Concrete Calculator
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Result
Concrete needed
- Bags to buy
- 147 × 80 lb bags
- In cubic feet
- 88 cu ft
- Approximate weight
- 13,200 lb (6.6 tons)
- Before waste allowance
- 2.96 cubic yards
- Ready-mix order
- 3.5 cubic yards
- At 3.26 cubic yards this is past the point where bags make sense — that is 147 80 lb bags to mix by hand. Ready-mix delivery is normally cheaper and far quicker above about one yard.
- At roughly 13,200 lb, check the load rating of any trailer or lift you are planning to use.
Bag count by bag size
The same pour, in each of the three common pre-mix bag weights.
| Bag size | Yield each | Bags needed | Total weight |
|---|---|---|---|
| 40 lb | 0.3 cu ft | 294 | 11,760 lb |
| 60 lb | 0.45 cu ft | 196 | 11,760 lb |
| 80 lb | 0.6 cu ft | 147 | 11,760 lb |
How do you calculate how much concrete you need?
To calculate concrete, multiply the length by the width to get the area in square feet, multiply by the thickness in feet, then divide by 27 to convert cubic feet into cubic yards. A 20 by 12 foot slab at 4 inches thick needs 2.96 cubic yards before waste.
cubic yards = (length_ft × width_ft × thickness_in ÷ 12) ÷ 27- length, width
- The slab's horizontal dimensions, in feet
- thickness
- Depth in inches, divided by 12 to reach feet
- 27
- Cubic feet in a cubic yard
Concrete volume is an area multiplied by a depth, and the only difficulty is unit consistency. Thickness is quoted in inches while length and width are in feet, so the thickness must be divided by 12 before multiplying — a 4-inch slab is 0.333 feet deep, not 4. The result is cubic feet, and because concrete is sold by the cubic yard, dividing by 27 gives the order quantity. A round column is the same calculation with a circular area: π times the radius squared, in square inches, divided by 144 to reach square feet. Footings are long, narrow slabs and use the identical formula. Steps are treated as a stack of solid boxes, which is what you pour unless they are formed hollow. Add a waste allowance last: concrete that falls outside the forms or fills a subgrade dip is gone, and running short mid-pour means a cold joint.
- Measure the length and width in feet and the thickness in inches.
- Multiply length by width to get the area in square feet.
- Divide the thickness in inches by twelve to convert it to feet.
- Multiply the area by that depth to get cubic feet.
- Divide cubic feet by 27 to get cubic yards.
- Add ten percent for waste, then round up to what you can order.
Worked example: A 20 ft by 12 ft patio slab, 4 inches thick
A 20 ft by 12 ft patio slab, 4 inches thick
The area is 20 × 12 = 240 square feet. Four inches is 4 ÷ 12 = 0.3333 feet, so the volume is 240 × 0.3333 = 80 cubic feet. Dividing by 27 gives 2.96 cubic yards before waste. Adding the standard 10% allowance brings it to 88 cubic feet, or 3.26 cubic yards. Ready-mix is sold in quarter-yard increments, so you would order 3.5 yards. In bags, 88 cubic feet at 0.60 cubic feet per 80 lb bag is 147 bags — about 11,760 lb of material to mix by hand, which is why nobody bags a pour this size. The delivered concrete weighs roughly 13,200 lb, or 6.6 tons. Note how sensitive the result is to thickness: taking the same slab to 6 inches raises it to 4.44 cubic yards before waste, a 50% increase for 2 inches.
What each concrete input means
- What are you pouring?
- The element you are pouring. The dimensions below change to the ones it needs.
- Length
- The longer horizontal dimension, in feet. For a footing, its total run.
- Width
- The shorter horizontal dimension, in feet. For a footing, the trench width.
- Thickness or depth
- In inches. Four inches suits a patio or walkway; a driveway carrying vehicles wants five to six.
- Diameter
- For a round column or post hole, the hole diameter in inches.
- Height
- For a column, how deep the hole is in inches. For steps, the rise of each step.
- How many?
- Number of identical columns, post holes or steps. Leave at 1 for a single slab.
- Bag size
- Pre-mixed bag weight. An 80 lb bag yields about 0.60 cubic feet. Bags only make sense for small pours — past about a cubic yard, ready-mix delivery is cheaper and far less work.
- Waste allowance
- Extra to cover spillage and an uneven subgrade. Ten percent is standard; an irregular excavation wants more.
What this concrete does not account for
- Reinforcement is not deducted. Rebar and mesh displace a negligible volume, so ignoring them is correct — but they are a cost and a requirement this calculator says nothing about.
- The subgrade is assumed flat and compacted. An uneven or soft base can absorb far more than the 10% waste allowance, and is the usual reason a pour runs short.
- Steps are treated as solid boxes. Steps formed over fill or with hollow risers need considerably less.
- Ready-mix suppliers have a minimum order, typically one cubic yard, and charge a short-load fee below it. A small pour can cost the same as a larger one.
- Bag yields vary slightly by manufacturer and mix. The figures here are the standard published yields for general-purpose pre-mix; check the bag.
- No allowance is made for the control joints, forms, gravel base or vapour barrier a durable slab needs.
Frequently asked questions
How many bags of concrete do I need for a slab?
Work out the volume in cubic feet, then divide by the bag yield. An 80 lb bag gives about 0.60 cubic feet, so a 240 square foot slab at 4 inches — 80 cubic feet — needs roughly 134 bags before waste.
How many cubic feet is an 80 lb bag of concrete?
About 0.60 cubic feet. A 60 lb bag yields roughly 0.45 cubic feet and a 40 lb bag about 0.30. It takes 45 of the 80 lb bags to make a single cubic yard.
How thick should a concrete patio be?
Four inches is the standard minimum for a patio or walkway carrying foot traffic. A driveway taking cars wants five to six inches, and anything carrying heavy vehicles needs an engineered thickness and reinforcement.
How many cubic yards is a 20x12 slab?
At 4 inches thick, 2.96 cubic yards before waste, or about 3.26 with a 10% allowance. The area is 240 square feet and the depth 0.333 feet, giving 80 cubic feet, which divided by 27 is the yardage.
Should I order ready-mix or use bags?
Bags make sense below about half a cubic yard. Past one yard, ready-mix delivery is normally cheaper and far quicker — a single yard is 45 eighty-pound bags, which is 3,600 lb to mix by hand.
How much does a cubic yard of concrete weigh?
About 4,050 lb, or just over two tons, at roughly 150 lb per cubic foot. That matters for trailer load ratings and for whether a site can take a delivery truck.
Sources and review
Last reviewed .