The formula everyone quotes — length × width × thickness ÷ 27 — is the easy 10% of this question. The harder 90% is picking the thickness, deciding bags vs truck, and not coming up 6 bags short at 7 pm. Here’s the whole picture.

Step 1: Pick the thickness first

Thickness drives everything — it linearly multiplies the volume, and it’s decided by what the slab carries, not by the area:

Project	Typical thickness
Sidewalk / garden path	4 in
Patio	4 in
Shed floor	4 in (5–6 in for heavy equipment)
Driveway, passenger cars	4–6 in
Driveway, trucks/RVs	6 in
Strip footing (house walls)	per code — commonly 8–16 in deep
Post hole / column footing	sized by post and frost depth, not a chart

Two honest notes. First, these are common practice, not engineering — anything load-bearing or attached to a house goes by local code, and your inspector’s word beats any website’s. Second, the base matters as much as the slab: 4 in over compacted gravel outlives 6 in over soft dirt, every time. On a driveway, spending the extra 2 in of excavation on a compacted base layer usually beats spending it on concrete.

Step 2: Get the volume right

Rectangles are L × W × (D ÷ 12) in feet — a 10 × 12 ft patio at 4 in is 40 ft³ = 1.48 yd³. Round posts and tubes are π × (d ÷ 2)² × h. Steps are stacked boxes. For irregular shapes, break them into rectangles and add — a walkway that’s 4 ft wide except for a 6 × 3 ft bump-out is two boxes, not a headache. The calculator handles all four shapes with the units converted either way.

Then add the waste margin. 10% is the standard — not because concrete gets wasted, but because real ground isn’t perfectly level, forms bow, and spillage is a law of physics. On a compacted, screeded, rectangular form you can shave it toward 5%; on hand-dug footings in soft soil, 12–15% isn’t pessimism, it’s calibration.

Step 3: Bags or truck — the real breakeven

Bag math first, because it’s honest math: an 80 lb bag of standard mix yields 0.60 ft³ (0.45 for 60 lb, 0.30 for 40 lb — the manufacturer’s published figures, cited on sources). So one cubic yard is exactly 45 × 80 lb bags, and a yard with 10% waste is ~50. Every yard in bags is a pickup-load of lifting and an afternoon of mixing.

The rough rule of thumb:

• Under ~0.5 yd³ (≈23 × 80 lb bags): bags win. Post holes, small pads, repairs — the truck can’t justify itself.
• 0.5–1 yd³: toss-up — price both. Bags if access is tight, you want multiple small pours, or the job spreads over weekends.
• Over ~1 yd³: the truck usually wins on effort and money, even with a short-load fee. 2 yd³ is ~100 bags — that’s not a project, it’s a workout program with a deadline.

Ready-mix ordering tips worth knowing: plants typically have a minimum (often 3–5 yd³) below which a short-load fee applies — ask the number before deciding. Order to the nearest quarter-yard, tell them it’s a DIY pour, and have the forms ready before you call — the truck waits for nobody, and waiting time bills by the minute. For the metric version of all this, 1 yd³ is 0.76 m³ and a 25 kg bag yields 0.0116 m³ (~86 bags to a cubic metre).

What your result doesn’t include

Volume isn’t the whole pour. Reinforcement (mesh or rebar), form boards, a gravel base, and curing all sit outside the calculator. And before any footing or post-hole digging: call 811 in the US a few business days ahead — free utility marking, and far cheaper than the alternative. The gravel calculator covers base material under slabs, and the paver calculator does the layered version — base, bedding, and bricks — for paths and patios.

Frequently asked questions

How do I figure out how much concrete I need?

Volume = length × width × (thickness ÷ 12), all in feet — then ÷ 27 for cubic yards. A 10 × 12 ft patio at 4 in is 40 ft³ = 1.48 yd³. Add ~10% for waste: order ~1.6 yd³, or 74 × 80 lb bags if you're mixing. The calculator does this live for slabs, footings, columns and steps.

How many 80 lb bags of concrete make a cubic yard?

Exactly 45 bags at the published 0.60 ft³ yield (27 ÷ 0.60). With the usual 10% waste margin you're buying ~50 bags per yard — and at 60–80 lb each, a yard in bags is roughly 3,600–4,000 lb of mixing. That's the number that sells ready-mix trucks.

When should I order ready-mix instead of bags?

The crossover is usually around 1 yd³: under that, bags are flexible (no truck access needed, pour at your pace); over that, bags are slower and often pricier than a truck even after the short-load fee most plants charge under ~4 yd³. Price both — the calculator's ready-mix line exists for exactly this comparison.

How thick should a concrete slab be?

Common residential practice: 4 in for sidewalks, patios and shed floors; 4–6 in for driveways (6 in for heavier vehicles); footings set by local code and soil — commonly 8–16 in deep for residential strip footings. Always check your local code for anything structural, and compact the base no matter what the depth.

How much extra concrete should I order?

~10% is the standard margin over a compacted, level base — it absorbs spillage, form wobble and ground that isn't as flat as it looked. Soft or irregular subgrade: go 12–15%. The expensive failure is coming up short — a second truck load or an unplanned cold joint mid-slab.

What is a 'short load' fee?

Ready-mix plants charge it when your order is under their minimum (commonly 3–5 yd³) — the truck costs the same to roll whether it carries 2 yards or 9. Ask the plant for their minimum and fee before comparing against bags; for a 1–2 yd³ job it often still pencils out.