Concrete Calculator (Slab, Footing, Column, Tube & Steps)
Add up every slab, footing, column and step — then get m³, yd³, bags and truckloads.
What are you pouring?
Every size has its own unit. You can also type units, fractions or feet and inches, such as 150 mm, 3 1/2 or 10' 6".
Waste, delivery and price
Enter the sizes of what you are pouring.
Volume of each element
| # | Element | Qty | m³ | ft³ | yd³ |
|---|
About the Concrete Calculator (Slab, Footing, Column, Tube & Steps)
Work out how much concrete to order for a whole job, not just one slab. Add each part of the pour — slabs and floors, walls, pad, strip, stepped and sloped footings, rectangular and round columns, Sonotubes, hollow tubes, steps on the ground, stair flights and curb and gutter — enter its sizes in metres, millimetres, feet or inches, and the calculator adds them up.
You get the volume in cubic metres, cubic feet (cft) and cubic yards with your waste allowance, the number of ready-mix truckloads for your supplier's load size, how many premix bags you would need instead (40–90 lb or 20–40 kg, with the yield editable), and the cost at your own price.
How to use it
- Pick Metric or US units, choose the type of the first element — for example Slab or floor — and type its sizes. Every size has its own unit picker, and you can also type
150 mm,3 1/2or10' 6". - Set Quantity for identical parts (four columns of the same size) and use Add element for every other part of the pour.
- Check the waste allowance, your supplier's ready-mix load size and the premix bag you would use. The bag yield fills in automatically; change it to the figure printed on your bag.
- Optionally enter a price per m³, yd³, ft³ or bag. Copy the result, or download the element table as CSV for your estimate.
Examples
1.8 m³ net → 1.89 m³ to order = 66.74 ft³ (cft) = 2.47 yd³ · 1 ready-mix load (6 m³) · 162 × 25 kg bags
40 ft³ net → 44 ft³ = 1.63 yd³ = 1.25 m³ · 74 × 80 lb bags at 0.60 ft³ each
4 × 3.14 ft³ = 12.57 ft³ = 0.47 yd³ · 21 × 80 lb bags
1.2 m³ (edge block) + 0.556 m³ (sloped part) = 1.756 m³ per footing
0.27 m³ (step triangles) + 0.604 m³ (waist slab) = 0.874 m³
Formulas used
- Slab, wall, pad or strip footing: length × width × thickness.
- Stepped footing: the sum of each step's length × width × depth.
- Sloped (trapezoidal) footing: the vertical block at the edge, L × B × D₁, plus the sloped part from the prismoidal formula
D₂ ÷ 6 × (L×B + l×b + 4 × (L+l)/2 × (B+b)/2). That formula is exact for a solid with a rectangular base and a parallel rectangular top; when the two are similar it gives the same answer as the frustum formulah ÷ 3 × (A₁ + A₂ + √(A₁·A₂)). - Round column or Sonotube: π ÷ 4 × diameter² × height. Hollow tube or ring: π ÷ 4 × (outer diameter² − inner diameter²) × height.
- Steps on the ground: each step is solid down to the ground, so n steps of rise R and tread T on a width W hold
W × R × T × n(n + 1) ÷ 2. If the top step is a deeper landing of depth P, the volume isW × R × (T × n(n − 1) ÷ 2 + P × n). - Stair flight on a waist slab: n step triangles of R × T ÷ 2 plus an inclined slab of thickness t and sloping length n × √(R² + T²), all times the width. Landings are separate slabs.
- Curb and gutter: length × (curb depth × (curb height + flag thickness) + gutter width × flag thickness).
Units, cft and cubic yards
The conversions use the exact definitions 1 ft = 0.3048 m and 1 in = 25.4 mm from NIST SP 811, so 1 m³ = 35.3147 ft³ = 1.30795 yd³, and 1 yd³ = 27 ft³ = 0.764555 m³. In India cft simply means cubic feet.
