Getting the amount of concrete right is the difference between a smooth Saturday pour and a panic run to the builder’s merchant halfway through the job. This concrete calculator takes the guesswork out of ordering: enter the length, width and thickness of your project, and it tells you how many cubic yards (or cubic metres) of concrete you need — plus how many ready-mix bags that works out to, so you can plan your mix or your order before you touch a shovel.
It matters because concrete is unforgiving. Order too little and you end up with cold joints and a weakened slab; order too much and you’ve paid for material you can’t use. The calculator also covers common project shapes beyond flat slabs — fence posts, footings and steps — so whether you’re setting deck posts or pouring a patio base, you get a realistic material estimate in a few seconds.
This page is for DIYers doing their first pour, but it’s just as handy for experienced hands checking a quick figure on site. If you’re new to mixing by hand, have a look at our guide on concrete mixing ratios explained first — knowing the difference between a 1:2:3 and a 1:2:4 mix will help you make sense of the bag counts below. And if your project is a flat slab, our article on how much concrete you need for a slab walks through the real-world factors — sub-base, reinforcement and waste — that can shift your final number.
How to Use the Concrete Calculator
- Choose your project shape. Pick slab/patio (rectangular), footing, circular post hole, or steps. Each shape asks for slightly different measurements.
- Enter the dimensions. For a slab, that’s length, width and thickness. Measure in feet or metres — the calculator converts automatically. Be honest about thickness: most patios need 4 inches (about 100 mm) of concrete, not the 2 inches you might hope for.
- Set your waste allowance. The default is 10%, which is a sensible margin for spillage, uneven excavation and formwork that isn’t perfectly square. Bump it to 15% if the ground is rough or you’re pouring into soil rather than clean forms.
- Choose bags or bulk. If you’re mixing by hand or with a small mixer, select the bag size you plan to buy (commonly 60 lb, 80 lb, or 25/20 kg bags) and the tool shows how many bags to buy. If a ready-mix truck is delivering, the cubic-yard figure is what you’ll order.
- Read the breakdown. Below the headline number you’ll see the cement, sand and gravel (aggregate) quantities behind it, estimated from a standard general-purpose mix — useful if you’re batching your own.
- Buy a couple of extra bags. The estimate is exactly that — an estimate. Having 1–2 bags spare is cheap insurance against a thin spot at the far end of the form.
How the Calculation Works
The core of the calculation is simple volume: length × width × thickness. Everything else is just unit conversion and practical allowances. For a rectangular slab measured in feet with thickness in inches, the tool converts thickness to feet, multiplies to get cubic feet, then divides by 27 to give cubic yards (concrete is sold by the cubic yard in the US, or by the cubic metre in the UK — 1 cubic yard ≈ 0.765 cubic metres).
For post holes the tool uses the cylinder formula (π × radius² × depth); for steps it multiplies each tread’s volume by the number of treads. The waste allowance is then added on top — so a raw volume of 1.0 cubic yard at 10% waste becomes an order quantity of 1.1 cubic yards.
Bag counts are derived from the typical yield of a bag: an 80 lb bag of pre-mixed concrete yields roughly 0.6 cubic feet, so it takes about 45 bags to make a cubic yard (60 lb bags yield about 0.45 cubic feet, so around 60 per cubic yard). Individual cement, sand and gravel quantities assume a standard 1:2:3 (cement:sand:gravel) general-purpose mix with an average cement density.
Worked example: a patio 12 ft long × 10 ft wide × 4 inches thick. Volume = 12 × 10 × (4/12) = 40 cubic feet = 1.48 cubic yards. Add 10% waste → about 1.63 cubic yards to order, or roughly 73 eighty-pound bags if you’re mixing by hand.
Tips for Accurate Results
- Measure the excavated area, not the finished drawing. Forms that bow outward and ground that isn’t perfectly level both add concrete you didn’t plan for.
- Dig to a consistent depth. A slab that’s 5 inches in the middle and 3 at the edges won’t just waste concrete — it’ll be weaker where it’s thin.
- Compact and level the sub-base before measuring thickness. Soft soil compresses under the pour, effectively deepening the form.
- Round up, never down. Suppliers sell ready-mix by the half yard in most areas; a slightly short order can stall a pour and weaken the joint.
- Account for reinforcement. Rebar and mesh displace a tiny amount of concrete, but at typical DIY scales the effect is negligible — don’t try to subtract it.
- Check bag yields on the product you actually buy. Yields vary slightly between brands and between standard and fast-setting mixes.
Frequently Asked Questions
How thick should a concrete slab be?
For a typical patio, footpath or shed base, 4 inches (100 mm) is the standard DIY thickness. Driveways that take car traffic should be 5–6 inches. Going thinner than 4 inches risks cracking under normal loads, and no calculator can fix a slab poured too thin.
Should I order ready-mix or bags?
As a rough rule: under about half a cubic yard (roughly 20–25 eighty-pound bags), mixing bags yourself is manageable and usually cheaper. Beyond a full cubic yard, a ready-mix delivery almost always makes sense — it arrives in one consistent batch and saves your back.
How much waste should I allow for?
10% is a solid default for clean formwork on level ground. Use 15% for irregular excavations, post holes dug in rocky soil, or your first ever pour. It’s always better to have a bag or two left over than to be a wheelbarrow short.
Can I use this calculator for a shed base or footings?
Yes — the slab setting covers shed bases, and the footing option handles strip and pad footings for posts, walls and deck supports. For structural footings that carry a building, always check local building codes and consider an engineer’s specification rather than relying on a DIY estimate alone.
Why does the calculator ask for a mix ratio?
The bag count for pre-mixed concrete bags is fixed, but if you’re batching your own cement, sand and gravel, the mix ratio determines how much of each raw material you need. A 1:2:3 mix suits most general DIY work like patios and footings; check our concrete mixing ratios guide if you’re unsure which mix your project needs.