# Concrete Slab Calculator > To calculate concrete for a slab, multiply length × width × thickness in matching units to get volume, then divide by your bag's yield and round up. A 10 × 10 ft slab at 4 in thick is 33.3 ft³ (1.23 yd³) — 56 bags of 80 lb mix, or 62 bags with 10% waste. In metric, a 5 × 4 m slab at 10 cm is 2.0 m³ — about 160 twenty-five-kilogram bags. This calculator turns slab dimensions into volume (cubic feet and cubic yards, or cubic meters) and a bag count for 80 lb, 60 lb, 40 lb, or 25 kg concrete mix, with an optional waste factor. It computes in your browser — nothing is uploaded — and switches live between imperial and metric. Bag yields assumed here: 0.60 ft³ of placed concrete per 80 lb bag, 0.45 ft³ per 60 lb, 0.30 ft³ per 40 lb, and about 12.5 L per 25 kg bag at roughly 2,000 kg/m³ mixed density. Brands differ — the yield printed on your bag wins. Past about one cubic yard (or one cubic meter), compare against a ready-mix short-load delivery before committing to hand-mixing: a single yard already means 45 bags of 80 lb mix. ## Formula `volume = length × width × thickness · bags = ⌈volume ÷ yield per bag⌉` 1. Convert thickness into the same unit as length and width (inches ÷ 12 → feet; centimeters ÷ 100 → meters). 2. Multiply length × width × thickness to get volume in cubic feet or cubic meters. 3. Divide cubic feet by 27 to get cubic yards — ready-mix is ordered in yards. 4. Divide the volume by your bag's yield (0.60 ft³ for 80 lb, 0.45 for 60 lb, 0.30 for 40 lb, ≈12.5 L for 25 kg) and round up to whole bags. 5. Add 5–10% waste for spillage, uneven subgrade, and slab-edge overpour. ## Worked example: a 10 × 10 ft patio slab, 4 in thick Given: - Length 10 ft - Width 10 ft - Thickness 4 in = 0.333 ft - 80 lb bags (0.60 ft³ yield per bag) - Waste factor 10% Steps: 1. Volume: 10 × 10 × 0.333 = 33.33 ft³ 2. Cubic yards: 33.33 ÷ 27 = 1.23 yd³ 3. Bags: 33.33 ÷ 0.60 = 55.6 → round up to 56 bags 4. With 10% waste: 33.33 × 1.10 = 36.67 ft³ → 36.67 ÷ 0.60 = 61.1 → 62 bags **Buy 62 bags of 80 lb mix — or, at 1.23 yd³, price a short-load ready-mix delivery before committing to hand-mixing roughly two and a half tons of bags.** ## FAQ ### How many 80 lb bags of concrete make a cubic yard? 45 bags. A cubic yard is 27 cubic feet and an 80 lb bag yields about 0.60 ft³ of mixed concrete, so 27 ÷ 0.60 = 45. For 60 lb bags it is 60 bags (27 ÷ 0.45), and for 40 lb bags 90 (27 ÷ 0.30). ### How many 25 kg bags of concrete make a cubic meter? About 80 bags. One 25 kg bag of dry mix yields roughly 12.5 L of placed concrete at ~2,000 kg/m³ mixed density, and 1,000 ÷ 12.5 = 80. Yields vary by brand — check the number printed on your bag. ### When should I order ready-mix instead of mixing bags? From about one cubic yard (or one cubic meter) upward. That volume already means mixing 45 or more 80 lb bags — over a metric ton and a half — and a slab wants a continuous pour. Many suppliers deliver small "short loads" for a modest fee. ### How thick should a concrete slab be? Common practice: 4 in (10 cm) for patios, walkways, and shed bases; 5–6 in (13–15 cm) for driveways and garage floors. Soil conditions, reinforcement, and local code govern the real answer — verify before pouring. ### Why add a waste factor at all? Subgrades are never perfectly flat, forms bow, and some mix always stays in the mixer and barrow. 5–10% extra is standard practice; running short mid-pour costs far more than an extra bag or two. --- _Bag yields vary by brand and mix — the yield printed on your bag wins. Slab thickness, base preparation, and reinforcement can be code-governed. Estimate only — verify against local code._ Interactive version (free, no signup): https://tenon.tools/calculators/concrete-slab/ This file is the markdown mirror of that page. All mirrors: https://tenon.tools/llms.txt