--- title: "Concrete Calculator | Method Calculator" description: "Estimate how much concrete you need for a slab, footing or round column in cubic yards and meters, plus the number of 40, 60 and 80 lb bags." canonical_url: "https://methodcalculator.com/concrete-calculator" meta: "og:description": "Estimate how much concrete you need for a slab, footing or round column in cubic yards and meters, plus the number of 40, 60 and 80 lb bags." "og:image": "https://methodcalculator.com/_og/s/o_0b8a2f8b3669bb46.png" "og:image:height": "600" "og:image:type": image/png "og:image:width": "1200" "og:title": "Concrete Calculator – Free Online Tool" "twitter:image": "https://methodcalculator.com/_og/s/o_0b8a2f8b3669bb46.png" --- > Index of every page on this site: [llms.txt](https://methodcalculator.com/llms.txt) # **Concrete Calculator** Enter the dimensions of your slab, footing or round column and get the volume of concrete in cubic yards, cubic feet and cubic meters, plus how many 40, 60 or 80 lb bags of pre-mixed concrete it takes, with a waste allowance built in. ## **Result** Concrete to order (incl. 5% waste) **1.30 yd³**
Without waste allowance
1.23 yd³ (33.33 ft³)
Cubic feet
35 ft³
Cubic meters
0.991 m³
80 lb bags
**59 bags**
60 lb bags
78 bags
40 lb bags
117 bags
Approximate weight (150 lb/ft³)
5,250 lb (2,381 kg)
Ready-mix
Order 1.5 yd³ ready-mix (rounded up to ¼ yd³)
## What this concrete calculator does Concrete is sold by volume: ready-mix trucks deliver it by the cubic yard (or cubic meter), and bagged mix is labelled with the cubic feet each bag yields. Before you pour a patio, shed base, footing, fence post or deck pier you need that volume, and getting it wrong is expensive in both directions. Order too little and the pour stops with a cold joint while you wait for more; order too much and you pay for concrete that ends up as a hardened lump. This calculator turns your measurements into cubic yards, cubic feet and cubic meters, adds a waste allowance and converts the result into a bag count. ## Concrete volume formula For a rectangular slab or footing measured in feet and inches: **Volume (ft³) = length (ft) × width (ft) × thickness (in) ÷ 12** For a round column, pier or post hole: **Volume (ft³) = π × (diameter (in) ÷ 24)² × height (ft)** Divide cubic feet by 27 to get cubic yards, or multiply by 0.0283 to get cubic meters. **Example:** a 10 ft × 10 ft patio slab 4 inches thick has a volume of 10 × 10 × (4 ÷ 12) = **33.33 ft³**, which is 33.33 ÷ 27 = **1.23 yd³**. Adding the recommended 5% waste allowance gives 35 ft³, or 1.30 yd³. An 80 lb bag yields 0.60 ft³, so bagged you would need 35 ÷ 0.60 = 58.3, rounded up to **59 bags**. At 150 lb per cubic foot the finished slab weighs about 5,000 lb. Metric measurements are converted first (1 m = 3.2808 ft, 1 cm = 0.3937 in) and then run through the same formula, so a 3 m × 3 m × 10 cm slab works out to 0.9 m³, or 1.18 yd³. ## Bags or ready-mix? Bagged concrete is convenient for small jobs, but the numbers add up fast: a single cubic yard takes 45 bags of 80 lb mix, which is 3,600 lb to carry, mix and place by hand before the first batch starts to set. As a rule of thumb, **order a ready-mix truck for anything over about 1 cubic yard**. Trucks deliver in quarter-yard increments, and most suppliers charge a short-load fee for orders under 3 to 4 yards, so check the price for your exact quantity. Between half a yard and a yard, a rented towable mixer with bags is a reasonable middle ground. | Bag size | Yield per bag | Bags per cubic yard | Bags per cubic foot | | --- | --- | --- | --- | | 40 lb | 0.30 ft³ | 90 | 3.4 | | 60 lb | 0.45 ft³ | 60 | 2.3 | | 80 lb | 0.60 ft³ | 45 | 1.7 | ## Typical slab thicknesses | Project | Typical thickness | Notes | | --- | --- | --- | | Sidewalk, path, patio | 4 in (10 cm) | Foot traffic only; wire mesh optional | | Shed or garage floor | 4 – 5 in | Thicken edges to 8 – 12 in if there is no separate footing | | Driveway (cars) | 5 – 6 in (12 – 15 cm) | Rebar or mesh recommended | | Driveway (trucks, RVs) | 6 – 8 in | Reinforced, on compacted gravel | | Strip footing | 8 – 12 in deep × 16 – 24 in wide | Set by local code and soil | | Fence post hole | 8 – 12 in diameter, ⅓ of post length deep | Below the frost line in cold climates | ## How to use this calculator 1. Pick the shape: a slab or footing for rectangular pours, or a round column for piers, sonotubes and post holes. 