What does the concrete calculator do?
The Concrete Calculator estimates how much concrete you need for any of four common project shapes — slab, footing, round column, or stairs — and tells you how much to order from a ready-mix plant or how many 60-lb / 80-lb bags to buy at a home center. A configurable waste allowance (default 10%) is automatically baked in.
Outputs include cubic yards (the standard US ordering unit), cubic metres (international), total cubic feet, and bag counts for the two most common bag sizes. The widget supports both imperial (ft/in) and metric (m/cm) inputs.
How to use the calculator
- Pick the shape: slab, footing, round column, or stairs.
- Pick units — imperial (ft/in) or metric (m/cm).
- Enter the dimensions. Fields adapt to the shape (length × width × thickness for slabs/footings; height + diameter for columns; width + tread + riser + step count for stairs).
- Set the waste allowance — 10% is standard.
- Read the output: cubic yards / cubic metres of concrete, plus equivalent in 60-lb and 80-lb bags.
The four shapes in detail
Slab
A flat, rectangular pad of concrete. Used for: patios, sidewalks, driveways, garage floors, basement floors, sheds. Volume = length × width × thickness. Standard thicknesses:
- Sidewalks: 4 inches (10 cm)
- Patios: 4 inches
- Driveways: 4-6 inches (depending on vehicle weight)
- Garage floors: 4-6 inches
- Heavy-duty (truck parking, industrial): 6-8 inches
Footing
A long rectangular trench of concrete that supports walls or columns. Used for: foundation walls, retaining walls, deck post supports. Same volume formula as slab (length × width × depth) but the geometry is usually long and narrow. Local building codes specify minimum dimensions based on soil bearing capacity and load — 8" wide × 12" deep is a typical residential footing.
Round column
A cylindrical post — for deck supports, fence posts, light pole bases, structural columns. Volume = π × r² × height where r is half the diameter. Common diameters: 8", 10", 12", 16" for residential decks; 24"+ for heavier structural use.
Stairs
Multi-step concrete structure. Each step is a rectangular volume; the calculator sums all steps. Inputs: stair width, tread depth (horizontal), riser (vertical), and number of steps. Standard residential stair dimensions: 11" tread, 7" riser. Total volume includes the cumulative riser-stack underneath each tread.
Worked examples
Example 1 — Patio slab
12 ft × 10 ft × 4 in slab. Volume = 12 × 10 × (4/12) = 40 ft³ = 1.48 yd³. With 10% waste, order 1.63 yd³. Or 89 60-lb bags. Or 67 80-lb bags. Above 1 yd³, ready-mix is more practical than bagged.
Example 2 — Driveway
30 ft × 12 ft × 5 in. Volume = 30 × 12 × (5/12) = 150 ft³ = 5.56 yd³. With 10% waste, order 6.11 yd³. Definitely ready-mix territory — you don't want to mix 330 60-lb bags by hand.
Example 3 — Round footing for a deck post
4 ft tall × 12 in diameter. Radius = 0.5 ft. Volume = π × 0.5² × 4 = 3.14 ft³ = 0.12 yd³. With waste, 0.13 yd³. About 7 60-lb bags. A small enough job that bagged concrete is the right call.
Example 4 — Concrete steps
4 ft wide × 11" tread × 7" riser × 3 steps. Volume of step 1 = 4 × (11/12) × (7/12) = 2.14 ft³. Step 2 doubles the riser height: 4 × (11/12) × (14/12) = 4.28 ft³. Step 3 triples it: 6.42 ft³. Total = 12.84 ft³ = 0.48 yd³. With waste, ~0.52 yd³ or ~28 60-lb bags. Bagged or small ready-mix delivery both work.
Bagged vs ready-mix — when to switch
Bagged (60-lb or 80-lb sacks)
Sold at any home center for $5-$7 per bag. Easy to pick up, easy to store. Mix in a wheelbarrow or rented mixer. Good for:
- Small projects under ~1 yd³ (under ~60 60-lb bags)
- Multiple small pours over time (fence posts, deck footings done one at a time)
- Tight access where a concrete truck can't reach
- Jobs where labor cost doesn't matter (DIY, weekend project)
Cost for 1 yd³ via 60-lb bags: ~60 bags × $5 = $300. Plus your time (mixing 60 bags takes hours).
Ready-mix (truck delivery)
From a concrete plant. Truck shows up, you have ~60-90 minutes to place the concrete before it sets. Good for:
- Larger projects above ~1 yd³
- Jobs where a uniform pour matters (no cold joints between batches)
- Projects where time matters more than money
- Sites accessible to a 10-yard truck (most residential driveways are fine)
Cost for 1 yd³ ready-mix: ~$150-$200 plus delivery. Often the same total cost as bagged but with vastly less labor.
