Concrete Calculator
Most calculators stop at cubic yards. This one keeps going: it applies your waste allowance, rounds to a quantity the plant can actually batch, counts the bags, plans the truckloads and prints an order sheet you can hand to the yard.
Build your pour
Add every pour that comes off the same truck — slab, footings, post holes — and get one consolidated order.
Appears on the printed and copied order sheet.
Patios, walkways, garage floors, driveways, shed pads — any flat rectangular pour.
4 in for patios and walkways; 5–6 in for driveways and garage floors.
Identical copies of this item.
Project settings
Applied to the total — you do not have to do this in your head.
ACI 318 takes reinforced normal-weight concrete at 150 lb/ft³ — the extra ~5 lb/ft³ over plain concrete accounts for the steel. 150 lb/ft³ is 4,050 lb per cubic yard, the familiar "about two tons a yard".
Every size is still shown in the results.
Prices optional
Prices vary by region and by day. Use a quote from your own supplier.
Your supplier's terms optional
Minimums and short-load charges are set by each plant and vary by region — the placeholders are typical practice, not a standard. Enter your supplier's actual terms and they will be used instead.
How to calculate concrete
Concrete is sold by volume, so every calculation comes down to the same three steps: get every dimension into the same unit, multiply for volume, then convert to the unit your supplier sells in.
Volume = length × width × thickness. With all three in feet that gives cubic feet. Cubic yards = cubic feet ÷ 27, because a cubic yard is 3 ft × 3 ft × 3 ft. That 27 is exact, which is why the calculator above keeps every imperial measurement in feet internally — it means no rounding creeps into the headline figure.
The step most calculators skip is the one that decides what you actually buy. Your calculated volume is a real number; your order is a number the plant can batch. Ready-mix is normally batched in quarter-yard increments, so 1.36 yd³ becomes an order of 1.5 yd³ — and it rounds up, never down, because running short mid-pour leaves a cold joint you cannot undo.
A worked example
A 10 ft × 10 ft patio at 4 inches thick:
- Thickness in feet: 4 ÷ 12 = 0.333 ft
- Volume: 10 × 10 × 0.333 = 33.3 ft³
- Cubic yards: 33.3 ÷ 27 = 1.23 yd³
- Plus 10% waste: 1.36 yd³
- Rounded to the next quarter yard: order 1.5 yd³
- Or in bags: 36.7 ft³ ÷ 0.60 = 62 × 80 lb bags
Note that the bag count is worked out from the waste-adjusted volume, not the raw 1.23 yd³. Counting bags off the net figure is a common way to end up two bags short on a Saturday afternoon.
Odd shapes
Break the pour into rectangles, circles and triangles, work each out separately, and add them up. That is exactly what the Build Project list above does — add one item per section and the totals take care of themselves. For a slab on ground that slopes, measure the thickness at several points and use the average.
How thick should the concrete be?
Thickness drives the volume more than anything else, so it is worth getting right before you order. These are common residential figures; a structural engineer or your local building department has the final word, particularly for anything load-bearing.
| Application | Typical thickness | Notes |
|---|---|---|
| Walkways and garden paths | 4 in | Foot traffic only. |
| Patios and pool decks | 4 in | Thicken to 5 in over soft ground. |
| Residential driveways (cars) | 4–5 in | 5 in where vehicles turn. |
| Garage floors | 5–6 in | Jack stands and loaded shelving. |
| Driveways for trucks or RVs | 6 in | Reinforcement is worth it. |
| Shed and hot-tub pads | 4–6 in | 6 in for a filled hot tub. |
Bags or ready-mix?
The honest answer is that volume decides this more often than price does. One cubic yard is forty-five 80 lb bags. A confident crew can mix that in a day with a mixer on site. Two cubic yards is ninety bags, and that is a different proposition entirely — a long, heavy day with a real risk of inconsistent batches across the pour.
Price pulls the other way at small volumes. Many plants set a minimum somewhere around 5–7 cubic yards, and below it a short-load charge is common — often in the region of $100–$250, though that is typical practice rather than a standard and varies by supplier and region. It can easily wipe out ready-mix’s per-yard advantage on a small job, so it is worth asking before you compare quotes. Enter your supplier’s actual minimum and charge in the calculator and it will use those instead of the typical figures.
