Concrete Calculator: Cubic Yards & Bags (Free)

Pick what you are pouring, enter the size, and get cubic yards and the number of bags to buy. Results update as you type.

How the concrete calculator works

Every shape comes down to a volume in cubic feet. The calculator adds your waste allowance, converts to cubic yards (÷ 27) and cubic meters, and divides by each bag's published yield to count bags, rounding up.

Slab: L × W × T(in) ÷ 12
Footing: L × W(in) ÷ 12 × D(in) ÷ 12
Wall: L × H × T(in) ÷ 12
Column: π × (D(in) ÷ 24)² × H
Post hole: (hole area − post area) × (depth − gravel)
Steps: W × Run × Rise × n(n + 1) ÷ 2
Cubic yards = cubic feet ÷ 27
Bags = cubic feet ÷ yield per bag

Example: a 10 × 10 ft patio 4 inches thick is 100 × 4 ÷ 12 = 33.3 ft³, or 1.23 yd³. With 10% extra that is 1.36 yd³: 62 bags of 80-lb mix or 82 bags of 60-lb mix. See the 10×10 slab page for other thicknesses.

How much does a bag of concrete make?

Manufacturer-published yields used by the calculator
Bag	Yield	Bags per volume	Covers at 4 in (10 cm)
80 lb	0.60 ft³	45 per yd³	1.80 sq ft
60 lb	0.45 ft³	60 per yd³	1.35 sq ft
50 lb	0.375 ft³	72 per yd³	1.13 sq ft
40 lb	0.30 ft³	90 per yd³	0.90 sq ft
25 kg	0.010 m³	100 per m³	0.10 m²
20 kg	0.008 m³	125 per m³	0.08 m²

US bag yields come from QUIKRETE's Concrete Mix data sheet; metric bags from Hanson's. Specialty mixes (fast-setting, high-strength) can differ, so check the bag. Details on the methodology page and the bags in a yard table.

How far a yard goes

One cubic yard covers 81 sq ft at 4 inches thick, 65 sq ft at 5 inches and 54 sq ft at 6 inches. More in how much a yard of concrete covers.

Bags or ready-mix?

A cubic yard is 45 bags of 80-lb mix. Bags suit posts, tubes, steps and small pads; for slabs and walls over about a yard, a ready-mix truck usually saves a lot of work. Suppliers often have a minimum order and a short-load fee for small loads, so ask before you choose. Enter your own prices in the calculator to compare. See ready-mix vs bags.

Tips before you pour

Measure the inside of the forms, not the outside.
Check the base for low spots: an extra half inch over a whole slab adds up fast.
Keep the waste allowance. Running out mid-pour leaves a weak cold joint; leftover bags keep for the next job if they stay dry.
Hire or rent a mixer for more than a dozen or so bags, and line up helpers before the first bag is opened.

Common slab sizes

Frequently asked questions

How do I calculate how much concrete I need?

Multiply length × width × depth in feet to get cubic feet, then divide by 27 for cubic yards. For a slab with the thickness in inches, use length × width × thickness ÷ 12 ÷ 27. Add 5 to 10% for waste.

How many bags of concrete do I need for a 10x10 slab?

At 4 inches thick with 10% extra: 62 bags of 80-lb mix, 82 bags of 60-lb mix or 98 bags of 50-lb mix. That is 1.36 cubic yards.

How many 80-lb bags of concrete are in a yard?

45. An 80-lb bag makes about 0.60 cubic feet, and a cubic yard is 27 cubic feet.

How much concrete do I need for a fence post?

A 4×4 post in a 10-inch hole 24 inches deep takes about 0.92 ft³: 2 bags of 80-lb mix or 3 bags of 50 or 60-lb mix. Use the post hole option for your hole size.

How much extra concrete should I order?

Five to ten percent is common. The calculator adds 10% by default; use more for small pours, rough bases or hand-dug holes.

Does the calculator work in metric?

Yes. Switch to meters and cm to get cubic meters and 20 or 25 kg bags. Metric bag yields vary more between brands, so check your bag.

Concrete Calculator