Concrete Calculator
Work out exactly how much concrete you need for a slab, footing, column, or fence posts — in cubic yards and bags — before you buy a pound more than the job takes.
Volume estimates only — not structural design. Footing sizes, reinforcement, and slab thickness for anything load-bearing are governed by local code and site conditions; check with your building department. Call 811 before digging — locating buried utilities is free and required in most states.
4″ is standard for patios and walkways; 4–6″ for driveways.
Add up all runs of footing for the project.
Sonotube sizes: 8″, 10″, 12″ are most common.
Rule of thumb: 3× the post width.
Typically ⅓ of the above-ground post height, below frost line.
A “4×4” post actually measures 3.5″.
Count the risers — a stoop 28″ above grade with 7″ risers is 4 steps.
Code generally caps residential risers at 7¾″, and every riser must match within ⅜″.
10″ minimum in most codes; 11–12″ walks far better.
Extra depth at the top. Counted solid, full height — it adds up fast.
You need
Enter dimensions
| Bagged mix | Bags needed |
|---|---|
| 80 lb bags | — |
| 60 lb bags | — |
| 50 lb bags | — |
| 40 lb bags | — |
Ready-mix commonly runs about $125–$165 per yard delivered; call local suppliers for real quotes.
What Will This Concrete Cost?
Concrete has a pricing quirk worth knowing before you order: small loads cost more per yard than big ones. Most plants charge a short-load fee under about 4 yards, which is why a tiny pour can cost nearly as much as a large one — and why bags sometimes win.
Contractor pricing covers forming, pouring, screeding, and finishing. Excavation, base gravel, and rebar are usually quoted separately.
Estimated cost
Enter dimensions above
| Option | Amount |
|---|---|
| Ready-mix delivered | — |
| Bagged concrete | — |
| Labor & finishing | — |
National ranges for planning. Ready-mix prices swing with fuel and plant distance more than almost any other building material.
Ready-mix or bags? The break-even
Ready-mix runs $125–175 per cubic yard delivered. An 80 lb bag yields 0.6 cu ft and costs $5–8, which works out to roughly $225–360 per cubic yard — before you count the mixing. Bags look cheap and aren't, per yard.
- Under about ½ yard (roughly 22 bags): bags win. No delivery fee, no short-load penalty, and you mix at your own pace.
- ½ to 2 yards: genuinely a toss-up. Short-load fees can push a 1-yard delivery to $200+, but mixing 45 bags by hand is a long, hard day.
- Over 2 yards: order ready-mix. Hand-mixing at that volume risks cold joints, because concrete you placed 40 minutes ago has already started setting.
- Any slab you care about: ready-mix. A consistent batch from a plant beats 60 buckets of varying water content, and consistency is what makes concrete last.
What contractors charge to pour and finish
| Project | Installed cost | Notes |
|---|---|---|
| Basic 4″ slab, broom finish | $6–12 per sq ft | National average near $8 |
| Concrete driveway | $8–20 per sq ft | Thicker pour, often reinforced |
| Patio, plain finish | $6–12 per sq ft | Stamped or stained runs higher |
| Stamped or decorative | $12–25 per sq ft | Finish work is the added labor |
Labor alone is typically $3–5 per square foot of that. Regional spread is wide — coastal markets average close to $8.50 per square foot for a plain slab while central states sit nearer $5.35, a gap of more than a third for identical work.
How to Calculate Concrete
Concrete is ordered by volume — in the US, by the cubic yard (27 cubic feet). Every shape on this page comes down to the same three steps:
- Measure in one unit. Convert everything to feet (divide inches by 12).
- Find the volume. Multiply the dimensions for your shape (formulas below).
- Convert to yards and add waste. Divide cubic feet by 27, then add 5–10% so you don't run short mid-pour.
Concrete volume formulas
Slab or footing: volume = length × width × thickness
Round slab or column: volume = π × (diameter ÷ 2)² × depth
Post hole: volume = hole volume − post volume, × number of posts
Worked example — 10 × 10 ft patio, 4″ thick: 10 × 10 × 0.333 = 33.3 cu ft. Divide by 27 → 1.23 cubic yards. With 5% waste → 1.30 yards, or about 59 bags of 80 lb mix.
How Many Bags of Concrete Do You Need?
Bagged concrete mix yields a fixed volume per bag once mixed with water. These yields come from manufacturer specifications (Quikrete/Sakrete standard mix):
| Bag size | Yield per bag | Bags per cubic yard | Bags per cu ft |
|---|---|---|---|
| 40 lb | 0.30 cu ft | 90 | 3.3 |
| 50 lb | 0.375 cu ft | 72 | 2.7 |
| 60 lb | 0.45 cu ft | 60 | 2.2 |
| 80 lb | 0.60 cu ft | 45 | 1.7 |
Always round up to whole bags. An unopened spare bag returns to the store; a half-poured slab doesn't.
