Concrete Slab Volume Calculator
Whether you are pouring a driveway, parking pad, patio, BBQ slab, sidewalk repair or shed footing, the first question is always the same: how much concrete? Plug in length, width, slab thickness and a waste factor — this tool returns the net volume, the volume to order (with waste built in) and the number of 25 kg, 40 kg or 80 lb pre-mix bags you need, so you do not have to squint at the yield printed on the back of every brand of bag.
Trade allowance is typically 5–10%; bump to 15% on uneven ground, long pump runs, or when re-pouring from a striked formwork.
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Concrete to order
1.32 m³ ≈ 1.73 yd³ · 46.6 ft³
Pre-mix bags (rounded up)
25 kg bags
113
40 kg bags
71
80 lb bags
78
Yields are scaled from the Quikrete-style 80 lb → 0.6 ft³ baseline; the actual yield printed on each bag can differ by ±10%, so cross-check before ordering.
Measure net dimensions (subtract column footings and trenches). Place within 60 min of mixing and keep the slab moist for 24 h+ after pouring to develop strength.
Formula
V = length × width × (thickness mm ÷ 1000) (m³) | V_total = V × (1 + waste % ÷ 100) | bags = ⌈V_total ÷ yield_per_bag⌉
- · Trade rules of thumb: 5–10 % waste for a clean rectangular pour on level ground, 10–15 % for uneven base, long pump runs or re-pours from a striked formwork. Over-ordering by a wheelbarrow is cheaper than calling another short-load truck.
- · Typical slab depths: footpath ≈ 75 mm, garden path or parking pad ≈ 100 mm, single-car driveway ≈ 100–125 mm, light-truck driveway ≈ 150 mm, structural floor slab ≥ 150 mm (always size with the engineer’s drawing for structural pours).
- · Bag yields follow the Quikrete / Sakrete industry baseline of one 80 lb (36.3 kg) bag = 0.60 ft³ ≈ 0.017 m³; scaling that gives 25 kg ≈ 0.0117 m³ and 40 kg ≈ 0.0187 m³ per bag. Real-world yields can vary by ±10 % between brands — always cross-check against the yield printed on the bag you buy.
- · For a ready-mix truck: the minimum drum load in most markets is 6 m³ (under that you pay a short-load fee); placed concrete must be deposited within 60 minutes of batching before it starts to stiffen.
- · Cure the slab: keep it moist (sprinklers, wet hessian or curing compound) for at least 24 hours after placing. Concrete reaches ~70 % of design strength by day 7 and the full 100 % by day 28.
- · Quick conversions: 1 m³ ≈ 1.308 yd³ ≈ 35.31 ft³ ≈ 1000 L.
Frequently asked
How many bags of pre-mix concrete do I really need?
First find the net volume: length (m) × width (m) × thickness (mm ÷ 1000) = m³. Then add 5–10 % waste and divide by the yield printed on your bag. Using the Quikrete 80 lb = 0.60 ft³ (≈ 0.017 m³) baseline that this tool ships with, one cubic metre needs ≈ 60 × 80 lb bags, ≈ 86 × 25 kg bags or ≈ 54 × 40 kg bags. As a worked example: a 4 m × 3 m × 100 mm slab with 10 % waste is 1.32 m³, which needs 78 × 80 lb bags or 113 × 25 kg bags.
When should I stop bagging and order ready-mix instead?
Rule of thumb: a solo DIYer with a mixer and two wheelbarrows can place about 0.5–1 m³ of bagged concrete in a working day. Past 1 m³ the labour adds up and the batch-to-batch water/cement ratio gets sloppy enough to risk cold joints. So once you cross roughly 1 m³ (≈ 60 × 80 lb bags) you should price a ready-mix truck. In Hong Kong, most ready-mix plants have a 0.5-truckload (~3 m³) minimum, with mini-mix or volumetric 2 m³ trucks available for smaller jobs.
Why do bag yields vary so much between brands?
Bagged concrete yield depends on three things: (1) the aggregate-to-sand ratio in the dry mix, (2) the water/cement ratio when you mix it, and (3) how well you consolidate (vibrate) it after placing. A 25 kg "sand mix" might only yield 0.0095 m³, while a "general purpose concrete mix" of the same weight can reach 0.012–0.013 m³. The tool uses the most-cited industry baseline (80 lb = 0.60 ft³). Before ordering, cross-check the yield wording on the back of the specific brand you are buying — it is also published on each manufacturer’s data sheet.
My slab has footings or cut-outs — how do I account for them?
Calculate each rectangular block separately and add them up. For example a main slab 4 m × 3 m × 0.1 m = 1.2 m³, plus four column footings 0.4 m × 0.4 m × 0.5 m × 4 = 0.32 m³, totals 1.52 m³. For cut-outs (manholes, equipment bases), subtract the cut-out volume from the main slab. The tool sizes one rectangular pour at a time — open several tabs for compound pours and sum the volumes.
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