Concrete Calculator
Pick your pour, a slab or wall, a round column or footing, a circular slab or tube, or a flight of stairs, and get the concrete volume in cubic yards, cubic metres and cubic feet, the weight, how many 40, 60 or 80 lb bags it takes, and an optional total cost with a waste allowance built in.
Formula
Worked example
A 10 ft × 10 ft slab at 4 in is 100 × 0.333 ft = 33.3 ft³, about 1.23 yd³ (0.94 m³). Add 10% waste to ~1.36 yd³, roughly 60 bags of 60 lb pre-mix, so a ready-mix short load is the better call here.
How the calculation works
Every shape reduces to a volume. A slab or wall is length times width times thickness. A round column or hole is the area of a circle, pi times radius squared, times its height. A circular slab or tube is the outer-circle area minus the inner-circle area, times the length. A staircase is the stacked wedge of each step (run times rise, halved and summed) plus the platform, times the width. All dimensions are converted to metres before multiplying, then the result is expressed in cubic metres, cubic feet, and cubic yards at once. Weight uses the standard density of cured, normal-weight concrete, 2,400 kg/m3 (about 150 lb/ft3), the accepted baseline for structural ready-mix.
Bags versus ready-mix, and the cost estimate
Bagged concrete lists a yield on the sack: a 40 lb bag makes about 0.30 ft3, a 60 lb bag about 0.45 ft3, and an 80 lb bag about 0.60 ft3 of mixed concrete. The calculator divides your volume (with waste added) by the chosen bag yield and rounds up so you know how many sacks to buy. Once you cross roughly one cubic yard (27 ft3), ready-mix delivery is usually cheaper and far less work than mixing dozens of bags by hand. Turn on the cost estimate to price the job either by the cubic yard or metre for ready-mix, or by the bag, the total is a planning figure since local prices, short-load fees, and delivery vary.
Quantity, waste, and getting the order right
Set the quantity field above one when you are pouring several identical piers, footings, or columns and the calculator multiplies the volume through. Always add 5 to 10 percent for waste: sub-grade that is not perfectly level, formwork that bows slightly, spillage at the chute, and the concrete left clinging to the mixer all eat into your pour. The calculator adds your chosen waste percentage before working out bags and cost, so the figures you see already include that cushion. Round the final ready-mix order up to the nearest quarter or half yard, since most suppliers sell in those increments.
What affects the real amount, and limitations
An uneven or over-excavated base raises the effective thickness and the volume with it, thickened slab edges, footings, and step-downs add concrete that a flat slab figure misses. For an L-shaped or irregular pour, split it into rectangles, columns, and circles, run each through the calculator, and add the results. The 2,400 kg/m3 density is nominal: lightweight mixes run roughly 1,440 to 1,920 kg/m3 and heavyweight mixes can top 3,200 kg/m3. This tool sizes the material only. Slab thickness, reinforcement, and sub-base preparation for any load-bearing pour must be set by a qualified engineer.
Pre-mix bag yields and typical slab thickness
| Item | Value | Notes |
|---|---|---|
| 40 lb bag | 0.30 ft³ (0.0085 m³) | About 90 bags per cubic yard |
| 60 lb bag | 0.45 ft³ (0.0127 m³) | About 60 bags per cubic yard |
| 80 lb bag | 0.60 ft³ (0.0170 m³) | About 45 bags per cubic yard |
| Patio / driveway | 4 in (100 mm) | Typical residential slab |
| Structural floor | 6 in (150 mm)+ | Confirm with an engineer |
| Normal-weight density | 2,400 kg/m³ (150 lb/ft³) | Used for the weight figure |
Bag yields are nominal mixed-concrete volumes; thickness guidance is for residential work.
Frequently asked questions
How many bags of concrete do I need for a slab?
Find the volume in cubic feet, add 5 to 10 percent for waste, then divide by the yield printed on the bag. A 40 lb bag yields about 0.30 ft3, a 60 lb bag about 0.45 ft3, and an 80 lb bag about 0.60 ft3. This calculator does that automatically and shows the bag count for all three sizes. Above roughly one cubic yard (27 ft3) ready-mix delivery is usually cheaper than bagged concrete.
How do I calculate concrete for a round column or footing?
The volume of a cylinder is pi times the radius squared times the height. Choose the round column or footing shape, enter the diameter and the height or depth, and the calculator works out the volume, weight, bags, and cost. Set the quantity field to pour several identical piers in one estimate.
How thick should a concrete slab be?
A 100 mm (4 in) slab is the typical minimum for residential driveways, patios, and sidewalks without heavy vehicle traffic. Garage floors and light commercial slabs are commonly 125 to 150 mm (5 to 6 in). Structural slabs that carry significant loads need a thickness and reinforcement schedule set by a structural engineer from actual load calculations.
How do I convert cubic metres to cubic yards for concrete?
One cubic metre equals about 1.308 cubic yards, so multiply your cubic metre figure by 1.308. Going the other way, one cubic yard is 0.7646 cubic metres. US ready-mix is almost always sold by the cubic yard, while suppliers in Canada, the UK, and Australia usually quote cubic metres. This calculator shows all three units at once.