Concrete Calculator
Calculate your Concrete needed in seconds with the free Concrete Calculator. No registration needed.
Slab dimensions
Concrete needed
What is a Concrete Calculator?
A concrete calculator tells you exactly how much concrete you need for a slab, column, or other pour. Instead of guessing or doing manual math, you enter the dimensions and the tool returns cubic feet, cubic yards, and bag counts. This is useful for driveway slabs, patio foundations, footings, deck posts, and any DIY or construction project where ordering the right amount of concrete saves you money and waste.
How to calculate concrete manually (the formula)
The basic formula for concrete volume is: Volume = Length × Width × Thickness. All measurements must be in the same unit. For a rectangular slab in feet:
Cubic feet = Length (ft) × Width (ft) × Thickness (ft)
Cubic yards = Cubic feet ÷ 27
To convert inches to feet, divide by 12. For example, 4 inches becomes 0.333 feet. Round columns use Volume = π × r² × height.
Example calculation
You are pouring a patio slab that is 10 feet long, 10 feet wide, and 4 inches thick.
Step 1: Convert thickness to feet: 4 in ÷ 12 = 0.333 ft
Step 2: Calculate cubic feet: 10 × 10 × 0.333 = 33.3 ft³
Step 3: Convert to cubic yards: 33.3 ÷ 27 = 1.23 yd³
Step 4: Add 10% waste: 1.23 × 1.1 = 1.35 yd³
Step 5: Convert to 80 lb bags: 33.3 ÷ 0.6 = 55.5 bags (round to 56)
You would order about 1.25 cubic yards of ready-mix or 56 bags of 80 lb concrete.
Common mistakes
- Mixing inches and feet — entering thickness in inches without converting to feet gives wildly wrong results. Always convert inches to feet first.
- Forgetting to account for waste — ground is rarely perfectly level. Order 5–10% extra to avoid running short mid-pour.
- Using the wrong bag yield — 60 lb and 80 lb bags produce different volumes. Check the bag label for exact yield.
- Ignoring slab shape — the formula changes for round columns, curved walkways, or irregular shapes. Break complex areas into simple rectangles.
- Not checking local building codes — minimum thickness requirements vary by region and use. A 4-inch slab may not be enough for a driveway.
Frequently asked questions
How do I calculate how much concrete I need?
Multiply the slab length by the width by the thickness (all in feet) to get cubic feet. Then divide by 27 to convert to cubic yards. For a 10 ft by 10 ft by 4 inch slab: 10 × 10 × 0.333 = 33.3 cubic feet, which is about 1.23 cubic yards.
How many bags of concrete do I need for a slab?
Divide your total cubic feet by the volume per bag. A 60 lb bag yields about 0.45 cubic feet. An 80 lb bag yields about 0.6 cubic feet. For 33.3 cubic feet, you would need about 74 bags of 60 lb or 56 bags of 80 lb.
How do you calculate concrete in cubic feet vs cubic yards?
Cubic feet is length × width × thickness in feet. To convert to cubic yards, divide cubic feet by 27 (since 1 cubic yard = 27 cubic feet). Most ready-mix concrete suppliers quote in cubic yards.
Should I order extra concrete for waste?
Yes. Add 5–10% extra to account for spillage, uneven subgrade, and slight miscalculations. For a 10 cubic yard pour, order 10.5 to 11 cubic yards.
What is the formula for calculating concrete volume?
Volume = Length (ft) × Width (ft) × Thickness (ft). Convert inches to feet by dividing by 12. For a round column, use Volume = π × r² × height.
How thick should a concrete slab be?
A standard patio or walkway slab is 4 inches thick. A driveway should be at least 5–6 inches. A garage or heavy-duty slab needs 6–8 inches. Always check local building codes.