🏗️ Construction
Concrete Calculator
Use this free concrete calculator to get fast, accurate results.
📝 Your Assumptions
This concrete calculator turns slab dimensions into cubic yards and material quantities. Enter length, width, and depth in feet and inches to see how much concrete you need. It's built for DIYers pouring patios, contractors estimating driveways, and anyone ordering ready-mix or mixing bags.
How to Use This Calculator
Three inputs give you volume and materials in seconds.
1
Measure the length in feet
Enter the longest side of your pour in feet. A typical two-car driveway runs 20 to 24 feet, while a small patio might be 10 feet. Round to the nearest foot for a quick estimate.
2
Measure the width in feet
Enter the shorter side in feet. A standard sidewalk is 3 to 4 feet wide, and a garage slab is often 20 feet. Multiply length by width to get the square footage.
3
Enter the depth in inches
Type the thickness in inches, not feet. Most residential slabs are 4 inches thick, driveways are 5 to 6 inches, and footings can be 8 to 12 inches. The calculator converts inches to feet automatically.
4
Read the volume in cubic yards
The calculator multiplies length × width × depth (in feet) and divides by 27 to get cubic yards. A 10 ft × 10 ft slab at 4 inches deep equals 1.23 cubic yards. Ready-mix concrete is ordered in cubic yards.
5
Check the materials list
The output shows how many 80-lb bags or 60-lb bags you need if mixing by hand. One 80-lb bag yields about 0.6 cubic feet, so a 1.23 cubic yard slab needs roughly 55 bags. For anything over 1 cubic yard, ready-mix is usually cheaper.
What Your Results Mean
Each number ties back to ordering, mixing, and finishing your concrete.
Volume in cubic yards
This is the amount of concrete you order from a ready-mix plant. A 12 ft × 12 ft slab at 4 inches deep is 1.78 cubic yards. Order 5% to 10% extra to cover spillage and uneven subgrade.
80-lb bag count
Each 80-lb bag of concrete mix yields about 0.6 cubic feet. A 10 ft × 10 ft slab at 4 inches deep (1.23 cu yd) needs about 55 bags. Bagged mix costs more per yard but works for small pours under 1 cubic yard.
60-lb bag count
A 60-lb bag yields about 0.45 cubic feet, so you need more bags for the same volume. The same 1.23 cubic yard slab needs about 74 bags. Use 60-lb bags when lifting 80-lb bags is difficult.
Depth matters most
Doubling thickness doubles volume. A 10 ft × 10 ft slab at 4 inches is 1.23 cubic yards; at 6 inches it jumps to 1.85 cubic yards. Always confirm depth with your local code — driveways often require 5 to 6 inches.
Waste factor
Add 5% to 10% to your volume for waste. For 1.78 cubic yards, add 0.09 to 0.18 cubic yards, bringing your order to 1.87–1.96 cubic yards. Ready-mix trucks charge for short loads, so rounding up is safer than running short.
Weight per yard
Concrete weighs about 4,050 lbs per cubic yard. A 1.23 cubic yard slab weighs roughly 4,980 lbs. Check that your subgrade and formwork can handle the load before pouring.
Key Terms
Cubic yard
A volume unit equal to 27 cubic feet, used to order ready-mix concrete. A cube 3 ft × 3 ft × 3 ft is one cubic yard.
Ready-mix
Concrete batched at a plant and delivered by truck in a rotating drum. Ordered in cubic yards, typically with a 3 to 5 yard minimum.
Bagged mix
Pre-blended concrete sold in 40-lb, 60-lb, or 80-lb bags. You add water and mix on site, best for small pours under 1 cubic yard.
Subgrade
The compacted soil or gravel layer beneath a concrete slab. It must be level and stable to prevent cracking and uneven settling.
Formwork
Wood or metal forms that hold wet concrete in place until it cures. They define the slab's shape, thickness, and edges.