The crossover point
Most home contractors switch to ready-mix at about 1 yd³. Below that, bagged often wins on convenience and cost. Above that, ready-mix usually wins on labor and consistency. The exact crossover depends on your local prices, your tolerance for hand-mixing, and your time value.
The waste allowance — why 10% (or more)
Concrete that you don't use is wasted (it sets in the truck or in unused bags). Concrete you run short of means a second pour, which either creates a visible cold joint or requires you to break out the partial pour and start over. Both are bad. The waste allowance hedges against running short.
Standard allowances:
- 10%: most projects. Slab on flat ground with rectangular form.
- 15%: irregular shapes (curved patios, rounded paths), heavy rebar reinforcement (takes up space inside the form).
- 20%: hand-mixing (more spillage), uneven subgrade (the form thickness varies), first-time DIY.
- 5%: only if you're an experienced concrete contractor with precise forms and a known-good supplier.
Concrete strength — what to ask for
When ordering ready-mix or buying bagged concrete, you'll see strength ratings like "3000 PSI" or "4000 PSI." This is the compressive strength after 28 days of curing. Common uses:
- 2500-3000 PSI: residential slabs, sidewalks, patios. Standard "ready-mix" if you don't specify.
- 3500-4000 PSI: driveways, garage floors, basements, structural footings.
- 4500-5000 PSI: commercial floors, structural columns, marine applications.
- 6000+ PSI: high-rise buildings, heavy industrial loads, freeze-thaw exposed structures.
Most home centers' bagged concrete is 3000-4000 PSI by default. Check the bag.
Setting and curing time
Concrete behavior over time:
- 0-1 hour: workable. You can pour, screed (level), and float (smooth) the surface. Truck-delivered concrete sets faster than bagged.
- 1-4 hours: initial set. Surface is firm enough to walk on lightly but not strong enough to support weight.
- 24 hours: walk-on hardness for foot traffic. Forms can be removed for vertical pours (walls, columns).
- 3 days: ~50% of final strength. Light vehicle traffic possible.
- 7 days: ~70% of final strength.
- 28 days: 100% rated strength. The "PSI" rating refers to this point.
Concrete continues to gain strength slowly for years after, but 28 days is the design number.
Cure tips
Concrete needs water to cure properly. Letting it dry too fast (in sun, wind, or low humidity) creates surface cracks and reduces final strength. Mitigations:
- Cover with plastic sheeting or wet burlap for the first 7 days.
- Mist with water several times a day in dry climates.
- Pour during cooler parts of the day (early morning or evening) in summer.
- Avoid pouring when temperature will drop below 40°F (5°C) in the next 48 hours.
Common questions and concerns
"I ran out of concrete mid-pour. What do I do?"
Bad situation. Best mitigation: pre-position extra bagged concrete as a backup. If you genuinely run out, pour what you have flat and roughly level, let it set, then pour the rest the next day. The cold joint will be visible but structurally OK if the surfaces are clean. For aesthetics, expansion joints planned in advance can hide a planned cold joint.
"My patio cracked after a year. What went wrong?"
Cracks happen — concrete is a brittle material and small surface cracks are normal. Big cracks usually come from: (1) inadequate thickness, (2) no expansion joints (concrete needs joints every 8-12 ft to control where cracks form), (3) inadequate curing (dried too fast), (4) ground settlement underneath, or (5) freeze-thaw cycles in cold climates. Reinforcement with wire mesh or rebar reduces crack severity.
"How much does concrete weigh?"
About 4000 lb per cubic yard (or 2400 kg per cubic metre). A typical 4×4 ft slab 4" thick is ~5.3 ft³ × 150 lb/ft³ = ~795 lbs. Worth knowing for transport (can my truck handle it?) and structure (can the existing surface support it?).
"Can I pour over an existing slab?"
Yes, but it requires preparation. Roughen the existing surface (sandblast or chip), apply a bonding agent, then pour the new layer at least 2 inches thick. For a clean separation (no bond), use a slip sheet or sand layer between old and new — this lets them move independently and reduces cracking.
What the calculator gives you, summarized
- Cubic yards — the standard US ordering unit for ready-mix concrete.
- Cubic metres — international ordering unit.
- Total cubic feet — useful for crosschecking and detailed planning.
- 60-lb bag count — for hand-mixing small jobs.
- 80-lb bag count — for slightly larger DIY jobs.
- Configurable waste — 10% default, adjustable for irregular shapes or heavy reinforcement.
- Four common shapes — slab, footing, round column, stairs.
- Both unit systems — imperial (ft/in) and metric (m/cm).
Pick a shape, enter dimensions, get the order. The right tool for any DIY or contractor sizing a concrete pour.