Concrete Calculator

Estimate cubic yards, bags, weight, cost, and rebar for slabs, columns, walls, and stairs.

Reference table

Length	Concrete Volume	Bags Needed	Total Weight
0.5	0.07 yd³	4 bags (80 lb)	272 lbs
1	0.14 yd³	7 bags (80 lb)	543 lbs
1.5	0.21 yd³	10 bags (80 lb)	815 lbs
2	0.28 yd³	13 bags (80 lb)	1,086 lbs
2.5	0.34 yd³	16 bags (80 lb)	1,358 lbs
3	0.41 yd³	19 bags (80 lb)	1,630 lbs
4	0.55 yd³	25 bags (80 lb)	1.1 tons (2,173 lbs)
5	0.68 yd³	31 bags (80 lb)	1.4 tons (2,716 lbs)
6	0.82 yd³	37 bags (80 lb)	1.6 tons (3,259 lbs)
7	0.96 yd³	43 bags (80 lb)	1.9 tons (3,802 lbs)
8	1.09 yd³	49 bags (80 lb)	2.2 tons (4,346 lbs)
9	1.23 yd³	56 bags (80 lb)	2.4 tons (4,889 lbs)
10	1.36 yd³	62 bags (80 lb)	2.7 tons (5,432 lbs)
12	1.63 yd³	74 bags (80 lb)	3.3 tons (6,519 lbs)
15	2.04 yd³	92 bags (80 lb)	4.1 tons (8,148 lbs)

💡 Concrete Tips

Always add 5-10% extra for waste, spillage, and uneven subgrade — running short mid-pour is costly.
Standard concrete reaches 90% strength in 28 days. Keep it damp during the first week for best curing results.
For projects over 1 cubic yard, order ready-mix delivery — it saves hours of mixing and ensures consistent quality.
Use rebar or wire mesh on slabs over 4 inches thick and any driveway or load-bearing surface to prevent cracking.

How to Calculate Concrete Volume

Calculating concrete volume depends on the shape of your project. For rectangular slabs and footings, multiply length × width × thickness (depth). For cylindrical columns and post holes, use π × radius² × height. For stairs, each step forms a wedge shape — volume is calculated as width × run × rise × steps, plus a solid base underneath. Convert the result to cubic yards by dividing cubic feet by 27. One cubic yard of concrete weighs approximately 4,000 lbs (about 2 tons). Always order 5-10% extra to account for waste, spillage, and uneven ground. Ready-mix concrete is sold by the cubic yard and typically costs $125-170 per yard depending on your location and mix specifications.

Bags vs. Ready-Mix Concrete

Pre-mixed concrete bags come in 40, 60, and 80 lb sizes. An 80 lb bag yields approximately 0.6 cubic feet (0.022 cubic yards). You need about 45 bags of 80-lb concrete to make one cubic yard. For projects under 1 cubic yard, bags are practical for DIY mixing. For larger projects, ready-mix delivery from a concrete truck is more economical ($125-170/yard vs $200-300/yard from bags) and ensures consistent quality. The minimum truck delivery is typically 1 cubic yard. Mixing concrete properly requires the right water ratio — too much water weakens the concrete, while too little makes it difficult to work with.

Important Considerations

Never order the exact amount — add 5-10% waste factor for spillage and uneven subgrade
Concrete should be poured above 40°F (4°C) — cold weather can permanently damage curing concrete
Standard residential slab thickness is 4 inches for patios, 5-6 inches for driveways and garages
Concrete can be walked on in 24-48 hours but needs 28 days to reach full strength
Rebar spacing of 12 inches is standard for driveways; 18 inches for patios and walkways
For stairs, standard rise is 7-8 inches and run is 10-12 inches per building codes

Bag Size Yields

40 lb bag: yields ~0.30 ft³ (0.011 yd³) — easiest to carry, most bags needed
60 lb bag: yields ~0.45 ft³ (0.017 yd³) — good balance of weight and yield
80 lb bag: yields ~0.60 ft³ (0.022 yd³) — most economical per cubic foot
1 cubic yard needs: ~90 bags (40 lb), ~60 bags (60 lb), or ~45 bags (80 lb)
Bags are ideal for projects under 0.5 cubic yards — above that, consider ready-mix delivery
Quick estimate for 4-inch slabs: square footage ÷ 81 = cubic yards needed

Concrete Calculation Examples

Step-by-step examples for common projects

10×10 ft Patio Slab (4 in)

Volume = 10 × 10 × (4/12) = 33.33 ft³
Convert: 33.33 ÷ 27 = 1.23 yd³
Add 10% waste: 1.23 × 1.10 = 1.36 yd³
80 lb bags: 1.36 ÷ 0.022 = 62 bags
Weight: 1.36 × 4,000 = 5,440 lbs
Cost: 1.36 × $150 = ~$204

1.36 yd³ = 62 bags (80 lb) ≈ $204

4 Concrete Steps (36 in wide)

Step volume = (36/12) × (11/12) × (7/12) = 1.60 ft³ each
Stacked: step 1 carries all 4 rises = 4 × 1.60 ft³ base
Total wedge volume ≈ 10 × (28/12) × (44/12) × 0.5 = 42.8 ft³
Convert: 42.8 ÷ 27 = 1.59 yd³
Add 10% waste: 1.59 × 1.10 = 1.74 yd³
80 lb bags: ≈ 80 bags

1.74 yd³ = ~80 bags (80 lb) ≈ $262

Frequently Asked Questions

How much concrete do I need for a 10×10 slab?

A 10×10 foot slab at 4 inches thick requires approximately 1.23 cubic yards, which is about 56 bags of 80 lb concrete (before waste factor). With a 10% waste factor, order about 62 bags or 1.36 cubic yards. For a 6-inch thick slab, you'll need approximately 1.85 cubic yards or 84 bags of 80 lb concrete.

How many bags of concrete are in a cubic yard?

One cubic yard requires approximately 45 bags of 80 lb concrete, 60 bags of 60 lb concrete, or 90 bags of 40 lb concrete. These numbers can vary slightly by brand, so always check the bag yield on the packaging. For large projects over 1 cubic yard, ready-mix delivery is usually more practical and cost-effective.

How much does a cubic yard of concrete weigh?

A cubic yard of standard ready-mix concrete weighs approximately 3,700-4,100 lbs (about 2 tons). The exact weight depends on the mix design and moisture content. Lightweight concrete weighs about 2,800 lbs per cubic yard, while heavy-duty structural concrete can weigh up to 4,400 lbs per cubic yard.

Should I use bags or ready-mix concrete?

Use bags for small projects under 0.5-1 cubic yard (fence posts, small repairs, steps). Use ready-mix truck delivery for anything over 1 cubic yard — it's more economical ($125-170/yd vs $200-300/yd from bags), saves hours of manual mixing, and ensures consistent quality. Most suppliers have a minimum delivery of 1 cubic yard.

How thick should my concrete slab be?

Standard residential thicknesses: 4 inches for patios, walkways, and sidewalks. 5-6 inches for driveways and garage floors that support vehicles. 6-8 inches for heavy-duty applications like RV pads or commercial floors. Footings are typically 12+ inches deep. Thicker slabs should include rebar or wire mesh reinforcement.

How do I calculate concrete for stairs?

Concrete stairs are calculated as stacked wedges. Each step adds a rise (typically 7 in) on top of the previous one, so the bottom step supports the full height. The formula accounts for the cumulative volume: width × run × (sum of rises from 1 to N steps). Standard residential stairs have a 7-inch rise and 11-inch run. Always add 10% waste for stairs since the formwork is more complex.