Concrete Calculator
Work out how much concrete to order, in cubic yards or metres and in bags, for a slab, wall, footing, round pad, column or set of steps.
Result
Buy 1.63 yd³
Forms bow, trenches wander and some concrete never leaves the mixer or the chute, so this includes 10% extra.
- Bags of premix instead
- 74Estimate
- Rounded up to whole bags
- Minimum if everything goes perfectly
- 1.48 yd³ (40 ft³)
https://calcgallery.com/concrete/concrete-calculator/
Results are estimates for planning only. Actual needs vary with site conditions, products and workmanship. Always confirm quantities with your supplier and follow local building codes and manufacturer instructions. Read the full disclaimer
How to use the Concrete Calculator
- Pick the shape that matches the pour. Walls and footings are rectangular blocks too, so they use the first option with a height or depth where a slab has its thickness.
- Enter the measurements the drawing labels for that shape. Every field has its own unit picker, so feet and inches can sit side by side.
- When several pieces are the same size, such as a row of piers, enter how many there are rather than adding them up by hand.
- Choose a bag size sold near you. Add a price per bag or a ready-mix price if you want the cost as well.
- Read the cubic yards or cubic metres first, since that is the number a ready-mix supplier asks for. The bag count is there for pours small enough to mix yourself.
How it is calculated
Every shape comes down to one volume for a single piece:
Slab, wall or footing length × width × thicknessRound slab or pad π × (diameter ÷ 2)² × thicknessRound column or tube π × (diameter ÷ 2)² × heightSquare column width × depth × heightSteps step width × tread × riser × n × (n + 1) ÷ 2The steps line makes sense from the side: the bottom step runs the full depth of the flight and each one above stops a tread shorter, so three solid steps hold 1 + 2 + 3, or six, blocks of one tread by one riser.
That volume is multiplied by how many matching pieces there are, and the waste allowance goes on top:
To order = piece × pieces × (1 + waste)Bags = ceil( to order ÷ yield of one bag )A bag’s yield is the volume of set concrete it makes; change it under More options if your bag prints a different figure. Each cost line is a quantity times the price you entered, nothing added.
Choosing the shape
Slab, wall or footing covers anything with square corners. For a wall, the length runs along the ground, the height goes in the second field and the wall’s thickness goes in the third. For a strip footing, use the trench length, its width and the depth of concrete in it.
Round slab or pad is for circular patios, hot tub bases and fire pit pads. Measure straight across in two directions at right angles; if the two differ, use the larger.
Round column or tube form is for piers, sonotubes and anything poured into an augered hole. If a post will sit in the concrete, the fence post concrete calculator subtracts it for you.
Square or rectangular column is for boxed piers and columns cast in plywood forms.
Steps is for a straight flight of equal steps. If the steps are poured against an existing wall or over compacted fill, the true volume is smaller than this.
Curb and gutter is not included. Curb profiles differ too much from one street standard to the next to fit one formula.
Yards, metres and bags
Ready-mix is sold by the cubic yard in the United States and by the cubic metre in most other places, which is why those units lead the result. Suppliers batch in set steps and many charge extra below a minimum load, so round your order up to the next amount they will send rather than down.
Bags make sense for small, scattered work: a few piers, a step, a pad for an air conditioner. Each bag has to be mixed with water and placed while the previous one is still workable, so the bag count is also a measure of labour.
Measuring for a pour
Measure the inside of the forms, not the outside of the boards. Check the depth from the top of the forms down to the base in several places and use the deepest reading, since every hollow fills with concrete too. For a tube, include every inch that will be filled, above and below ground.
Practical tips
Order for the forms you actually built. Re-measure once the forms are set and braced, since they rarely end up exactly where the plan put them.
Local codes vary. Depth, reinforcement and whether a permit is needed depend on where you are and what the concrete carries. This page counts material only; check your local building code, and ask a qualified professional about anything structural.
