Concrete Calculator: How Much You Need for Slabs, Posts, and Patios

Ordering too little concrete leaves you short on pour day, scrambling for a second delivery while the first batch sets. Ordering too much wastes money on material you cannot return. A concrete calculator takes the guesswork out of your next project by telling you exactly how many cubic yards or cubic meters you need based on your project dimensions. Here is how the math works for the most common projects — slabs, fence posts, and patios — plus the waste factor professionals always add.

The Basic Formula: Length × Width × Depth

For any rectangular pour, the volume is simple: multiply length by width by depth, all in the same unit. The tricky part is converting to what suppliers actually sell, which is cubic yards in the US and cubic meters elsewhere.

• Cubic feet: length (ft) × width (ft) × depth (ft)
• Cubic yards: cubic feet ÷ 27 (one cubic yard = 27 cubic feet)
• Cubic meters: length (m) × width (m) × depth (m)

A 10-foot by 12-foot patio slab at 4 inches thick (0.33 ft) is 10 × 12 × 0.33 = 39.6 cubic feet, which is 39.6 ÷ 27 = 1.47 cubic yards.

Common Projects and How to Calculate Them

Project	Method	Example
Slab (patio, driveway, floor)	L × W × thickness	10′ × 12′ × 4″ = 1.47 yd³
Fence post footings	Volume per hole × number of holes	12 holes, 8″ dia × 30″ deep ≈ 0.52 yd³ total
Walkway	L × W × thickness (same as slab)	20′ × 3′ × 4″ = 0.74 yd³
Column / pier	π × r² × height	10″ dia × 4′ tall ≈ 0.08 yd³ each

For round footings, use the cylinder formula (π × radius² × height). For a fence with 12 holes that are 8 inches wide and 30 inches deep, each hole holds about 1.16 cubic feet, so total is roughly 14 cubic feet — just over half a cubic yard.

Always Add 10% Waste

Professionals never order the exact calculated amount. Uneven ground, spilled material, and the concrete stuck in the chute and wheelbarrow all eat into your total. The standard buffer is 10% — for small DIY jobs, 15% is safer.

Using the patio example above: 1.47 yd³ × 1.10 = 1.62 cubic yards to order. Round up to the nearest quarter yard that suppliers will actually deliver.

Bagged Mix vs. Ready-Mix Delivery

• Bagged concrete (60 or 80 lb bags): One 80 lb bag yields about 0.6 cubic feet. For the 1.62 yd³ patio, that is 43.7 cubic feet ÷ 0.6 ≈ 73 bags. Best for projects under 1 cubic yard.
• Ready-mix truck: The minimum delivery is usually 1 cubic yard (often with a short-load fee below 3–4 yards). Best for anything larger than a yard.

Project Size	Recommendation
Under 0.5 yd³	Bagged mix (mixing by hand or in a small mixer)
0.5 – 1 yd³	Bagged, or rent a small mixer and work fast
Over 1 yd³	Ready-mix delivery (check short-load minimums)

Quick Project Estimates

Project	Dimensions	Concrete Needed (with 10%)
Small patio	10′ × 12′ × 4″	1.62 yd³ (~73 bags)
Driveway apron	20′ × 10′ × 4″	2.71 yd³ (~122 bags)
Walkway	20′ × 3′ × 4″	0.81 yd³ (~37 bags)
12 fence posts	8″ dia × 30″ deep	0.57 yd³ (~26 bags)
4×4 deck pier	10″ dia × 4′ tall	0.36 yd³ (~16 bags)

Ready to price out your pour? Enter your project dimensions in the concrete calculator to get the exact volume, the bag count, and a realistic order amount — before you call the supplier.