How to use this calculator
- Measure the pour. For a slab, patio or footing, enter the length and width. If the shape is not a simple rectangle — an L-shaped patio, a walkway that turns a corner — split it into rectangles and add a section for each. The calculator adds the areas together.
- Set the thickness. Four inches is standard for patios and walkways. The preset buttons cover the common cases.
- Pick your bag size and check the waste allowance. Ten percent is the default, and the reasoning is in the waste section below.
You can type feet-and-inches the way you would write it on a scrap of paper: 12' 6", 12 ft 6 in and 12-6 all mean the same thing. Switch to metres and centimetres with the toggle if that is how your tape reads.
Every number stays in the URL, so you can copy the link and send the result to whoever is picking up the material.
The formula
Concrete is sold by volume, but you measure a slab in two dimensions and a thickness. The conversion is where most mistakes happen, because the thickness is in inches while everything else is in feet.
volume (cubic feet) = length (ft) × width (ft) × thickness (in) ÷ 12
cubic yards = cubic feet ÷ 27
bags = cubic feet ÷ bag yield (cu ft), rounded up
Worked example — a 10 ft × 12 ft patio, 4 inches thick:
- Area: 10 × 12 = 120 sq ft
- Thickness in feet: 4 ÷ 12 = 0.333 ft
- Volume: 120 × 0.333 = 40 cubic feet
- In cubic yards: 40 ÷ 27 = 1.48 cubic yards
- With a 10% allowance: 40 × 1.10 = 44 cubic feet, or 1.63 cubic yards
- In 80 lb bags: 44 ÷ 0.6 = 73.3, rounded up to 74 bags
That last line is the one worth sitting with. Seventy-four bags is roughly 5,900 lb of material, four or five trips in a half-ton pickup, and several hours of mixing. This is exactly the size of job where ready-mix delivery stops being a luxury.
Round post holes and columns use the same idea with a different area:
volume (cubic feet) = π × (diameter in inches ÷ 24)² × depth (ft) × number of holes
The ÷ 24 converts a diameter in inches into a radius in feet in one step.
Concrete bag yields
A bag of pre-mixed concrete already contains cement, sand and aggregate — you add water. The yield is the volume of finished concrete one bag makes, and it is printed on the bag.
| Bag size | Yield per bag | Bags per cubic yard |
|---|---|---|
| 90 lb | 0.675 cu ft | 40 |
| 80 lb | 0.60 cu ft | 45 |
| 60 lb | 0.45 cu ft | 60 |
| 50 lb | 0.375 cu ft | 72 |
| 40 lb | 0.30 cu ft | 90 |
These are the yields published by QUIKRETE for their general-purpose Concrete Mix (data sheet no. 1101); the 90 lb bag is regionally available. High-strength, fast-setting and crack-resistant formulas differ, and so do other brands. Check the label before you buy in quantity — being wrong by 0.05 cu ft per bag across 70 bags is a missing half-yard of concrete.
How thick should the slab be?
| Thickness | Typical use |
|---|---|
| 4 in | Patios, walkways, shed floors, and driveways used only by cars |
| 5 in | Driveways that see the occasional heavier vehicle |
| 6 in | Trucks, RV and trailer pads, workshop floors |
| 8 in+ | Footings, commercial slabs, heavy equipment |
Four inches is the minimum for a patio, and the industry range for residential driveways is four to six inches: four is enough for passenger cars, and you move up when trucks or an RV will park on it. Heavy point loads — a hot tub, an outdoor fireplace — also push a patio to five or six.
Thickness is not the only thing carrying the load. A 4-inch slab on a properly compacted gravel base will outlast a 6-inch slab poured on soft ground. Most residential slabs sit on 4 to 6 inches of compacted gravel, which is a separate material to price up.
Local building codes often set a minimum thickness and may require reinforcement or a permit for anything structural. Check before you pour, not after.
How much waste should you add?
The default here is 10%. Industry guidance is to order 10 to 15 percent extra, and the reason is not padding for its own sake:
- Subgrade is never flat. A slab that averages a quarter-inch deeper than planned across 200 sq ft is another 4 cubic feet of concrete.
- Forms move. Wet concrete is heavy, and form boards bow outward as you fill them. The finished slab is almost always slightly larger than the one you measured.
- Spillage is real. Some mix stays in the mixer, some ends up on the ground, some sets in the wheelbarrow while you are screeding.
- Running short is expensive. Stopping a pour to fetch more material leaves a cold joint — a visible seam that is also a weak point. Ordering an extra wheelbarrow’s worth costs a few dollars; a cold joint costs a slab.
For bagged mix, keep unopened bags dry and most stores will take them back. For ready-mix, order to the nearest quarter yard above your number and have somewhere in mind for the surplus — a spare pier, a step, a mower ramp.
This is an estimate, not an engineering calculation. It gives you a shopping quantity. Structural work — footings that carry a building, slabs that support vehicles, anything a permit covers — should be sized by someone who has seen your site.