Concrete Slab Material Calculator
Find the concrete for a floor, pad or shed base, in m³ or 25 kg bags. A table below lists the cement and batch gravel to mix your own.
Measure your project
Your answer
We added 10% extra.
If you buy bags
Each 25 kg bag makes about 0.01 m³ (Kilsaran data sheet).
If you order a lorry (ready-mix)
Ask for 1.00 m³. Roadstone and Hanly take orders from 1 m³, so we price at least that.
This is a guide. For a house floor, garage, drive or foundation, check your local building rules or ask a builder or engineer.
Which costs less?
You also skip mixing 83 bags by hand. Have the edges ready before it comes.
Prices from October 2026. Ready-mix €143.01 with 13.5% VAT (Hanly Quarries only). Bags €12.80 (B&Q Ireland only), taken to include 23% VAT.
Quick answers
- A 3 × 2.5 m shed base
- At 100 mm thick, 0.83 m³ with 10% extra, or 83 bags of 25 kg. Mixing your own: 2 tonne bags of batch gravel, and 7 bags of cement for the weakest floor mix.
- House floor (TGD C)
- At least 150 mm of concrete on at least 200 mm of T2 hardcore.
- Shed base
- 100 mm of concrete on 100–150 mm of packed hardcore, says Summerhouse24. Use 150 mm for bigger buildings.
- Damp-proof sheet (DPM)
- 300 μm (1200 gauge) or more under a house floor (TGD C). Summerhouse24 uses 1200 gauge under a shed.
- Screed
- TGD C shows at least 65 mm where the insulation sits on top of the slab.
How to work it out
- Measure the length and width in metres. Multiply them for the area in square metres (m²).
- Pick the thickness in mm, and divide it by 1,000 for metres.
- Multiply the area by that thickness to get m³. Then add 10% for spills.
- Bags: divide by 0.01 and round up. To mix your own, use the table below.
3 × 2.5 = 7.5 m²
7.5 × 0.1 = 0.750 m³
0.750 + 10% = 0.825 m³
0.825 ÷ 0.01 = 82.5 → 83 bags
Material calculator for a concrete slab: what to buy
Mixing it yourself? Buy cement in 25 kg bags and batch gravel in tonne bags of 1,000 kg. Kenny Fuels says its batch gravel holds washed 20 mm pebbles, grit and sand. So you don't need sand as well.
| Slab | m³ (+10%) | Cement, at least | Batch gravel | Cost, from |
|---|---|---|---|---|
| Pad 1 × 1 m, 100 mm | 0.11 | 1 bag | 0.20 t → 1 bag | €77 |
| Shed base 3 × 2.5 m, 100 mm | 0.83 | 7 bags | 1.51 t → 2 bags | €201 |
| Patio 5 × 4 m, 100 mm | 2.20 | 18 bags | 4.03 t → 5 bags | €507 |
| Bigger base 4 × 3 m, 150 mm | 1.98 | 16 bags | 3.62 t → 4 bags | €420 |
Cement: €9.35 a bag, the middle of 8 prices from 7 sellers. Gravel mix or batch gravel: €67.65 a tonne bag, the middle of 11 sellers. With VAT where the seller says so. Part bags round up.
The cement here is TGD C's least for a plain floor: its weakest mix, 20N. Summerhouse24 uses C20 or C25 for a shed base, and that takes more cement. Ask your merchant how much, or order ready-mix.
The batch gravel is our own rough estimate. Water is at most about 21 litres per bag of cement (TGD C). How we worked it out.
Buying sand and gravel apart? Summerhouse24 mixes a shed base by hand at 1 : 2 : 3: cement, sand and gravel. Our sand calculator and gravel calculator count the tonne bags.
Concrete floor: hardcore, DPM and radon barrier
TGD C is the Irish guide for ground floors. Here is what it asks for in a house floor, from the ground up.
| Layer | TGD C |
|---|---|
| Hardcore: T2 stone, 4/40 mm | 200 mm or more |
| Blinding: T3, 0/4 mm | 50 mm at most |
| DPM (plastic sheet) | 300 μm (1200 gauge) or more |
| Concrete slab | 150 mm or more |
| Screed, if insulation is on top | 65 mm or more |
Lay the hardcore in packed-down layers of up to 225 mm. Seal the DPM joints.
- Hardcore: TGD C asks for T2 stone, 4/40 mm, from S.R. 21 Annex E. It lets radon gas move to a sump. If you need more fill below it, Roadstone names T1, 0/31.5 mm crushed rock.
- Radon barrier: TGD C says it is normally laid instead of the DPM.
- Under a timber floor: at least 100 mm of concrete on 100 mm of hardcore.
