Concrete Calculator
The Concrete Calculator estimates how much concrete you need for a slab, footing, or pad based on its dimensions, and converts that volume into bags of pre-mixed concrete. Works in U.S. or metric units. For larger pours, most contractors add a 5–10% waste allowance on top of the calculated volume to account for an uneven subgrade, spillage, and minor over-excavation. This makes the tool equally useful for a DIY homeowner pricing out a shed pad in bags and a contractor sizing a ready-mix truck order, since the same volume figure feeds both.
Switch between U.S. Customary and Metric units.
How This Tool Works
Pick your units, enter your slab's length, width, and thickness, then choose a bag size to see how many bags to buy — or the cubic yards (or cubic meters) to order for a ready-mix delivery.
Formula & Method
Volume = length × width × thickness. Cubic yards = cubic feet ÷ 27. Bags = volume ÷ bag yield — a 60 lb bag yields about 0.45 cu ft, and a 25 kg bag yields about 11.75 L.
Example Calculation
A 10×10 ft patio slab at 4 inches thick needs 33.3 cubic feet (1.23 cubic yards) of concrete — about 56 bags of 80 lb concrete. In metric, a 3×3 m slab at 10 cm thick is about 0.94 m³. A larger 12×20 ft driveway slab poured 5 inches thick needs 100 cubic feet (3.70 cubic yards) — big enough that most suppliers recommend a ready-mix truck instead of the roughly 167 bags of 80 lb premix it would take by hand.
Frequently Asked Questions
How thick should a concrete slab be?+
Standard patios and walkways use 4 inches (about 10 cm), while driveways for vehicles typically need 5–6 inches (13–15 cm) for durability.
Can I calculate concrete in cubic meters and kg bags?+
Yes — switch the unit toggle to Metric and enter your slab in meters with the thickness in centimeters. Volume is returned in cubic meters and bags are sized in kilograms.
When should I order ready-mix instead of bags?+
For pours over about 1 cubic yard (roughly 0.75 m³), ready-mix delivery is usually faster and more cost-effective than mixing dozens of bags by hand.
Does this include a base layer?+
No, this calculates concrete volume only. Most slabs also need a compacted gravel base, calculated separately with our Gravel Calculator.
Should I order extra concrete to cover waste?+
Yes — most pros add 5-10% on top of the calculated volume for larger pours to cover an uneven subgrade, spillage, and minor over-excavation. For a small pour like a single fence post, it's simpler to just round up to the next whole bag instead.
How long before I can walk or drive on the new concrete?+
Concrete typically sets enough to walk on after about 24-48 hours, but needs roughly 7 days of curing before it's ready for regular foot traffic and closer to 28 days to reach its full rated strength before heavy vehicle loads like a car or truck.
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<a href="https://everyfix.net/tools/concrete-calculator/">Concrete Calculator</a> by <a href="https://everyfix.net">EveryFix</a>Related Tools
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