Concrete Calculator
Calculate cubic yards, cubic feet, bags needed, and estimated cost for any concrete shape - slabs, columns, stairs, rings, and more.
Whether you're pouring a backyard patio, building a fence line, or handling a commercial job site, this free concrete calculator tells you exactly how much concrete you need - in cubic yards, cubic feet, cubic meters, and number of pre-mix bags. No guesswork. No short pours. Just accurate numbers.
We support six common concrete shapes: rectangular slabs, circular slabs, columns, hollow tubes/rings, stairs, and curb-and-gutter. Toggle between imperial and metric, adjust the waste factor, and get an optional cost estimate in seconds. Bookmark this page - you'll be back.
What Is Concrete (And Why Do You Need to Measure It Carefully)?
Concrete is ordered by the cubic yard. Ordering wrong is expensive. Here's what you need to know.
Concrete is a mixture of cement, sand, aggregate (gravel or crushed stone), and water. Once mixed, it starts to harden fast. That means you need to have the right amount ready before the truck shows up. Order too little and you're left scrambling. Order too much and you're paying for concrete you don't need and have to dispose of.
Ready-mix concrete is sold by the cubic yard (27 cubic feet). Pre-mix bags are sold by weight (40 lb, 60 lb, or 80 lb), and each bag yields a specific volume of mixed concrete. Getting your measurements right before you order saves you time, money, and a lot of stress on pour day.
- Slabs & FootingsPatios, driveways, foundations, and garage floors - the most common concrete application.
- Columns & PiersRound post footings, deck piers, and structural columns use cylindrical forms.
- Stairs & StepsOne of the trickiest shapes to calculate, the volume grows non-linearly with each step.
- Curb & GutterRoad edges and drainage channels require precise calculations to avoid waste on long runs.
How to Calculate Concrete Volume
The math behind each shape, explained simply
Every shape has its own volume formula. Here's a quick breakdown:
Shape | Formula | Key Tip |
|---|---|---|
| Rectangular Slab | L x W x Thickness | Convert thickness to feet (divide inches by 12) |
| Circular Slab | pi x r2 x Thickness | Radius = Diameter divided by 2 |
| Column / Cylinder | pi x r2 x Height | Same as circular slab but vertical |
| Tube / Ring | pi x (R2 - r2) x H | Subtract the hollow inner cylinder |
| Stairs | W x Run x Rise x n(n+1)/2 | Each step adds a progressively larger wedge |
| Curb & Gutter | (Curb + Gutter Area) x Length | Two rectangular prisms combined |
After calculating volume in cubic feet, divide by 27 to get cubic yards (the unit ready-mix is sold in). Our calculator does all of this automatically for every shape.
The Waste Factor: Why You Should Always Order a Little Extra
Every concrete pour loses some volume. Here's how to account for it.
No concrete job pours perfectly. Between spillage, uneven subgrade (your ground isn't perfectly level), form overfill, and mixing losses, you'll always use slightly more concrete than pure math suggests. This is called the waste factor, and ignoring it is one of the most common (and costly) mistakes on pour day.
Job Type | Recommended Waste Factor | Reason |
|---|---|---|
| Flat slab (patio, driveway) | 5-10% | Minimal waste on flat pours |
| Footings and foundation walls | 10% | Some overpour into forms is common |
| Columns and piers | 5% | Forms contain the pour well |
| Stairs and steps | 10-15% | Complex shapes waste more |
| Curb & gutter | 10-15% | Long runs have more variability |
Our calculator defaults to 10% waste, which is the most common industry standard. You can adjust it up or down depending on your job.
Pre-Mix Bag Sizes: How Much Concrete Is In Each Bag?
Bag concrete is convenient for small jobs, but know what you're getting.
Bag Weight | Yield (Mixed) | Bags per Cubic Yard | Best For |
|---|---|---|---|
| 40 lb | ~0.30 cu ft | ~90 bags | Very small repairs, post holes |
| 60 lb | ~0.45 cu ft | ~60 bags | Small slabs, general repairs |
| 80 lb | ~0.60 cu ft | ~45 bags | Larger DIY jobs, fence posts |
For any pour larger than about 0.5-1 cubic yard, ready-mix concrete delivered by truck is almost always more economical than bags. The break-even point depends on your local pricing, but bags typically cost 2-3x more per cubic yard than ready-mix at scale.
Tips for Ordering Concrete Like a Pro
Small things that make a big difference on pour day
- 1
Measure twice, calculate once
Walk the slab area with a tape measure and double-check your numbers. Measure depth with a stake. A 10% error in your measurement can mean a short load.
💡 Tip: For irregular shapes, break the area into rectangles and calculate each one separately, then add them up.
- 2
Call ahead and confirm minimums
Most ready-mix plants have a minimum order of 1 cubic yard. Orders under 3-4 yards often incur a short-load fee. Call your local supplier before finalizing your order.
- 3
Plan your pour time
Ready-mix concrete starts to set within 90 minutes of mixing. Have your crew, tools, and forms ready before the truck arrives. Delays = bad concrete.
💡 Tip: Hot weather speeds up set time; cold weather slows it down. Plan accordingly.
- 4
Round up, not down
If your calculator says 4.6 cubic yards, order 5. You can always have leftover concrete form small pads or blocks. Running short means a partial cure, which is a structuaral problem.