Under a house floor, ask for certified S.R. 21 Annex E fill, as Roadstone says, not plain 804. For Clause 804 under a drive or path, use our gravel calculator.
Shed base in Ireland: concrete, hardcore and DPM
We found no Irish rule that says a shed needs a concrete base. Summerhouse24 Ireland, a garden-building seller, gives this guide for one:
- 100 mm of concrete on 100–150 mm of packed-down hardcore.
- 150 mm of concrete for bigger, heavier buildings.
- A 1200-gauge DPM, and A142 or A193 steel mesh.
- A C20 or C25 mix. For which grade to order, see our concrete calculator.
| Base | m³ (+10%) | 25 kg bags | Weight of bags |
|---|---|---|---|
| 2 × 2 m | 0.44 | 44 | 1.1 t |
| 3 × 2.5 m | 0.83 | 83 | 2.1 t |
| 4 × 3 m | 1.32 | 132 | 3.3 t |
Sizes are examples. Roadstone says its small lorry loads, from 1 m³, suit shed bases.
Screed on a concrete floor: when you need it
TGD C shows two ways to lay a floor. In one, the insulation goes on top of the slab. Then a screed of at least 65 mm covers it. In the other, the insulation goes under the slab, and no screed is shown.
Keohane Readymix says its flowing screed is often 60–90 mm deep. It suits underfloor heating. A 10 × 8 m floor at 65 mm needs 5.2 m³ of screed.
On the day you pour
- Dig out, then lay the hardcore in layers. Pack each layer down.
- Lay the DPM or radon barrier, and seal the joints.
- Fix timber boards to form the edges, level with the top of the slab.
- Pour, level and finish. Kilsaran says drives and yards are usually brushed for grip.
- Keep the edges on for 7 days, as Summerhouse24 says.
Questions people ask
Does a garden shed need a concrete base?
We found no Irish rule that says it must. Summerhouse24, a garden-building seller, calls a solid base essential for a timber shed in Ireland's damp weather.
How much concrete do I need for a shed base?
For a 3 × 2.5 m base, 100 mm thick: 0.83 m³ with 10% extra. That is 83 bags of 25 kg, or 1 m³ of ready-mix.
How thick should a concrete floor slab be?
For a house, TGD C asks for at least 150 mm. Roadstone says many home slabs, such as patios and shed bases, are 100 mm.
Do I need to screed a concrete floor?
Only if the insulation goes on top of the slab. Then TGD C shows a screed of at least 65 mm over it. With the insulation under the slab, it shows none.
How much concrete do I need for a foundation?
Measure the trench: length × width × depth, in metres. Pick "Foundation trench" above. Kilsaran says foundations use C25/30 (30N) or stronger. For the size, check your local building rules or ask a builder or engineer.
What goes under a concrete slab?
Under a house floor, TGD C asks for at least 200 mm of T2 hardcore, which lets radon gas out. Then an optional fine blinding layer, and a 300 μm DPM.
Does a concrete slab need steel mesh?
Summerhouse24 puts A142 or A193 mesh in a shed base. TGD C allows a house floor with no steel if the mix is strong enough. Check your local building rules or ask a builder or engineer.
How long before a shed can go on a new base?
Summerhouse24 says to take the timber edges off after 7 days. It says to wait 7–10 days before the shed goes on.
Where our numbers come from
- Concrete per bag: Kilsaran KPRO Multi-Purpose 25 kg data sheet
- 25 kg concrete bag price: B&Q Ireland (Kilsaran, €12.80)
- 20 kg concrete drymix: Goodwins (€8.95; 23% VAT shown)
- Ready-mix prices: Hanly Quarries, Roscommon (€143/m³ C25/30)
- Ready-mix terms: Hanly (1 m³ steps, small load charge may apply)
- Ready-mix full load: Hanly terms (5 m³; small load charge below)
- Ready-mix terms: Roadstone (1 m³ minimum, small-load charge under 6 m³)
- VAT on ready-mix concrete: Revenue (reduced rate)
- House floor slab rules: TGD C (150 mm concrete, 200 mm hardcore)
- Shed base: Summerhouse24 Ireland guide (10 cm concrete on 10–15 cm gravel)
- Screed depth: Keohane Readymix (60–90 mm self-levelling)
- Which concrete grade: Kilsaran (floors C25/30, drives C28/35)
- Hardcore under floors: S.R. 21 Annex E grades (Roadstone FAQ)
- Batch gravel tonne bag: Kenny Fuels, Wexford (€65 inc VAT, 1,000 kg)
- Cement price: the middle of 8 prices from 7 Irish sellers (25 kg, Oct 2026)
- Tonne bag prices: the middle price from Irish sellers (Oct 2026)
Made by BuildersCount. Last checked 4 Oct 2026.
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