Quick answer
Concrete Calculator: direct answer
Concrete Calculator helps you estimate concrete volume, cubic yards, bags, weight, and cost for slabs, walls, footings, and post holes. It is best for slabs, footings, walls, and post holes and returns cubic feet, cubic yards, whole bag count for material planning.
Use this calculator when you know project shape such as slab, wall, footing, or post hole, length, width, thickness, height, depth, or diameter, quantity when the same shape repeats. The estimate uses this rule: cubic feet = length x width x thickness in feet; cubic yards = cubic feet / 27.
Inputs
- Project shape such as slab, wall, footing, or post hole
- Length, width, thickness, height, depth, or diameter
- Quantity when the same shape repeats
- Waste percentage
- Bag size, bag cost, and ready-mix cost when pricing is needed
Outputs
- Cubic feet
- Cubic yards
- Whole bag count
- Estimated material weight
- Estimated material cost
Formula
How this estimate works
cubic feet = length x width x thickness in feet; cubic yards = cubic feet / 27
In plain terms, calculate the concrete volume from the shape dimensions, convert cubic feet into cubic yards, then add waste before comparing ready-mix and bagged options.
A 20 ft by 10 ft slab at 4 in thick with 10% waste needs about 2.72 cubic yards of concrete.
Use cases
When to use this calculator
Use length, width, and thickness to estimate patio, walkway, shed pad, and equipment pad concrete.
Switch shapes when estimating rectangular footings or round post holes instead of a flat slab.
Compare cubic yards, bag count, total weight, and rough cost before choosing how to buy material.
Worked example
Estimate a 12 ft by 10 ft patio slab
For a small patio, the calculator turns the slab dimensions into cubic feet, cubic yards, bag count, and rough material weight so you can compare bagged concrete with ready-mix.
- Enter 12 ft for length, 10 ft for width, and 4 in for thickness.
- Use a 10% waste factor if the base is uneven or forms are not perfectly square.
- Review both cubic yards and bag count before deciding whether hand mixing is practical.
Planning reference
Concrete estimate reference points
Use these examples to sanity-check the calculator before ordering. They assume a simple rectangular slab and do not include footings, turndowns, reinforcement, or delivery fees.
How to calculate concrete
For a slab, multiply length by width by thickness. Convert thickness from inches to feet first, then divide cubic feet by 27 to get cubic yards. Add a waste factor when ordering because forms, subgrade variation, spills, and uneven surfaces can change the actual quantity.
Concrete bag estimates
Bag counts are approximate because premix yield varies by brand and mix. This calculator uses common estimated yields: 40 lb = 0.30 cu ft, 50 lb = 0.375 cu ft, 60 lb = 0.45 cu ft, and 80 lb = 0.60 cu ft.
Measurement tips for a better estimate
- Measure the finished concrete dimensions, not only the excavated area.
- Convert slab thickness carefully: 4 inches is 0.333 feet, not 4 feet.
- Measure several spots if the base is uneven and use the deepest practical thickness for ordering.
Common estimating mistakes
- Entering slab thickness in feet when the field expects inches.
- Forgetting to add waste for uneven subgrade, spills, form leakage, and over-excavation.
- Comparing bag cost with ready-mix cost without considering delivery minimums and mixing labor.
Ordering checks
Check these before using the result
- Confirm whether the estimate should include thickened edges, steps, footings, or post holes as separate shapes.
- Check ready-mix minimum delivery quantity, short-load fees, and scheduling before comparing against bag cost.
- Keep reinforcement, forms, gravel base, finishing, sealers, and labor outside the concrete volume number.
Assumptions used
- Normal-weight concrete is estimated at 150 lb per cubic foot.
- Premix bag yield is estimated from common 40, 50, 60, and 80 lb bag sizes.
- Waste factor is applied after the base volume is calculated.
Before you order materials
- Measure the finished thickness, not only the form height.
- Add extra for uneven subgrade, spills, and small ordering errors.
- Confirm local ready-mix minimums before choosing bags for a large pour.
Frequently asked questions
How much extra concrete should I order?
For small projects, 5% to 10% extra is common. Uneven ground, complex forms, and hand mixing can justify 10% to 15%.
When should I order ready-mix instead of bags?
If the result is dozens of 80 lb bags, ready-mix may be easier and more consistent. Compare local delivery minimums and bag costs.
Is concrete weight included?
Yes. The estimate uses 150 lb per cubic foot, which is a common rule of thumb for normal-weight concrete.