Concrete Calculator
How Much Concrete Do You Need?
Ordering too little concrete mid-pour is a real problem — you can't pause and wait for a new batch on most jobs. Ordering too much wastes money. Getting the calculation right before you start is the most important prep step for any concrete project, and this calculator handles the math in seconds.
How to Use This Calculator
Select your shape (slab/wall/footing or column/cylinder), enter your dimensions, set your waste factor (5% is standard for most projects; increase to 10% for irregular forms or complex pours), and hit Calculate Concrete. The results show cubic yards (for ready-mix ordering), cubic feet, and the number of 60 lb and 80 lb bags needed for smaller projects.
How the Volume Is Calculated
For slabs, walls, and footings:
For columns and cylinders:
There are 27 cubic feet in one cubic yard — which is why dividing by 27 converts from cubic feet to cubic yards. Always convert depth/thickness from inches to feet (divide by 12) before calculating.
Real-World Example
A standard 12 ft × 10 ft patio slab at 4 inches thick with 5% waste:
- Net volume: 12 × 10 × (4÷12) ÷ 27 = 1.48 cubic yards
- With 5% waste: 1.48 × 1.05 = 1.55 cubic yards
- In cubic feet: 1.55 × 27 = 41.9 cu ft
- 80 lb bags needed: 41.9 ÷ 0.6 = ~70 bags
- 60 lb bags needed: 41.9 ÷ 0.45 = ~93 bags
- Approximate weight: ~6,285 lbs (concrete weighs ~150 lbs/cu ft)
70 bags of 80 lb concrete is a significant physical undertaking — mixing that much by hand would be exhausting and time-consuming. For anything over about 0.5 cubic yards, consider renting a mixer or ordering ready-mix.
Bags vs. Ready-Mix — Which Should You Use?
Bagged concrete is best for small projects under 0.5–1 cubic yard. One 80 lb bag yields about 0.6 cubic feet; one 60 lb bag yields about 0.45 cubic feet. It's convenient — no minimum order, available at any hardware store, and you can work at your own pace. The trade-off is cost (more per yard than ready-mix) and the labor of mixing.
Ready-mix concrete is ordered by the cubic yard and delivered in a truck. It's more economical above about 1 yard, comes in specific mix designs for different applications, and eliminates the mixing labor. Minimum orders are typically 1 yard, and you pay for the full minimum even if you use less. Timing is critical — the truck needs to be able to pour and leave, so your forms and crew must be ready.
Common Thickness Guidelines
- Walkways and sidewalks: 4 inches
- Residential driveways: 4 inches (6 inches for heavy vehicles)
- Garage slabs: 4 inches with wire mesh or rebar
- Patio slabs: 4 inches
- Structural footings: per local building code — typically 8–12 inches, often twice the width of the wall above
- Fence post footings: typically 1/3 the total post length in depth
Frequently Asked Questions
Why add a waste factor?
Concrete pours always involve some waste — spillage during transport, concrete that sticks to the mixer, uneven subgrade that requires more material than calculated, and slight form irregularities. A 5% buffer is standard for clean, square projects. Add 10% for complex shapes, uneven ground, or if you're an inexperienced pour. Running out mid-pour creates a cold joint that weakens the slab — better to have a little extra.
Do I need rebar or wire mesh?
For most residential slabs, 6×6 wire mesh or #3 rebar on 18-inch centers is recommended to control cracking. Structural applications (driveways that will see heavy vehicles, footings) should use rebar per local codes. Decorative walkways and small patios often skip reinforcement, though they're more prone to cracking under frost heave or soil movement.
What PSI concrete should I use?
3,000 PSI is standard for residential driveways, patios, and slabs. 3,500–4,000 PSI for commercial driveways or areas with heavy loads. 4,000+ PSI for structural applications and footings in severe climates. Higher PSI concrete is denser, more durable, and more resistant to freeze-thaw cycles — worth the small additional cost for exposed exterior applications.
When is the best weather to pour concrete?
Ideal temperature range is 50–90°F (10–32°C). Below 40°F, concrete cures too slowly and can freeze before gaining strength — cold-weather additives and insulated blankets are required. Above 90°F, concrete cures too fast, increasing cracking risk — pour early in the morning and keep it moist during curing. Avoid pouring in direct rain, which dilutes the mix and weakens the surface.
How long does concrete take to cure?
Concrete reaches about 70% of its design strength in 7 days and 99% by 28 days. You can walk on it after 24–48 hours, drive on it after 7 days, and subject it to full loads after 28 days. Keeping it moist during the first week (misting or covering with wet burlap) significantly improves final strength and reduces surface cracking.