❓ Frequently Asked Questions
A 10 ft × 10 ft slab at 4 inches deep needs 1.23 cubic yards. The math: 10 × 10 = 100 sq ft, 4 inches = 0.333 ft, so 100 × 0.333 = 33.3 cubic feet, and 33.3 ÷ 27 = 1.23 cubic yards. If you pour 6 inches deep, it jumps to 1.85 cubic yards. Add 5% to 10% for waste, so order 1.35 to 1.5 cubic yards. Most ready-mix companies have a 3-yard minimum, so you may pay for 3 yards even if you only need 1.23. For a slab this size, bagged mix is an option: 1.23 cubic yards equals about 55 bags of 80-lb concrete. If you're also pouring a walkway, use our gravel calculator to estimate the base layer underneath.
Most residential driveways are 5 to 6 inches thick over a 4-inch compacted gravel base. A 20 ft × 24 ft driveway at 6 inches deep needs 8.89 cubic yards of concrete. At 5 inches, it drops to 7.41 cubic yards. The difference is about 1.5 cubic yards, or roughly $200 to $300 at $150 per yard. If you park heavy trucks or RVs, go with 6 inches and add rebar or wire mesh. For standard cars, 5 inches is usually enough. Always check local codes — some municipalities require 6 inches for driveway aprons. Use the concrete calculator to compare 5-inch and 6-inch volumes before ordering. For the gravel base, our gravel calculator will tell you how many tons you need.
It takes 45 bags of 80-lb concrete to make one cubic yard. Each 80-lb bag yields about 0.6 cubic feet, and 27 cubic feet ÷ 0.6 = 45 bags. For a 10 ft × 10 ft slab at 4 inches deep (1.23 cubic yards), you need about 55 bags. At $5 to $7 per bag, that's $275 to $385, plus the cost of mixing. Ready-mix concrete typically runs $140 to $170 per cubic yard, so 1.23 yards costs about $172 to $209 — cheaper and much faster. Bagged mix makes sense for pours under 0.5 cubic yards, like a small footing or fence post. Above 1 cubic yard, ready-mix wins on price and labor. If you're mixing bags, you'll need a wheelbarrow, hoe, and water source. For larger projects, consider renting a mixer.
Measure the footing's length, width, and depth in feet, then divide by 27. A 40 ft long footing that's 16 inches wide and 8 inches deep: convert 16 inches to 1.33 ft and 8 inches to 0.67 ft. Volume = 40 × 1.33 × 0.67 = 35.6 cubic feet, or 1.32 cubic yards. Add 10% for waste, so order 1.45 cubic yards. Footings usually require ready-mix because they're poured continuously. If you have multiple footings, calculate each one and add them together. For a 30 ft × 40 ft house with a continuous perimeter footing, the total length is 140 ft. At 16 inches wide and 8 inches deep, that's 4.61 cubic yards. Use the concrete calculator for each section, then sum the results. For the gravel base under the footing, our gravel calculator handles that step.
A 6-inch slab uses 50% more concrete than a 4-inch slab for the same area. A 10 ft × 10 ft slab at 4 inches needs 1.23 cubic yards; at 6 inches it needs 1.85 cubic yards. The cost difference is about $90 to $100 at $150 per cubic yard. Use 4 inches for patios, sidewalks, and shed floors that only see foot traffic. Use 5 to 6 inches for driveways, garage floors, and any surface that supports vehicles. A 20 ft × 20 ft garage slab at 6 inches needs 7.41 cubic yards, while 4 inches needs 4.94 cubic yards. Thicker slabs also need more rebar or mesh and a stronger mix. If you're unsure, check your local building code — many require 6 inches for driveways and 4 inches for patios. The concrete calculator lets you compare both depths instantly.
Ready-mix concrete costs $140 to $170 per cubic yard in most US markets, with higher prices in remote areas or for small loads. One cubic yard covers 81 square feet at 4 inches deep, 65 square feet at 5 inches, and 54 square feet at 6 inches. A 10 ft × 10 ft slab at 4 inches needs 1.23 cubic yards, costing about $172 to $209. Add $50 to $100 per yard for delivery and short-load fees if you order less than 3 yards. Bagged concrete costs more per yard: 45 bags of 80-lb mix at $6 each is $270 per yard. For a 20 ft × 24 ft driveway at 6 inches (8.89 cubic yards), ready-mix costs $1,245 to $1,511, while bagged mix would cost $2,400 and take hours to mix. For any pour over 1 cubic yard, ready-mix is the clear winner. Use our gravel calculator to price the base layer too.
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