Concrete Volume Chart — Slabs, Bags & yd³ Yield

How to Use This Chart

Find a slab close to your pour, read cubic yards, then add 5–10% waste before ordering. For odd shapes, stairs, continuous footings with steps, or thickened edges, use the concrete volume calculator instead of forcing a rectangle. Walkthrough of the formula and bags-vs-truck basics: concrete volume guide. If a pour already came up short or left a pile of leftover, switch to concrete volume troubleshooting — this chart will not diagnose a bad grade or a ft³/yd³ ticket mix-up.

Volume in cubic feet = length (ft) × width (ft) × thickness (ft). Convert to cubic yards by dividing by 27. Bag counts below assume 80 lb premix at 0.60 ft³ yield per bag with no waste — round up and add a waste multiplier before buying. Brand labels vary; always confirm the bag yield printed on the sack you will purchase.

These tables are sized for field estimating when you already know thickness and plan dimensions. They do not replace string-lining the subgrade. If the dirt is high in the middle and low at the edges, the “4″ slab” on the drawing is not a 4″ prism in the field — average measured thickness before you trust a row below.

Reading yd³ vs Bags Without Mixing Them Up

Every row shows net volume first. Ready-mix plants sell cubic yards (often in 0.25 or 0.5 yd³ steps). Bag counts are a separate supply path — do not tell the dispatcher “eighty bags” when you mean 1.78 yd³. Convert, add waste, then speak in the unit the supplier uses.

One cubic yard is always 27 ft³. At 0.60 ft³ per 80 lb bag, pure math is 45 bags per yard. Field yield is often a little worse once you account for spillage, incomplete emptying of the mixer, and thirsty subgrade on the first pass. That is why the waste multiplier sits after the bag math, not instead of it.

Slab Volume — 3.5″ Thick

Common for sidewalks, shed pads, and light-duty paths. Confirm local thickness requirements — some jurisdictions want 4″ minimum even for walks. If the path slopes to a drain, volume is not a flat prism; either average depth across stations or use the calculator with a conservative thickness.

*Bags = ceil(ft³ ÷ 0.60), no waste. At 1.56 yd³ net (12′ × 12′), a 10% waste bump is ~1.72 yd³ — usually a short-load ready-mix conversation if the truck can reach.

Slab Volume — 4″ Thick

Default residential patio and many garage approaches. A 10′ × 12′ × 4″ slab is 1.48 yd³ net — the classic example in the guide. With 10% waste you are near 1.63 yd³; that is almost always more pleasant as ready-mix than ~74–90 bags of hand mixing unless access forbids a truck.

Slab Volume — 5″ Thick

Often used for driveways and heavier residential garage slabs. Thickness jumps from 4″ to 5″ add 25% volume for the same footprint — easy to under-order if the drawing changed after the first estimate. Re-check the row when the inspector or engineer moves you up a thickness class.

Slab Volume — 6″ Thick

Heavy-duty pads, some shop floors, and thickened residential slabs. At these volumes bag counts become crew-killers: a 12′ × 12′ × 6″ slab is 2.67 yd³ net (~120 bags before waste). Treat anything past ~2 yd³ as a delivery problem first, bag problem only if access forces it.

Footings & Piers — Quick Rows

Footings are trench prisms: length × width × depth of the excavated cavity, not the slab footprint above. Over-excavation is the usual reason footing pours drink extra yards — measure the hole you dug. Round pier volume = π × r² × height; cardboard tubes often run slightly oversize versus the nominal diameter printed on the tube.

When combining footings + slab in one ticket, calc each section separately, sum yd³, then apply waste once. Do not order from the slab rectangle alone and hope the trenches “fit.”

Bag Yield Reference

One cubic yard = 27 ft³. At 0.60 ft³ per 80 lb bag, pure math gives 45 bags per yard — field yield is often slightly less due to spillage and uneven subgrade. If you are choosing bags vs a truck for the same yardage, read Ready-Mix vs Bagged after you know the number.

Waste Multipliers

Waste is not optional padding for bad math — it covers form bow, spill, pump/chute loss, rocky grade, and the first-pass absorption of dry subgrade. Skip it and you own the cold joint when the slab dies mid-pour.

Example: 12′ × 12′ × 4″ slab = 1.78 yd³ net. At 10% waste → 1.78 × 1.10 = 1.96 yd³ order. In bags: 80 bags × 1.10 = 88 bags — buy 90. Round ready-mix up to the plant’s increment after waste — never truncate 1.96 down to 1.75 if the grade is already thirsty.

Related