Concrete Calculator

How much concrete do you need?

Calculate concrete for slabs, footings, round columns, and post holes. Get cubic yards, bag counts (60 lb and 80 lb), and an optional cost estimate. QUIKRETE-spec bag yields.

Concrete Calculator

Slabs, footings, columns, and post holes. Yards or bags.

feet
feet
inches — standard driveway/patio: 4 in, garage floor: 6 in
QUIKRETE-spec bag yields
Slabs, footings, columns, post holes
Cubic yards and bags

How to Calculate Concrete

Concrete volume is measured in cubic yards for ready-mix orders, and in cubic feet when counting bags. The formulas differ by shape — a slab is a simple rectangle, a column is a cylinder, a post hole is a smaller cylinder. This calculator handles all four common shapes.

Bags vs Ready-Mix: When to Switch

For projects under about 1 cubic yard (27 cubic feet), bags are practical — no minimum order, no waiting for a truck, no leftover concrete. A 10 × 10 ft patio at 4 inches thick is exactly 1.2 cubic yards, right at the borderline. Above 1 cubic yard, call for ready-mix quotes. Ready-mix is typically $120–$200 per cubic yard delivered, which works out far cheaper per cubic foot than bags once you're past the truck minimum (usually 1 yard).

Bag economics: an 80 lb bag at $6 yields 0.60 cu ft, which works out to $10 per cubic foot, or $270 per cubic yard. Ready-mix at $150/yard is less than a third of that cost for large pours.

Standard Slab Thicknesses

Footing Depth and Width

Footings must extend below the frost line to prevent heaving. Frost line depth varies from near zero in Florida to 60 inches in northern Minnesota. Your local building department can tell you the frost line for your area — it's usually in the residential building code or available by phone.

A rule of thumb for continuous wall footings: the footing should be as wide as the wall it supports, and as deep as it is wide. A 12-inch wall typically sits on a 12 × 12-inch footing. Point load footings (for posts and columns) follow different sizing rules based on load calculations.

Concrete for Post Holes

The standard guidance is to dig holes 3 times the diameter of the post. A 4×4 post (actual: 3.5 inches) calls for a 10-inch diameter hole. A 6×6 post (actual: 5.5 inches) calls for a 16-inch hole. Each 80 lb bag fills approximately 0.60 cubic feet — see the Fence Post Depth Calculator for hole depth guidance, then use the post holes mode above for bag counts.

How Much Does Concrete Cost?

Ready-mix concrete runs $120–$200 per cubic yard delivered depending on mix design, location, and market conditions. Short-load fees apply when ordering less than the truck minimum (typically 5–7 yards) — expect $50–$100 for small loads. Bags at home centres cost $5–$7 for a 60 lb bag and $6–$9 for an 80 lb bag. Fibre-reinforced and high-strength mixes cost slightly more.

Frequently Asked Questions

Use the calculator above — select the shape, enter your dimensions, and the result shows both 60 lb and 80 lb bag counts automatically. As a rough guide: a 10 × 10 ft slab at 4 inches requires about 50 bags of 80 lb mix or 67 bags of 60 lb mix. For anything over 50 bags, get a ready-mix quote — it will likely be cheaper.

Select the slab or footing tab, enter your dimensions, and the calculator returns cubic yards directly. One cubic yard equals 27 cubic feet. It's the standard unit for ready-mix orders — when you call a concrete supplier, give them the cubic yard number from the calculator, rounded up to the nearest quarter or half yard. Always add 5–10% for waste and spillage.

For most residential applications: 4 inches for patios, walkways, and driveways (passenger vehicles); 6 inches for garage floors or driveways that will see heavy trucks. A 4-inch slab reinforced with wire mesh is the minimum for any load-bearing application. For structural slabs, footings, or anything in a regulated building, consult your local building code or an engineer.

Standard concrete weighs approximately 3,700 to 4,050 lb per cubic yard (roughly 150 lb per cubic foot), depending on aggregate size and moisture content. Normal weight concrete — the most common type for residential use — averages about 145 lb per cubic foot. Lightweight concrete (using pumice or expanded shale aggregate) runs 95–115 lb per cubic foot and is used primarily for roof decks and upper-floor applications where structural weight matters.

An 80 lb bag of QUIKRETE or Sakrete yields 0.60 cubic feet. One cubic yard is 27 cubic feet. Divide: 27 ÷ 0.60 = 45 bags per cubic yard. That's 45 × 80 lb = 3,600 lb of bagged concrete to pour one cubic yard, versus a ready-mix truck delivering the same volume for roughly $150. Bags make sense for small projects; ready-mix wins on anything approaching 1 yard or more.

A Sonotube is a round cardboard tube form used to pour concrete columns — typically for deck footings and support posts. Common sizes are 8, 10, 12, and 16 inches in diameter. Use the Column tab in the calculator above: enter the tube diameter in inches and the pour height in feet. As an example, a 12-inch Sonotube 4 feet tall requires about 0.31 cubic yards, or roughly 14 bags of 80 lb concrete. Always add a few extra bags for spillage.

Concrete achieves about 70% of its design strength within 7 days and reaches full cure at 28 days. For practical purposes: foot traffic is typically safe after 24–48 hours; vehicle traffic after 7 days; full loads after 28 days. Don't rush curing — keep the surface moist for the first 7 days (wet curing) to prevent surface cracking, especially in hot or windy conditions. Temperature matters: concrete placed below 40°F slows dramatically; below 32°F it can freeze before setting and must be protected.

Fence Post Depth Calculator

Get the right burial depth, then use post holes mode above.

Gravel Calculator

Base gravel before pouring a slab or driveway.

Fence Cost Calculator

Posts, pickets, rails, and total fence cost estimate.