Bags per cubic yard
| Bag size | Yield | Bags per yd³ | Bags per ½ yd³ |
|---|---|---|---|
| 40 lb | 0.3 ft³ | 90 | 45 |
| 50 lb | 0.375 ft³ | 72 | 36 |
| 60 lb | 0.45 ft³ | 60 | 30 |
| 80 lb | 0.6 ft³ | 45 | 23 |
| 90 lb | 0.66 ft³ | 41 | 21 |
Yields are the figures published on the manufacturers’ technical data sheets, not estimates. Note that the 90 lb bag does not sit on the same yield-per-pound line as the others, which is why a smaller bag's yield should never be scaled from it.
What one cubic yard covers
| Thickness | Coverage per cubic yard |
|---|---|
| 4 in | 81 sq ft |
| 5 in | 65 sq ft |
| 6 in | 54 sq ft |
| 8 in | 41 sq ft |
| 12 in | 27 sq ft |
The 4-inch row is where the old contractor shortcut comes from: divide your square footage by 81 and you have your yardage.
Ordering without getting caught out
- Never order the exact calculated volume. A little left over costs almost nothing. Running out halfway through a slab costs you a cold joint, and possibly the whole pour.
- Check the subgrade before you order, not after. A base that dips half an inch across a 400 sq ft slab is an extra sixth of a yard you did not plan for. That is what the higher waste presets are for.
- Measure posts at their actual size. A 4×4 is 3.5 inches and a 6×6 is 5.5. Across twenty fence posts the difference in displaced concrete is real.
- Ask about the plant minimum before you compare prices. A per-yard price means little if a short-load charge doubles the cost of a small order.
- Have everything ready before the truck arrives. Most suppliers allow a limited unloading window and charge waiting time beyond it.
- Order the mix, not just the volume. Strength, aggregate size, slump and any admixtures all need to be on the ticket.
Concrete weight and density
Weight matters for anything you have to carry, hoist or support. The calculator reports both the weight of what you order and the weight of the finished pour, using these figures:
| Mix | Density | Per cubic yard |
|---|---|---|
| Plain concrete (no reinforcement) | 145 lb/ft³ (140–150) | 3,915 lb |
| Reinforced concrete (with rebar) | 150 lb/ft³ (145–156) | 4,050 lb |
| Structural lightweight concrete | 110 lb/ft³ (90–120) | 2,970 lb |
Ranges are shown deliberately. Density varies with aggregate, moisture and mix design, and a single figure quoted to three decimals would be false precision. See our methodology for where each number comes from.
Waste allowances
| Site condition | Allowance | When to use it |
|---|---|---|
| Clean forms, level base | 5% | Well-formed slab on a compacted, screeded base. Little to lose but spillage. |
| Typical job | 10% | The usual default. Covers spillage, minor over-excavation and an uneven subgrade. |
| Rough or hand-dug | 15% | Hand-dug trenches, soft ground, or a base you could not screed accurately. |
Shapes this calculator handles
Each of these can be added to the same project as many times as you need, so a driveway, its footings and a run of post holes come out as one order:
- Slab / rectangle — Patios, walkways, garage floors, driveways, shed pads — any flat rectangular pour.
- Strip footing — Continuous spread footings under foundation walls, garages and additions.
- Round column / form tube — Deck footings, porch piers and any cylindrical form tube.
- Post hole — Fence, deck and mailbox posts. The volume of the post itself is deducted.
- Wall — Foundation walls, stem walls, retaining walls. Door and window openings are deducted.
- Stairs — Poured steps. Choose solid-on-grade or a suspended slab with a throat.
- Monolithic slab (thickened edge) — Slab and perimeter footing poured together. Common for garages, sheds and slab-on-grade homes.
- Circular slab / pad — Round patios, hot-tub pads, fire-pit bases, grain-bin pads.
- Square / rectangular column — Piers, posts and pilasters formed with straight forms.
- Tapered slab (trapezoid) — Driveways and walks that widen or narrow along their length.
- Curb & gutter — Cast-in-place curb with an integral gutter flag: cross-section × run, the way DOT quantities are taken off.
- Tube / ring (annulus) — Circular walls, manhole surrounds and tree rings — a cylinder with the middle removed.
- Belled pier — Drilled shaft with a flared base. The bell is treated as a conical frustum.
Frequently asked questions
How do I calculate how much concrete I need?
