Concrete Calculator

Concrete Calculator: How Many Yards and Bags Do You Need?

If you are pouring a patio slab, setting fence posts or pricing a driveway, the first question is always the same: how much concrete do I need — and how many bags of concrete do I need if I mix it myself? This calculator answers both. Enter your dimensions once and it works out the volume in cubic yards, cubic feet and cubic metres, the number of bags at the size you choose, an estimate of the weight, and what the same job would cost as ready-mix, using prices you enter yourself.

How much concrete do I need? Quick table for common slabs

These are the slabs people ask about most. Every figure below is for a slab 4 in thick, includes 10% waste, and counts bags at the 80 lb size:

Slab size (4 in thick)	Cubic yards (with 10% waste)	80 lb bags
8 × 8 ft	0.87 yd³	40
10 × 10 ft	1.36 yd³	62
12 × 12 ft	1.96 yd³	88
10 × 20 ft	2.72 yd³	123
24 × 24 ft	7.82 yd³	352

A note on the biggest row: at 24 × 24 ft you are looking at 352 bags. Mixing 352 bags is a full day of mixer loads — lifting, mixing and placing several tons of concrete bag by bag — and at that size ready-mix delivered by truck is the sensible choice.

The formula, step by step

Concrete is sold by volume, so every calculation on this page is really one idea: work out the volume of the slab or hole, then convert it into the units you order in. Nothing here needs more than a tape measure and a few divisions.

• Rectangular slab: volume = length × width × thickness, with everything in feet. Thickness is usually measured in inches, so divide it by 12 first — 4 in is 0.333 ft.
• Circular slab: volume = π × radius² × thickness. The radius is half the diameter, and π is about 3.1416.
• Post holes: volume = π × radius² × depth for one hole, then multiply by the number of holes.
• Convert: cubic yards = cubic feet ÷ 27. Cubic metres = cubic feet ÷ 35.31.
• Add waste: multiply by 1.10 for a 10% allowance, so you order a little more than the neat geometry says.
• Bags: divide the volume with waste by the yield of one bag — 0.60 cu ft for an 80 lb bag — and always round up, because you cannot buy part of a bag.

The calculator does exactly these steps, in exactly this order, which is why the table above and the worked example below match its results. If your own hand calculation disagrees with a supplier's quote, one of these steps — usually thickness or waste — is where the difference is hiding.

Worked example

Take the most common job on this page: a 10 × 10 ft slab, 4 in thick, for a shed base or patio.

• Step 1 — volume: 10 ft × 10 ft × (4 ÷ 12) ft = 33.3 cu ft.
• Step 2 — cubic yards: 33.3 ÷ 27 = 1.23 yd³ net.
• Step 3 — add waste: 1.23 × 1.10 = 1.36 yd³ with 10% waste.
• Step 4 — bags: the volume with waste is 36.7 cu ft, and 36.7 ÷ 0.60 = 61.1, rounded up to 62 × 80 lb bags.
• Step 5 — weight check: 33.3 cu ft × 150 lb per cu ft is about 5,000 lb of concrete — worth knowing before you plan to move it in a small trailer.

So the headline answer for that slab is 1.36 yd³ to order, or about 62 bags to buy. It also sits right on the bags-versus-truck borderline discussed next.

Bags vs ready-mix: which is cheaper and when

Bags suit small jobs. There is no delivery to book, no minimum order and no truck clock running: you mix only what you can place, when you are ready. For a few post holes, a small equipment pad or a repair, bags are usually the cheaper and simpler route overall — even though the concrete itself costs more per yard — because a ready-mix delivery brings fees of its own.

The crossover is roughly 1 yd³. Beyond that, ready-mix is usually cheaper per yard, faster and more consistent: the plant batches the whole pour to one recipe, and a truck places in minutes what would take hours of mixing bag by bag. Concrete that arrives in one pour also sets as one slab, rather than as a patchwork of mixer loads placed at different times.

One catch: under about 7 yd³ many suppliers add a short-load fee of roughly $50–200, because a part-full truck still costs them the same trip. That fee can wipe out the saving on a small load, so ask about it before ordering, and compare the delivered price — truck plus any fee — against the bag cost from the calculator, not the per-yard price alone.

How thick should a slab be?

• 4 in for patios, walkways and sheds. This is the standard for foot traffic and garden buildings on a prepared, level subgrade.
• 5–6 in for driveways and garage floors. Vehicles, turning wheels and point loads from jacks and stands all ask more of a slab, and many builders add mesh or rebar at this thickness.
• Footings and load-bearing work follow code. Their depth and width are set by your local building code and the ground underneath, not by a rule of thumb.

Whatever thickness you choose, measure the forms rather than trusting the plan. A slab dug to 5 in where 4 in was intended needs about 25% more concrete — which is another reason the waste allowance in the calculator matters, and why it should never be set to zero.

Common mistakes

• Ordering with no waste allowance. Forms bow under wet concrete and the subgrade is never perfectly level, so the neat volume is always a little less than the real one. Running short mid-pour leaves a weak cold joint and an expensive second delivery.
• Guessing thickness. A slab is rarely the same depth everywhere. Measure inside the forms at several points and use the average, not the shallowest reading.
• Forgetting how much a small error matters. A half-inch of thickness error is about 11% of the volume of a 4 in slab. Small measuring slips become whole extra bags — or a shortfall — very quickly.
• Ordering bags for a job that needed a truck. If the bag count runs into the hundreds, stop and price ready-mix. Hundreds of bags mean hours of lifting and mixing, and a pour that goes in slowly, in batches, instead of all at once.

US, UK and Australia: yards, metres and bag sizes

The US orders ready-mix in cubic yards and buys bagged concrete in 40, 60 and 80 lb bags. The UK and Australia order ready-mix by the cubic metre (m³) and buy 20 or 25 kg bags. The Metric mode of the calculator covers both systems — the same formulas, with metric units and bag sizes. Prices vary too much by town and supplier for any calculator to hard-code them honestly, so the price fields are yours to fill in: enter your own local prices and the comparison uses those, not a guess.

Disclaimer

This calculator works out volumes only — it is not a structural design. Slab thickness and reinforcement for load-bearing work, foundations, retaining walls over 4 ft and anything structural need an engineer and usually a permit under your local code.

Concrete guides

Prefer to work things out on paper first? These four guides explain the maths behind the calculator, with worked examples for slabs, footings, post holes and bag counts.

• How Much Concrete Do I Need?
• Concrete Calculator Formula
• How Many Bags of Concrete Do I Need?
• How to Calculate Materials for a Concrete Slab — Every material for a slab in one list — concrete, gravel base, rebar or mesh, and formwork, with a full worked example.

Frequently asked questions