Bags vs. Ready-Mix: Which Should You Order?
The break-even point is around one cubic yard:
- Under 1 yard — bagged mix wins. A yard takes ~45 bags of 80 lb mix; mixing that by hand or in a small mixer is a full, hard day.
- 1 yard or more — ready-mix delivery usually wins on both effort and price, typically $125–$165 per yard delivered. Note that most plants charge a short-load fee for orders under 3–4 yards.
- In between — some suppliers offer towable mixing trailers (about 1 yard) that split the difference.
Why Steps Take So Much Concrete
People estimate a stoop by counting steps and multiplying. That is not how the volume works. The bottom step spans the whole run, the next spans one tread less, and so on — so the cross-section grows triangularly, not linearly:
Volume = width × tread × rise × N(N+1)/2
Four steps is not four times one step — it is ten times. A 4 ft wide stoop with 7″ risers and 11″ treads comes to 21.4 cu ft for four steps against 2.1 cu ft for one. That is 36 bags of 80 lb mix, or most of a cubic yard, for a stoop most people would call small.
This is why almost nobody pours steps solid. Build a form, fill the interior with compacted gravel or rubble, and pour a shell 4–6″ thick over it. You get the same steps for a fraction of the concrete. Use the figure here as the solid-pour ceiling — the number you would need if you filled every cubic inch — then decide how much of it to replace with fill.
Code, briefly: risers are generally capped near 7¾″ and treads at 10″ minimum, and every riser in a flight must match within ⅜″. Uneven risers are the classic trip hazard and the classic inspection failure.
Tips Before You Pour
- Order for the measured slab, not the plan. Measure formed dimensions after forms are set — forms move.
- Check thickness in several spots. Subgrade high spots silently eat volume; that's what the waste factor is for.
- Concrete sets on its schedule, not yours. Have help, tools, and finishing gear staged before the truck arrives.
- Cold joints weaken slabs. If in doubt between two quantities, order the larger.
Frequently Asked Questions
How many bags of concrete do I need for a 10×10 slab?
At 4″ thick, a 10 × 10 slab is 33.3 cu ft (1.23 cubic yards) — about 56 bags of 80 lb mix, 74 of 60 lb, or 112 of 40 lb, before adding 5–10% for waste.
How many 80 lb bags make a yard of concrete?
One 80 lb bag yields about 0.60 cu ft, so roughly 45 bags make one cubic yard.
Should I order ready-mix or use bags?
At about 1 cubic yard or more, ready-mix is usually less work and competitive on price (watch for short-load fees). Under a yard, bags are the practical choice.
How thick should a concrete slab be?
4″ for patios and walkways, 4–6″ for passenger-car driveways, 6″+ where heavy vehicles park — always over a compacted base, and local code governs.
How much concrete per fence post?
A 4×4 post in a 10″ hole, 24″ deep, needs about 0.85 cu ft after subtracting the post — about 1½ bags of 80 lb mix per post.
Why add extra for waste?
Uneven subgrade, flexing forms, and mix left in the equipment all consume concrete. 5–10% extra is standard practice; running short mid-pour creates a weak cold joint.
Do I need rebar, or is wire mesh enough?
Wire mesh is enough for patios, walkways, and sheds. Add rebar for driveways and anything carrying vehicle weight — it's most important along thickened edges, which is where slabs actually crack first.
How long before I can walk or drive on a new slab?
Light foot traffic at 24–48 hours. Full design strength — safe for vehicles — takes the full 28-day cure. Loading it early is the single most common way a DIY slab underperforms its rated strength.
How do I cure a slab properly?
Keep it damp for 5–7 days: cover with plastic sheeting, spray on a liquid curing compound, or mist it several times a day. Skipping this step can cut final strength by 30% or more even with a correct mix.
How much does a concrete slab cost?
$6–12 per sq ft installed for a basic 4″ broom-finish slab, averaging near $8. Driveways run $8–20; stamped or decorative $12–25.
How much is a yard of concrete?
$125–175 delivered. Orders under about 4 yards usually carry a short-load fee that adds 20–50% per yard.
Is it cheaper to use bags or ready-mix?
Bags cost roughly $225–360 per cubic yard versus $125–175 for ready-mix — so bags only win on small pours where delivery and short-load fees dominate. Under about half a yard, bag it; over two yards, order the truck.
References
- Quikrete — Concrete Mix product data sheet (bag yields): quikrete.com
- Portland Cement Association — concrete basics and placing guidance: cement.org
- American Concrete Institute — residential concrete guidance (ACI 332): concrete.org
Embed This Calculator
Run a blog, a supplier site, or a class page? You're welcome to put this calculator on your own site — free, no signup, no API key. Copy the snippet, paste it into your HTML, done.
The only condition: keep the “Powered by MeasureQuik” link visible. It's how we keep the calculators free. Full details on the embed page.