United Kingdom · Construction
Concrete Bag CalculatorHow many 25 kg bags do you need? Enter your dimensions in metres and get your count instantly.
Example — 3 × 2 m shed base, 100 mm
0.600
cubic metres
A 3 × 2 m shed base at 100 mm deep needs about 50 × 25 kg bags. Add 5% waste and buy 53 — a manageable half-day job with a hired mixer.
How to calculate how many bags of concrete you need
Working in metric keeps the maths simple. Because all dimensions are in metres, multiplying them together gives you cubic metres directly — no conversion factor required. The only thing to watch is that UK building drawings almost always show slab depth in millimetres, not metres. Divide by 1,000 before you start.
Step 1 — volume
Volume (m³) = Length (m) × Width (m) × Depth (m)
Step 2 — bag count
Bags = (Volume × waste factor) ÷ bag yield (m³)
A standard 25 kg bag of ready-mix concrete covers around 0.012 m³. One cubic metre therefore needs roughly 83–84 bags at exact volume. The calculator adds a waste factor on top — the default is 5%, which accounts for spillage and the small amount of mix left on tools and in the drum. Increase it to 10% if the ground is uneven or you're mixing in cold weather, where concrete can stiffen faster than expected.
Worked example: 4 × 3 m patio slab at 100 mm
A 4 × 3 m patio is one of the most common weekend concrete projects in the UK. At a standard 100 mm depth on a prepared hardcore base, here's how to work through the numbers:
Convert depth to metres
100 mm ÷ 1,000 = 0.10 m
Calculate volume
4 × 3 × 0.10 = 1.2 m³
Add 5% waste
1.2 × 1.05 = 1.26 m³
25 kg bag count
1.26 ÷ 0.012 = 105 bags
105 bags is a significant hand-mix job — at least a full day with a hired electric mixer. At this volume, it's worth getting one or two ready-mix quotes. Many UK suppliers will deliver from 1 m³, and the cost per m³ is considerably lower than bagged concrete at this scale.
UK concrete bag sizes explained
UK builders' merchants and DIY sheds stock bagged concrete mix in two main sizes. Both contain the same type of mix — the difference is purely the amount per bag:
20 kg
≈ 0.0095 m³
~105 bags per m³
Easier to lift and carry, particularly useful on confined sites or for solo workers. Slightly more expensive per m³ than 25 kg bags due to packaging.
25 kg
≈ 0.012 m³
~83–84 bags per m³
The most common size in the UK. Better value per m³ and fewer bags to buy. Weighs around the limit for safe solo lifting — a sack truck helps on bigger jobs.
General-purpose bagged concrete mix is suitable for most domestic jobs — shed bases, paths, and fence posts. For structural or exposed applications where a specific strength class is required (C20, C25, C30), you'll typically need to order ready-mix with the correct BS designation rather than relying on bagged general-purpose mix.
Ready-mix vs bagged concrete — which is right for your job?
Use bags when…
- • Your pour is under 0.5–1 m³
- • A mixer lorry can't access the site
- • You want to work across several sessions
- • You're doing fence posts, repairs, or small footings
- • The job is straightforward and non-structural
Use ready-mix when…
- • Your pour is 1 m³ or more
- • You need a specified BS mix class (C20, C25, C30)
- • Consistent strength throughout is critical
- • You're under time pressure on a full pour
- • Cost per m³ matters — bags are 3–5× more expensive
A useful rule of thumb: if you need more than two bulk bags of ballast, get a ready-mix quote. Most UK volumetric suppliers deliver from 0.5 m³ and can mix to the exact specification you need on arrival.
How much do bags of concrete cost in the UK?
Prices vary by retailer and region, but these are approximate 2025–2026 figures at UK builders' merchants and DIY sheds:
20 kg bag
£3–5
~£315–525/m³
25 kg bag
£4–6
~£335–500/m³
Ready-mix C25
£90–140
per m³ + delivery
Bagged concrete works out at roughly £335–525 per equivalent m³ — three to five times the cost of ready-mix. For a 1.2 m³ patio pour, that difference could be £250–400. The calculator defaults to £110/m³ as a mid-range estimate; update this in the "Adjust waste & price" panel to match your local supplier's rate.
Common mistakes on UK concrete bag jobs
Entering depth in millimetres instead of metres
The most frequent input error. Entering 100 instead of 0.10 gives a result 1,000 times too large. UK drawings show depth in mm — always divide by 1,000 before calculating.
Underestimating the bag count for a large pour
83 bags per m³ sounds manageable until you price it up and carry them. For anything over 0.5 m³, get a ready-mix quote first — the cost difference is often larger than expected.
Mixing in cold or frosty conditions
Concrete needs temperatures above 5°C to cure properly. Pouring in late autumn or winter without frost protection measures can lead to a weak, crumbly slab that needs to be broken out.
Skipping the sub-base
A concrete slab poured directly onto soft or uneven soil will crack as the ground moves. A 75–100 mm compacted hardcore layer is standard under any domestic slab in the UK.
Frequently asked questions
How many 25 kg bags per cubic metre?
A 25 kg bag covers approximately 0.012 m³, so you need about 83–84 bags per m³ at exact volume. With a 5% waste allowance, budget for 88 bags per m³.
How do I work out cubic metres from millimetres?
Divide each millimetre measurement by 1,000 to get metres, then multiply length × width × depth. For example: 4 m × 3 m × 100 mm becomes 4 × 3 × 0.10 = 1.2 m³.
What is the difference between 20 kg and 25 kg bags?
Both contain the same general-purpose mix. A 20 kg bag yields about 0.0095 m³ and a 25 kg bag around 0.012 m³. The 25 kg bag gives better value per m³ but is heavier to lift.
Can I use bagged concrete for a driveway in the UK?
Technically yes for very small driveways, but at 83+ bags per m³ a typical single driveway (2–3 m³) would require 250–300 bags. Ready-mix is far more practical and cost-effective at that scale.
How deep should a shed base be in the UK?
100 mm (0.10 m) on a 75–100 mm compacted hardcore sub-base is standard for a domestic shed base. For larger outbuildings or on soft ground, 150 mm is more appropriate.
When should I use ready-mix instead of bags in the UK?
Once your pour reaches 0.5–1 m³ or more, ready-mix is usually cheaper per m³ and far quicker to pour. Most UK volumetric suppliers will deliver from 0.5 m³ with no minimum-order penalty.
Disclaimer
This calculator provides estimates only and should be used for planning purposes. Actual quantities and costs may vary based on ground conditions, sub-base preparation, material yield, and local pricing. Always verify your order with the supplier before purchasing.