With your numbers
- Volume = 10 ft × 12 ft × 4 in = 40 ft³
- With 10% waste = 44 ft³
- Bags = 44 ft³ ÷ 0.6 ft³ per bag, rounded up to 74
- To order = 1.63 yd³
Quick reference
Forms bow, trenches wander and some concrete never leaves the mixer or the chute, so this includes 10% extra.
| Job | Concrete to order, waste included |
|---|---|
| 10 × 10 ft slab, 4 in thick | 1.36 yd³ |
| 10 × 12 ft slab, 4 in thick | 1.63 yd³ |
| 20 × 20 ft garage floor, 6 in thick | 8.15 yd³ |
| Wall 20 ft long, 4 ft high, 8 in thick | 2.17 yd³ |
| 8 ft round patio, 4 in thick | 0.68 yd³ |
| 12 in sonotube, 4 ft deep | 0.13 yd³ |
| Four 12 in tube forms, 4 ft each | 0.51 yd³ |
| 12 × 12 in column, 8 ft tall | 0.33 yd³ |
| 3 steps, 4 ft wide, 7 in risers | 0.52 yd³ |
| 3 × 3 m slab, 100 mm thick | 0.99 m³ |
| 2.5 m round pad, 100 mm thick | 0.54 m³ |
| Four 300 mm tube forms, 1.2 m each | 0.37 m³ |
Where the numbers come from
The fixed figures this calculator relies on, and the published source each one was checked against. How the calculators are built and checked
2 sources
Quikrete, Concrete Mix No. 1101 data sheet (Oct 2022), Yield: 0.30, 0.375, 0.45 and 0.60 ft³ for 40, 50, 60 and 80 lb bags, https://www.quikrete.com/PDFs/DATA_SHEET-Concrete%20Mix%201101.pdf; Sakrete High-Strength Concrete Mix (Oldcastle APG): 0.6 ft³ per 80 lb, https://orders.oldcastleapg.com/dry-mixes-masonry/dry-mixes/sakrete-high-strength-concrete-mix-80-lb-gray
Used for
- Volume of set concrete per kilogram of dry premix. Equivalent to 0.60 ft³ per 80 lb bag, 0.45 per 60 lb, 0.375 per 50 lb, 0.30 per 40 lb, about 0.0094 m³ per 20 kg bag.
NRMCA, CIP 31 "Ordering Ready Mixed Concrete": order 4% to 10% more than the plan estimate, https://www.nrmca.org/wp-content/uploads/2021/01/31pr.pdf; US Army FM 5-428 Concrete and Masonry (1998), para 3-20: add 10% up to 200 yd³
Used for
- Slab on ground allowance: 10% extra
Common questions about the Concrete Calculator
- How many bags of concrete do I need?
- 74 bags of 80 lb premix, or 1.63 cubic yards of ready-mix, for the page’s default 10 by 12 foot slab 4 in thick: 40 ft³ plus the 10% allowance is 44 ft³, and 44 ÷ 0.60 ft³ per bag rounds up to 74. In metric, a 3 by 3 m slab 100 mm thick is 0.99 m³ with the allowance, or 106 bags of 20 kg. Every other shape works the same way once its volume is known: a 12 in tube form 4 ft high holds 3.14 ft³, which is 6 bags.
- How do I work out cubic yards of concrete?
- Multiply length, width and thickness in feet to get cubic feet, then divide by 27, because a cubic yard is a cube 3 ft on each side. Convert inches first: a 4 in pour is a third of a foot deep, not 4. The calculator converts every field before it multiplies, so mixed units are safe.
- Should I buy bags or order a truck?
- Look at how many bags the job takes and how long mixing them would keep you busy. On a big pour the first batches begin to stiffen before the last ones go in. Enter both prices here to compare the concrete itself, then ask the supplier what delivery adds.
- How much concrete goes in a sonotube?
- A tube form is a cylinder, so the volume is π × radius² × height. Choose the round column shape, enter the tube’s nominal diameter and the full height including the part below ground, and set how many tubes you are filling.
- My pour is an odd shape. What do I do?
- Split it into pieces this page can draw, work each one out, and add the volumes together. A half-round end is half of a round slab of the same diameter. Add the volumes before you count bags, so the rounding up happens once.
- Does the ready-mix cost include delivery?
- No. It is the volume to order multiplied by the price you type in, nothing more. Suppliers commonly add delivery, a surcharge on small loads and a charge if the truck waits, and these differ from one yard to the next, so ask for them with the quote.