- 5
Keep records for your invoice
If you're a contractor, document your concrete calculations and supplier receipts. Invoice Mama makes it easy to attach material costs to your client invoices so you get reimbursed accurately.
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You just calculated your concrete order. Now imagine sending the invoice for that job in 10 seconds flat, from your phone, on the job site, before you even finish cleaning up. That's what Invoice Mama does for concrete contractors, general contractors, and construction pros.
- AI-generated invoices in under 10 seconds
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- Bill clients from the job site on your phone
Concrete Calculator FAQs
Common questions from contractors, DIYers, and homeowners, answered honestly.
How much does a cubic yard of concrete cover?
One cubic yard of concrete (27 cubic feet) covers 81 square feet at a 4-inch thickness, 54 square feet at 6 inches, or 324 square feet at a 1-inch thickness. The formula is: coverage = 27 divided by thickness in feet. So for a standard 4-inch driveway slab, divide 27 by 0.333 = 81 sq ft per yard.
How many bags of concrete do I need?
It depends on the bag size. An 80 lb bag yields about 0.60 cubic feet, a 60 lb bag yields about 0.45 cubic feet, and a 40 lb bag yields about 0.30 cubic feet. Divide your total cubic feet by the yield per bag and always round up. For example, a 10 ft x 10 ft slab at 4 inches thick = 33.3 cubic feet, which requires about 56 bags of 60 lb concrete.
What is a waste factor and why do I need it?
A waste factor accounts for spillage, uneven subgrade, form overfill, and mixing losses that always happen on a real job site. The standard industry waste factor is 5-10% for slabs and footings, and up to 15% for complex shapes like stairs or curbs. If you skip the waste factor and run short, you'll have to order more concrete, often at a premium rush delivery cost. Always better to order a little more.
When should I use ready-mix concrete vs. pre-mix bags?
Use ready-mix concrete (delivered by truck) for large pours over 1-2 cubic yards. It's faster, more consistent, and usually cheaper per yard at scale. Use pre-mix bags for small projects under 1 cubic yard like fence posts, small slabs, or repairs. Minimum ready-mix orders are typically 1 cubic yard, and most plants charge a short-load fee for orders under 3-5 yards.
How do I calculate concrete for a circular slab or round patio?
Use the formula: Volume = pi x radius squared x thickness. For example, a circular patio 10 feet in diameter and 4 inches thick: radius = 5 ft, so Volume = 3.14159 x 25 x 0.333 = 26.2 cubic feet = 0.97 cubic yards. Our calculator handles this automatically - just enter the diameter and thickness.
How do I calculate concrete for stairs?
Stair calculations are more complex because each step represents an additional wedge of concrete. The formula is: Volume = width x run x rise x n(n+1)/2, where n = number of steps. For 5 steps that are 4 ft wide, 12 inches deep, and 7 inches tall: Volume = 4 x 1 x 0.583 x 15 = 35 cubic feet. Add a platform and use a 10% waste factor. Our calculator does all of this automatically.
How thick should a concrete slab be?
Typical slab thicknesses are: 4 inches for residential driveways and patios, 5-6 inches for driveways with heavy vehicle traffic or commercial use, 3.5-4 inches for sidewalks and walkways, and 6-8 inches for industrial floors or heavy equipment areas. Thicker slabs require more concrete and rebar but carry significantly higher loads.
What does concrete cost per cubic yard?
Ready-mix concrete typically costs between $120 and $200 per cubic yard in the US, depending on your location, mix design, and current market conditions. Pre-mix bags cost about $5-$8 per 80 lb bag, which works out to roughly $250-$380 per cubic yard, significantly more expensive than ready-mix for large pours.
Do I need rebar or wire mesh in my concrete?
For most residential slabs 4 inches thick or less, fiber mesh mixed into the concrete or welded wire fabric (6x6 WWF) is sufficient. For structural elements, driveways, or anything carrying vehicular loads, rebar is recommended. Typical rebar patterns: #4 bars on 12-inch centers for driveways, #3 bars on 18-inch centers for walkways and patios. Always check local building codes.
How long does concrete take to cure?
Concrete gains strength gradually over time. It reaches about 70% of its final strength after 7 days and over 90% by 28 days, the standard cure period referenced in engineering specs. You can typically walk on it after 24-48 hours and drive on it after 7 days. Keep concrete moist during the first week by covering it with plastic sheeting to prevent cracking from rapid evaporation.
Can I mix concrete myself instead of ordering it?
Yes, for small projects. A standard hand-mix ratio is 1 part cement : 2 parts sand : 3 parts gravel, plus enough water to achieve a workable consistency. For anything over about 0.5 cubic yards, pre-mix bags or ready-mix gives better and more consistent results. Machine mixing (a rented drum mixer) produces more uniform concrete than hand mixing.
What is the minimum concrete order for a ready-mix truck?
Most ready-mix suppliers require a minimum order of 1 cubic yard, though some will deliver as little as 0.5 yards with a short-load surcharge. Orders under 3 yards typically incur a short-load fee of $50-$150. If you only need a small amount, pre-mix bags are often more economical despite the higher cost per yard.
Can't find what you're looking for? Contact us and mama will help you out!
Invoice Mama helps concrete and construction contractors create professional invoices and estimates in seconds. From the job site, on your phone — bill clients before the truck even leaves.