Concrete Calculator
Find how much concrete a slab, footing, column, tube, curb and gutter or steps need. Mix units per field, add a waste allowance and get cubic yards, cubic metres, litres, 40 to 80 lb bags, truck-ready totals and a cost estimate.
Leave blank for 1.
Spillage and uneven ground. Blank counts as 0.
In your own currency.
Enter the dimensions to see the volume.
About the concrete calculator
The Concrete Calculator works out how much concrete a job needs, in cubic yards, cubic feet, cubic metres and litres, and then converts that volume into a count of premixed bags. It handles eight shapes, so the same page covers a flat slab, a round column, a hollow tube, a triangular wedge, a trapezoid, a set of concrete steps, a curb and gutter run and any area you already know. Every length has its own unit, so you can type feet in one field and centimetres in the next without converting anything first.
Concrete is sold by volume, and the main job of a calculator like this is to turn your measurements into the number the supplier asks for. Ready-mix is ordered by the cubic yard in the United States, or by the cubic metre in most other places. Bagged mix is sold by weight, and each bag fills a known amount of space. The tool shows both views so you can decide which suits the job.
How to calculate concrete
Steps
- Pick the shape that matches the pour: Rectangle, Circle, Tube, Triangle, Trapezoid, Stairs, Curb and gutter or Known area.
- Enter each dimension and choose its unit (ft, in, yd, m, cm or mm). The depth field is labelled Thickness or height.
- Set the Quantity if you have several identical pieces, such as four columns. The default is 1.
- Leave the waste allowance at 10 percent or change it to suit the site.
- Read the order volume, the bag counts and, if you entered a price per yd³, m³ or bag, the estimated cost.
The formulas
Volume by shape
Rectangle (slab, footing, wall):
Circle (round column or pier, solid):
Tube (hollow cylinder with an outer and inner diameter):
Triangle (a wedge with base b, height h and depth t):
Trapezoid (parallel sides a and b, distance h between them, depth t):
Stairs (n steps, each with a rise and a run, across a given width):
Curb and gutter (curb depth c and height h standing on a gutter flag of width w and thickness f, over a length L):
Known area:
Order quantity and bags
What is a cubic yard?
A cubic yard is a cube that measures 3 ft by 3 ft by 3 ft, so . That is the number to keep in mind: divide cubic feet by 27 to get cubic yards. A cubic metre is a little larger, at about 1.31 cubic yards. If you only need to switch units, the Volume Converter does that directly.
Bag yields used here
Bag counts are rounded up, because you cannot buy part of a bag. The calculator assumes these yields.
- 40 lb bag: 0.30 ft³
- 60 lb bag: 0.45 ft³
- 80 lb bag: 0.60 ft³
- 25 kg bag: 0.0125 m³
These match the figures printed on most bags, but the label on the product you buy is the number to trust.
Worked example: a 10 ft by 10 ft slab
A slab is 10 ft long, 10 ft wide and 4 in thick. Four inches is one third of a foot, so the net volume is 10 x 10 x 0.3333 = 33.33 ft³, or 1.23 yd³. With a 10 percent allowance the order is 36.67 ft³, which is 1.36 yd³, 1.04 m³ or 1,038.3 litres. In bags that is 123 of 40 lb, 82 of 60 lb, 62 of 80 lb or 84 of 25 kg. At 6.50 per 80 lb bag the 62 bags cost 403.00. As ready-mix at 150 per yd³ the same 1.36 yd³ comes to about 203.70, which is why larger pours are usually delivered rather than mixed from bags.
Worked examples for the other shapes
- Round column: 2 ft diameter and 3 ft tall holds 9.42 ft³ each, so four columns come to 37.70 ft³ before waste.
- Tube: 2 ft outer diameter, 1.5 ft inner diameter and 10 ft long is 13.74 ft³. This suits a hollow pipe-shaped pour or a ring wall.
- Triangle: base 6 ft, height 4 ft and depth 6 in is 6 ft³.
- Trapezoid: parallel sides of 10 ft and 6 ft, 5 ft apart and 6 in deep is 20 ft³.
- Stairs: 4 steps with a 7 in rise, 11 in run and 36 in width is 16.04 ft³.
- Curb and gutter: a curb 6 in deep and 4 in high on a gutter flag 12 in wide and 6 in thick, run for 10 ft, is (0.5 x 0.8333 + 1 x 0.5) x 10 = 9.17 ft³.
- Known area: 250 ft² at 3 in deep is 62.5 ft³. The Area Calculator is handy for finding an irregular area first.
- Mixed units: 3 m long, 2 m wide and 10 cm thick is 0.6 m³.
Why add waste
Ground is rarely perfectly flat, forms bow slightly under the weight of wet concrete, and some mix is lost to spillage and what stays in the truck or barrow. A typical allowance is 5 to 10 percent, and the calculator starts at 10. Raise it if the subgrade is rough or you over-dig, and lower it for a tight, level formwork. Ordering slightly too much is usually cheaper than a second delivery for a small shortfall.
Accuracy and limits
The result is an estimate based on the measurements you enter. It does not know about reinforcement, voids, slopes you did not measure or the exact yield of a particular mix. Many ready-mix suppliers have a minimum load and a short-load fee, so confirm quantity and price with the supplier before you order. For structural work, such as footings that carry a building, columns or steps with a code requirement, check dimensions and mix with a qualified engineer and your local building rules.
Privacy
Nothing is uploaded; every calculation runs in your browser and never leaves your device.
Related tools
To estimate fill under a slab, try the Gravel Calculator. For a quick flat-area measurement use the Area Calculator, and if you are buying bags the Concrete Bag Calculator focuses on bag counts and small jobs. The Length Converter helps when plans mix feet and metres.
Frequently asked questions
How do I calculate how much concrete I need?
Multiply length by width by thickness for a slab, using the same unit for all three, then convert to cubic yards by dividing cubic feet by 27. The calculator also covers columns, tubes, triangles, trapezoids, stairs, curb and gutter and a known area, and adds a waste allowance on top.
How many cubic feet are in a cubic yard?
There are 27 cubic feet in a cubic yard, because a cubic yard is a cube that measures 3 ft by 3 ft by 3 ft. To convert, divide cubic feet by 27, so 36.67 ft³ is about 1.36 yd³.
Which shapes can I calculate?
Rectangle, circle (solid column or pier), tube (hollow cylinder), triangle, trapezoid, stairs, curb and gutter, and known area times depth. Each shape has a quantity field, so several identical pieces can be added in one go.
Can I mix units, such as metres and inches?
Yes. Every length has its own unit selector for feet, inches, yards, metres, centimetres or millimetres, and the tool converts them before multiplying. For example, 3 m by 2 m by 10 cm gives 0.6 m³.
How much extra concrete should I order?
The calculator defaults to a 10 percent waste allowance to cover uneven ground, over-excavation and spillage. Raise it for rough subgrades and confirm the final quantity, minimum load and any short-load fee with your supplier.
How is the stairs volume worked out?
The tool treats the steps as a stack of blocks, so volume equals width times run times rise times n(n+1)/2 for n steps. Four steps with a 7 in rise, 11 in run and 36 in width come to about 16.04 ft³.
Is my data sent anywhere?
No. The slab, column and bag sums all run in your browser, so your dimensions, waste allowance and prices are never uploaded or stored on a server. They stay on this device and are not shared with anyone.