2. Choose US or metric units and enter the dimensions. Thickness is in inches (or centimeters) while length and width are in feet (or meters). 3. Set the quantity if you are pouring several identical pieces, such as 8 post holes. 4. Keep the 5% waste allowance for a formed pour on a prepared base; choose 10% for footings dug into uneven ground. 5. Read the cubic yards to order from a ready-mix supplier, or the bag count for a DIY mix. ## Tips and common mistakes - **Measure the actual excavation,** not the plan. A trench dug 14 inches wide instead of 12 needs 17% more concrete. - **Compact the base.** A 4-inch slab on 4 inches of compacted gravel resists settling and cracking far better than one poured on soil. - **Build the forms level and brace them.** Forms that bow outward during the pour quietly consume your waste allowance. - **Do not confuse inches and feet** for thickness. Entering 4 ft instead of 4 in overstates the volume twelve times over. - **Count the post itself.** A post hole holds less concrete once the post is in it; the calculator ignores that, which keeps you on the safe side. - **Keep it wet.** Concrete reaches its design strength by curing, not drying; mist it or cover it for several days. For odd shapes, break the pour into rectangles and cylinders and add the pieces, or find the footprint with the [area calculator](https://methodcalculator.com/area-calculator) and the volume with the [volume calculator](https://methodcalculator.com/volume-calculator). The [unit converter](https://methodcalculator.com/unit-converter) handles any other unit changes. ## **Frequently asked questions ** ### **How many 80 lb bags of concrete make a cubic yard?** An 80 lb bag yields about 0.60 cubic feet of mixed concrete, and a cubic yard is 27 cubic feet, so you need 27 ÷ 0.6 = 45 bags per cubic yard. The equivalent is 60 bags of 60 lb or 90 bags of 40 lb. That is roughly 3,600 lb of dry mix to carry and mix, which is why larger jobs are usually poured from a truck. ### **How thick should a concrete slab be?** Four inches is standard for patios, sidewalks and shed floors. Driveways and garage floors that carry cars should be 5 to 6 inches, and anything that takes trucks or heavy equipment usually needs 6 inches or more with reinforcement. Footings are sized by the local building code, commonly 8 to 12 inches deep. ### **Why add a waste factor?** Real excavations are never perfectly flat or square, forms bow slightly, and a little concrete is always left in the mixer, the wheelbarrow and on the tools. Running short mid-pour creates a cold joint that weakens the slab, so ordering 5 to 10 percent extra is far cheaper than a second delivery or an emergency run for more bags. ### **What is the difference between concrete and cement?** Cement is the grey powder that acts as the glue. Concrete is cement mixed with sand, gravel and water. Bagged "concrete mix" already contains the sand and gravel, so you only add water, while a bag of pure cement needs aggregate added before it becomes concrete. ### **How much does concrete weigh?** Normal-weight concrete weighs about 150 lb per cubic foot, or roughly 4,050 lb (2 tons) per cubic yard, and about 2,400 kg per cubic meter. The calculator uses that figure to estimate the weight of your pour, which matters when you are checking what a trailer, deck or floor can carry. ## **Related calculators ** - [**Volume Calculator**](https://methodcalculator.com/volume-calculator) - [**Area Calculator**](https://methodcalculator.com/area-calculator) - [**Unit Converter**](https://methodcalculator.com/unit-converter) - [**Electricity Cost Calculator**](https://methodcalculator.com/electricity-cost-calculator) ```json {"@context":"https://schema.org","@graph":[{"@id":"https://methodcalculator.com/#website","@type":"WebSite","description":"Free online calculators for finance, health and fitness, math and everyday life. 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