vary slightly based on the exact product used and mixing conditions.
Factors That Affect Coverage
Right, now let’s get into the bits that might catch you out if you’re not careful when planning your construction projects!
1. Mix Ratio Matters
The ratio of cement, sand, and aggregate in your mix significantly affects coverage. Our standard ready mix concrete bags at Express Concrete are balanced for optimal strength and workability, but if you’re mixing your own concrete, keep this in mind!
The typical ratio for general purpose use is 1:2:4 (cement:sand:aggregate), but this can vary based on your project’s needs. Our experts are always on hand to advise on the right mix for your specific job.
2. Substrate Irregularities
Bumpy ground? You’ll need more concrete. It’s as simple as that. Uneven surfaces can increase your concrete requirements by up to 10-15%.
If you’re pouring onto rough ground, consider adding a bit extra to your calculations or levelling the ground first. Your future self will thank you!
3. Wastage
Let’s be honest – some concrete always seems to end up where it shouldn’t! Allow for about 5-10% extra for spillage, over-pouring, and the bit that inevitably gets stuck in the mixer or bucket.
Different Project Types and Their Requirements
Let’s look at some common projects and roughly how many 20kg bags you might need:
Small Concrete Slab (1m × 1m × 10cm)
Volume = 1m × 1m × 0.1m = 0.1 cubic metres
Number of 20kg bags needed = 0.1 ÷ 0.01 = 10 bags
Garden Path (5m × 1m × 7.5cm)
Volume = 5m × 1m × 0.075m = 0.375 cubic metres
Number of 20kg bags needed = 0.375 ÷ 0.01 = 37.5 bags (so round up to 38)
Not keen on lugging that many bags? That’s when our ready mix concrete delivery service starts to make a lot more sense! Our concrete calculator can help you determine exactly how much ready mix you’ll need.
How Does Our Ready Mix Compare to Bagged Concrete?
While this post focuses on 20kg bags, it’s worth mentioning that for larger construction projects, our high quality ready mix concrete delivered directly to your site is often more economical and definitely easier on your back!
According to the Concrete Society, using ready mix can reduce wastage by up to 15% compared to mixing bagged concrete on site. Plus, with our concrete calculator tool, you’ll get exactly the right amount for proper concrete cover every time.
Making Your Concrete Go Further
Want to make the most of every bag? Here are some smart tips:
- Proper storage: Keep unused bags in a dry place. Damp cement is basically useless!
- Accurate measuring: Use proper measuring tools rather than guessing water quantities.
- Consider using formwork: Good formwork prevents concrete from spreading where it’s not needed.
- Use aggregate for deep fills: For deep areas, consider using larger stones at the bottom to reduce the amount of concrete needed (a technique called “bulking”).
Our Express Concrete Promise
While calculating concrete quantities might seem a bit daunting, remember that at Express Concrete, we’re committed to making your construction projects as simple as possible. As the region’s trusted ready mix concrete supplier, our team is always ready to help you use our concrete calculator to determine exactly what you need – no more wasted materials and no frustrating mid-project trips back to the shop!
Frequently Asked Questions
How many 20kg bags of concrete do I need for a cubic metre?
To make 1 cubic metre of concrete, you’ll need approximately 100 bags of 20kg concrete mix. That’s quite a lot of heavy lifting, which is why many people opt for our high quality ready mix concrete delivered for construction projects requiring more than about 15-20 bags.
Can I mix different brands of concrete in the same project?
While it’s technically possible, we don’t recommend mixing different brands as they may have different setting times and strength characteristics. This could lead to an uneven finish or inconsistent strength. Stick with the same product throughout your project for best results.
How long does bagged concrete last in storage?
Properly stored in a dry place, unopened bags of concrete mix should last about 6-12 months. However, once the bag is opened, try to use it all as quickly as possible as it will absorb moisture from the air. If you notice lumps in the mix, that’s a sign it’s past its best and should be discarded.
According to research by the Building Research Establishment (BRE), cement products can lose up to 25% of their strength when stored improperly for extended periods.
Is it cheaper to mix my own concrete from cement, sand and gravel?
For very large projects, mixing from raw ingredients can be marginally cheaper. However, for most DIY projects, the convenience and guaranteed mix ratio of bagged concrete often outweighs the small cost savings. Plus, you’d need to factor in the cost of purchasing or renting a cement mixer for larger jobs.
How soon can I walk on newly poured concrete?
You can usually walk on concrete after 24-48 hours, but it continues to gain strength for up to 28 days. For vehicular traffic, you should typically wait at least 7 days, although this varies based on conditions and the specific mix used.
Wrapping Up
So there you have it – a 20kg bag of concrete mix covers approximately 0.25 square metres at a 50mm depth, but remember that your mileage may vary depending on the factors we’ve discussed.
Whether you’re laying a garden path, setting fence posts, or creating a new patio area, understanding concrete cover helps you budget your project properly and avoid those frustrating mid-job trips for more materials. For larger construction projects, our concrete calculator and ready mix concrete delivery will save you time, effort, and possibly a bit of backache!
Need more specific advice for your project? Our team at Express Concrete is just a click or call away. We’ve been helping DIYers and professionals alike get their concrete calculations spot on for years!
Until next time, happy concreting!