How to use the concrete calculator
- Under What are you pouring?, choose a slab, patio, footing or wall, or round column or post holes.
- Enter Length, Width and Thickness for a slab, or the Hole or column diameter, Depth or height and any Post width to subtract for round pours.
- Set the Number of identical pours and the Extra for spillage & uneven ground (10% by default).
- Under How will you buy it?, pick premixed bags and a Bag size, or a ready-mix truck.
- Optionally add a price per bag or per cubic yard to see an estimated material cost.
How it works
The calculator works out the volume of one pour, multiplies it by the number of identical pours and adds your extra allowance. For round holes it subtracts the square footprint of the post you set in the hole, so a 4×4 (3.5 in square) is removed from the concrete volume.
Bag counts use typical published yields for premixed concrete: 0.60 ft³ for an 80 lb bag, 0.45 ft³ for 60 lb, 0.375 ft³ for 50 lb and 0.30 ft³ for 40 lb. Yields vary slightly by product, so check the bag label. Ready-mix orders are rounded up to the next quarter cubic yard, which is how most plants take orders.
Slab volume (ft³) = length (ft) × width (ft) × thickness (in) ÷ 12
Hole volume (ft³) = (π × (diameter ÷ 2)² − post width²) × depth
Bags = total volume × (1 + extra %) ÷ bag yield, rounded up
Cubic yards = cubic feet ÷ 27
Worked example
With the default inputs, a 10 × 10 ft slab 4 in thick is 33.3 ft³. Adding 10% for spillage and uneven ground brings it to 36.7 ft³, or 1.36 cubic yards (1.04 m³).
In 80 lb bags that is 36.7 ÷ 0.6 = 61.1, rounded up to 62 bags. Ordering ready-mix instead, the calculator rounds 1.36 yd³ up to 1.5 yd³. At more than 60 bags, ready-mix is usually worth pricing.
Tips before you buy
- Measure thickness after the base is compacted; a slab that ends up half an inch thicker uses about 12.5% more concrete.
- Patios and walkways are usually 4 in thick and driveways 5–6 in. Footings and structural work should follow your local building code.
- For more than about one cubic yard, compare a ready-mix delivery with bags. Ask about short-load fees on small orders.
- Buy one or two spare bags for small jobs. Running short partway through a pour leaves a weak cold joint.
- Call 811 before digging post holes or footings so buried utility lines can be marked.
Frequently asked questions
How many 80 lb bags of concrete are in a cubic yard?
About 45. A cubic yard is 27 ft³ and an 80 lb bag yields about 0.6 ft³, so 27 ÷ 0.6 = 45 bags. With 60 lb bags (0.45 ft³) it takes 60 bags.
How many bags of concrete do I need for a 10×10 slab?
At 4 in thick, a 10 × 10 ft slab is 33.3 ft³. With 10% extra that is 36.7 ft³, or 62 bags of 80 lb mix (or 82 bags of 60 lb mix).
Is it cheaper to use bags or ready-mix concrete?
Bags are practical for small pours such as post holes and pads. Above roughly one cubic yard, a ready-mix truck is usually cheaper and much faster, though small loads often carry a short-load fee, so get a quote.
How thick should a concrete patio or driveway be?
Patios and walkways are commonly 4 in thick, and driveways 5–6 in. Requirements for driveways, footings and slabs that carry structures vary, so check your local building code.
How much concrete do I need for a fence post?
A 10 in hole 2 ft deep with a 4×4 post needs about 0.92 ft³, or roughly 1.0 ft³ with 10% extra. That is two 80 lb bags per post.
Why add extra concrete?
Ground is rarely perfectly level, forms bow slightly and some concrete is always lost to spillage. The default 10% allowance covers typical variation; use more if the subgrade is rough.
Results are estimates based on the measurements you enter and typical material properties. Products vary, so check quantities with your supplier and follow product labels and your local building codes. Read the full disclaimer. Last reviewed October 9, 2026.