Multiply length × width × thickness with every measurement in feet, then divide by 27 to get cubic yards. A 10 ft × 10 ft slab at 4 in thick is 10 × 10 × 0.333 = 33.3 cubic feet, or 1.23 cubic yards. Then add a waste allowance — the calculator above does all of this and rounds the result up to a quantity your supplier can actually batch.
How many cubic yards is a 10 × 10 slab?
At 4 inches thick, 1.23 cubic yards. At 5 inches, 1.54. At 6 inches, 1.85. With a 10% waste allowance on the 4-inch slab you would order 1.5 cubic yards, because ready-mix is batched in quarter-yard steps.
How many bags of concrete make a cubic yard?
Forty-five 80 lb bags, sixty 60 lb bags, seventy-two 50 lb bags, or ninety 40 lb bags. Those follow directly from the yields on the manufacturers’ data sheets: an 80 lb bag yields 0.60 cubic feet, and 27 ÷ 0.60 = 45.
Should I use bags or order ready-mix?
Volume decides it more than price. Under about one cubic yard, bags usually win — a possible short-load charge below the supplier minimum often cancels ready-mix’s per-yard advantage. Over about two cubic yards, you are looking at 90+ bags, which stops being a realistic hand-mix, and ready-mix is both cheaper and more consistent. Between the two it is a genuine judgement call, which is why the calculator says so rather than picking for you.
How much does a cubic yard of concrete weigh?
About 4,050 lb — roughly two tons — at the 150 lb/ft³ that ACI 318 uses for reinforced normal-weight concrete. Plain concrete is taken at 145 lb/ft³, or about 3,915 lb per yard. Lightweight mixes run far lower and vary with the aggregate, so get the figure from your supplier’s mix design.
How much waste should I add to a concrete order?
Five percent for a clean pour into tight forms on a screeded base, ten percent for a typical job, and up to fifteen percent for hand-dug trenches or soft, uneven ground. Never order the exact calculated volume: running out mid-pour leaves a cold joint, while a little left over costs you almost nothing.
How many cubic yards does a concrete truck hold?
A full load is usually about 10 cubic yards, with mixer drums rated between 8 and 12. NRMCA notes that a drum’s rated mixing capacity is roughly 63% of its gross volume, which is why a large-looking drum still delivers around 10 yards.
What is a short-load charge?
A surcharge some suppliers apply when you order less than their minimum, which is often somewhere around 5 to 7 cubic yards. Where it applies it commonly runs $100 to $250. Both the minimum and the charge are set by each plant and vary by region and over time — they are not standards. The calculator flags when your order falls below the typical threshold so you know to ask, and you can enter your own supplier’s minimum and charge to replace the defaults.
How many square feet does a yard of concrete cover?
Eighty-one square feet at 4 inches thick, 65 at 5 inches, 54 at 6 inches and 27 at 12 inches. The 4-inch figure is where the contractor shortcut “divide your square footage by 81” comes from.
Can I calculate a slab, footings and post holes together?
Yes — that is what the Build Project list is for. Add each pour as its own item and the calculator sums them into one net volume, one waste-adjusted order and one delivery plan. Most calculators handle a single shape at a time, which forces you to add the pieces up yourself.
Does the calculator account for the post inside a post hole?
Yes. Enter the post’s actual width — a 4×4 is 3.5 inches, not 4 — and its volume is subtracted from the hole. Over a run of fence posts that deduction is large enough to change how many bags you buy.
How accurate is this calculator?
The geometry is exact and the bag yields come straight from manufacturer data sheets. The variables are real-world ones: concrete density varies with the mix, subgrades are never perfectly flat, and plant minimums and prices differ by region. That is why densities are shown as ranges and why the order figure includes a waste allowance rather than pretending to three decimal places of precision.
Related
- Gravel Calculator — base stone under a slab or driveway, layer by layer, with compaction handled.
- Cubic Yard Calculator — plain volume for any material, when you only need the yards.
Sources
Bag yields come from the manufacturers’ published technical data sheets, densities follow the ACI 318 convention for normal-weight concrete, and delivery figures follow NRMCA guidance on ordering ready-mixed concrete. Every constant used on this page, with its citation and its range, is listed on the methodology page — which is generated from the same data the calculator uses, so the two cannot drift apart.
Last reviewed September 16, 2026.