You can mix units freely: a slab can be 12 ft long, 3.5 m wide and 150 mm thick. Type the unit in the box (6 in, 15 cm) or pick it from the list next to each size.
Premix bags and ready-mix loads
Bag yields for 40, 50, 60, 80 and 90 lb bags are the figures on the Quikrete Concrete Mix (No. 1101) data sheet: 0.30, 0.375, 0.45, 0.60 and 0.675 ft³. There is no single published figure for metric bags, so 20, 25 and 40 kg bags are estimated at the same 0.60 ft³ per 80 lb (about 0.47 litres of concrete per kg of dry mix) — a 25 kg bag shows about 11.7 litres. Brands differ, so type in the yield printed on the bag you buy.
Bags and truckloads are rounded up to whole numbers, and the result tells you how full the last truck is. Ready-mix suppliers set their own load sizes and minimum orders, so enter the load size they quote.
Sources
- NIST, SP 811 Appendix B.8 — conversion factors (foot, inch, cubic foot, cubic yard)
- QUIKRETE®, Concrete Mix (No. 1101) data sheet — yield per bag (40 lb 0.30 ft³, 50 lb 0.375 ft³, 60 lb 0.45 ft³, 80 lb 0.60 ft³, 90 lb 0.675 ft³)
- Prismoidal and frustum volume formulas: standard solid geometry
Limitations
- Volumes are for the shapes exactly as entered. They do not allow for rebar, embedded pipes, voids, sloping or soft ground, or formwork that bulges — that is what the waste allowance is for.
- Stair flights exclude landings and the extra concrete where a flight meets a landing or beam; add those as slabs.
- Bag yields vary by brand and mix. The metric bag yields are estimates scaled from a published 80 lb figure.
- This is a quantity estimate, not a structural design: thickness, concrete grade and reinforcement must come from your engineer or local building code.
Privacy
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Frequently asked questions
How do I calculate the concrete for a slab?
Multiply length × width × thickness in the same unit. A 4 m × 3 m slab 150 mm (0.15 m) thick holds 4 × 3 × 0.15 = 1.8 m³. In feet, 12 ft × 10 ft × 4 in (0.333 ft) is 40 ft³, or 40 ÷ 27 = 1.48 yd³. Add a waste allowance before you order.
How many bags of concrete do I need?
Divide the volume by the yield of one bag. An 80 lb bag of Quikrete Concrete Mix makes about 0.60 ft³, so a 40 ft³ slab needs 40 ÷ 0.60 = 66.7, which rounds up to 67 bags — 74 with 10% waste. For bigger pours, compare the cost with ready-mix.
What is cft in concrete?
Cft is cubic feet. 1 m³ = 35.31 cft, so 1.89 m³ of concrete is 66.7 cft. Sand and aggregate are also sold by the brass, which is 100 cft.
How much extra concrete should I order?
It depends on how even the base is, how rigid the formwork is and how the concrete is placed, so the allowance is yours to set — the default is 5%. Running short in the middle of a pour leaves a cold joint, so most people order a little more than the net volume.
How do I calculate a sloped (trapezoidal) footing?
Split it into the vertical block at the edge and the sloped part. The sloped part has a rectangular base L × B and top l × b, so its volume is D₂ ÷ 6 × (L×B + l×b + 4 × (L+l)/2 × (B+b)/2). A 2 m × 2 m base with a 0.6 m × 0.6 m top and a 300 mm high slope holds 0.556 m³; with a 300 mm vertical edge (1.2 m³) the footing holds 1.756 m³.
Why does a round column use π ÷ 4?
The area of a circle is π × r², and with the radius r = d ÷ 2 that becomes π ÷ 4 × d². A 300 mm column 3 m tall holds 0.7854 × 0.3² × 3 = 0.212 m³; a 12 in Sonotube 4 ft deep holds 3.14 ft³.