How Much Concrete for a Patio Slab?
Updated 2026-10-18 · Yardly
Background and Pain Points
Almost every patio project stalls at the same question: how much concrete for a patio slab do I actually need? It sounds simple—length times width times thickness—but the real-world answers people get online rarely match what shows up at the job site. One calculator says 2 cubic yards, a big-box employee says “about 90 bags,” and the ready-mix dispatcher asks questions about waste, subbase, and minimum loads that nobody warned you about.
The pain points are predictable:
- Bag math is confusing. A 12 ft × 14 ft slab at 4 inches is roughly 56 cubic feet. That’s about 93 bags of 80 lb mix before waste—far more than most people expect and a lot of mixing.
- Thickness assumptions vary wildly. Four inches is standard for foot traffic, but hot tubs, fire pits, heavy grills, and vehicle-adjacent slabs often need 5–6 inches with a thickened edge.
- Waste factors get skipped. Ordering the bare calculated volume almost guarantees a short pour, a cold joint, and a visible seam in the middle of your new patio.
- Bagged mix vs. ready-mix is a real decision. Bag counts and truck volumes aren’t interchangeable, and delivery minimums, short-load fees, and pump charges can swing the budget by hundreds of dollars.
- The subbase is invisible in most guides. You’ll also need excavated depth and compacted gravel, which means more material than the concrete volume alone suggests.
This guide walks through the calculation the way you’d actually do it in the driveway, with the conversions and overage built in.
Why It Matters (Core Value)
Getting the volume right is the difference between one clean pour and two compromised ones.
Under-ordering is the expensive mistake. If you run 15% short mid-pour, you either buy bagged mix on the fly and mix it into a slab that’s already setting (weakening the bond), or you stop and create a cold joint—a plane where the first batch cured before the second arrived. That joint is a future crack.
Over-ordering costs money twice: once for concrete you don’t use, and again for disposal, since cured concrete can’t go in the regular trash and most suppliers won’t take back a partial yard.
Getting the number right also lets you:
- Compare real costs between bagged mix and ready-mix instead of guessing.
- Confirm truck access, chute reach, and whether you need a wheelbarrow relay or a pump.
- Schedule the pour with enough help—concrete typically starts setting in 30–45 minutes in warm weather.
- Buy the correct amount of gravel base, rebar or mesh, and form lumber in the same trip.
How to Do It Right (Step-by-Step)
Step 1: Measure the footprint
Measure the actual pour area, not the outside of the forms. For rectangles, multiply length × width in feet. For L-shapes or angled patios, split the plan into rectangles and triangles (triangle area = ½ × base × height), calculate each, and add them. Round up slightly—an extra 6 inches of width is easy to include now and impossible to add later.
Step 2: Choose the slab thickness
| Use | Typical thickness |
|---|---|
| Foot traffic, patios, walkways | 4 in (with 4 in compacted gravel base) |
| Hot tubs, heavy fire pits, large planters | 5–6 in, often with thickened perimeter |
| Light vehicle parking | 5–6 in with rebar |
Edges are usually thickened to 6–8 inches deep and 8–12 inches wide, which adds volume. For a typical 12 × 14 patio, edge thickening can add 5–8%—fold that into your waste factor rather than calculating it separately.
Step 3: Convert to cubic feet, then cubic yards
The core formula:
Cubic feet = Length (ft) × Width (ft) × Thickness (ft) Cubic yards = Cubic feet ÷ 27
Convert thickness to feet by dividing inches by 12 (4 in = 0.333 ft).
Worked example—12 ft × 14 ft at 4 in:
- 12 × 14 × 0.333 = 56 cubic feet
- 56 ÷ 27 = 2.07 cubic yards
Quick reference at 4 inches thick:
| Patio size | Cubic feet | Cubic yards |
|---|---|---|
| 10 × 10 | 33.3 | 1.23 |
| 10 × 12 | 40.0 | 1.48 |
| 12 × 12 | 48.0 | 1.78 |
| 12 × 16 | 64.0 | 2.37 |
| 16 × 20 | 106.7 | 3.95 |
| 20 × 20 | 133.3 | 4.94 |
Step 4: Add the waste factor
This is the step most DIY calculators get wrong. Add 5% for a simple rectangle, flat site, and ready-mix delivery. Add 10% if any of these apply: irregular shape, thickened edges, uneven subgrade, pump placement, or bagged mixing (where spillage and bucket residue eat volume).
Back to the example: 2.07 × 1.10 = 2.28 cubic yards. Order 2.5 cubic yards—suppliers sell in quarter-yard increments, and the extra is cheap insurance.
Step 5: Convert to bags if you’re mixing by hand
Standard yields:
- 80 lb bag ≈ 0.60 cubic feet
- 60 lb bag ≈ 0.45 cubic feet
- 40 lb bag ≈ 0.30 cubic feet
For 56 cubic feet with 10% waste (61.6 cu ft): 61.6 ÷ 0.60 = 103 bags of 80 lb mix. That’s roughly 8,200 lb—two pallets—so plan for a truck, a trailer, or multiple trips. Also check the total weight against your vehicle’s payload rating.
Step 6: Don’t forget the base and reinforcement
Excavate 4 inches of compacted gravel below the slab, plus the slab thickness. One cubic yard of gravel covers about 81 square feet at 4 inches deep and weighs roughly 1.4 tons. Rebar or wire mesh doesn’t change concrete volume meaningfully, but it does affect cost and labor.
Step 7: Compare delivery options and order
Ready-mix is usually the better deal above about 1 cubic yard. Ask about minimum loads (often 1 yard), short-load fees for small pours, and whether a pump or extended chute is needed. Order 2–3 days ahead, confirm the pour time, and have at least two helpers and a finished surface plan before the truck arrives.
Common Mistakes and How to Avoid Them
- Skipping the waste factor. Even a perfect pour loses material to the forms, the mixer, and the wheelbarrow. Always