Concrete Slab Calculator
Work out the concrete a slab, footing, round pier, post-hole or set of steps needs — volume in cubic yards and cubic feet, 40/60/80 lb bag counts, and the ready-mix order.
Concrete in the slab, before waste
1.23 yd³
≈ 33.3 cu ft. To buy: 62 × 80 lb bags, or order 1.50 yd³ ready-mix — 10% waste applied.
In the slab
1.23 yd³
33.3 cu ft · before waste
Bags to buy
62
80 lb bags · 10% waste, rounded up
Ready-mix order
1.50 yd³
10% waste, up to the supplier increment & minimum
Waste allowance
10%
order sits 0.27 yd³ over the slab (waste + rounding up)
Bill of materials
- Concrete volume in place1.23 yd³ (33.3 cu ft)in place
- 80 lb concrete mix bag62to buy
- Ready-mix concrete order1.5 yd³to buy
- Waste allowance10%to buy
Footprint 100 sq ft. One cubic yard covers about 81 sq ft at 4 in.
Buy one route or the other — the bags or the delivery, not both.
Using these numbers
- The "in the slab" figure is the geometry, before any waste — it is what the forms will hold if nothing spills, nothing over-digs and the subgrade is dead level. It is shown so you can check it against your own measurements — it is not the amount to buy.
- The bag count and the ready-mix order are what to buy, waste included — both already carry the waste allowance and both are rounded up — bags to a whole bag, ready-mix to the supplier's increment and minimum. Ordering either figure as shown is ordering enough.
- Pick one route, not both — the two lines are alternatives — 62 bags of dry mix or a 1.5-yard delivery for the same pour, not 62 bags plus a truck. Bags suit a small pour you can mix as you go; ready-mix suits anything a wheelbarrow would make miserable.
- A small pour often orders more than it needs — if the ready-mix figure jumps to a round yard for a job that works out to a third of one, that is the supplier minimum, not a mistake. Below roughly a yard, most suppliers charge a short-load fee or will not deliver, so the order is set at the minimum they will bring.
- For an odd shape, add the pieces — an L-shaped slab is two rectangles; a thickened-edge slab is a slab plus its perimeter footing. Run each piece through the matching shape, then add the volumes — the bag and order figures scale with the total.
- This is a quantity, not a quote — the volume and the bag count are the durable part of planning a pour. What the concrete costs — per yard, delivered, or poured and finished — changes by region and by week, and is answered on the cost calculator, not here.
Methodology
Concrete is a poured volume, not a unit count. The calculator takes the shape you pick, works out the volume it holds in full precision, adds a waste allowance, and then reports two ways to buy that volume — whole bags of dry mix, or a ready-mix delivery rounded to how a supplier actually sells it. Every formula and every constant it uses is below, each with the source it came from.
The volume formula: length × width × thickness ÷ 27
A rectangular slab or footing holds length × width × thickness, all measured in feet. Divide that cubic-foot figure by 27 to get cubic yards, because a yard is three feet and 3 × 3 × 3 = 27. A 10 ft × 10 ft slab at 4 inches — which is one third of a foot — is 10 × 10 × 0.3333 = 33.33 cubic feet in the slab, or 33.33 ÷ 27 = 1.23 cubic yards, before any waste.
The order the multiplication happens in does not matter, and that is the built-in check: area × thickness must equal length × width × thickness to full precision. If the two disagree, a dimension was entered in the wrong unit. The calculator computes it both ways on every keystroke and the diagram redraws to the proportions you entered.
Thickness is carried as a decimal of a foot, never rounded on the way through. This matters because the number gets multiplied twice more — once by the waste factor, once by a bag yield — and a value rounded early drifts by a bag or two by the end. The only rounding is at the very end, when whole bags and a supplier-sized order are what you can actually buy.
volume_ft³ = length_ft × width_ft × thickness_ft
volume_yd³ = volume_ft³ ÷ 27
10 ft × 10 ft slab, 4 in thick (0.33333 ft):
10 × 10 × 0.33333 = 33.333 cu ft in the slab
33.333 ÷ 27 = 1.235 cu yd in the slabHow much one cubic yard covers
One cubic yard of concrete covers about 81 square feet at 4 inches thick, 65 square feet at 5 inches, and 54 square feet at 6 inches. The rule is 27 ÷ thickness in feet: at 4 inches, 27 ÷ 0.3333 = 81. Thicker slab, less area per yard — which is why the coverage figure on the page moves the moment you change the thickness.
Bag coverage follows the same rule from the bag's own cubic-foot yield. An 80 lb bag yields 0.60 cubic feet, so at 4 inches it covers 0.60 ÷ 0.3333 = 1.8 square feet. That is the arithmetic behind "how many bags for a 10 × 10 slab" — it is the slab's square footage divided by the per-bag coverage, then rounded up.
coverage_ft²_per_yd³ = 27 ÷ thickness_ft
4 in : 27 ÷ 0.3333 = 81 sq ft per cubic yard
5 in : 27 ÷ 0.4167 = 65 sq ft per cubic yard
6 in : 27 ÷ 0.5000 = 54 sq ft per cubic yardBags, ready-mix, and where one stops making sense
The bag count is the poured volume in cubic feet, times one plus the waste percentage, divided by the bag's yield, then rounded up to a whole bag. You buy whole bags and you round the breakage allowance up, so the ceiling is right here — it is not a display rounding feeding back into a calculation, it is the number of bags that leave the store. One cubic yard takes 45 × 80 lb bags, 60 × 60 lb bags, or 90 × 40 lb bags.
The ready-mix order starts from the same post-waste volume in cubic yards, then rounds up to the supplier's increment — commonly a quarter or a half yard — and finally takes whichever is larger, that figure or the supplier's minimum load. Many US suppliers will not deliver below roughly one cubic yard without a short-load charge, so a small pour often orders one yard even though it needs less. Both the increment and the minimum are regional facts the calculator is configured with, not something baked into the formula.
Bagged concrete is practical up to roughly half a cubic yard — about 23 × 80 lb bags. Past that, mixing by the bag is a lot of work for a pour that a truck would place in minutes, and ready-mix costs less per yard at volume. The calculator shows both routes side by side so the crossover is visible rather than assumed. What a delivery or a pour costs is the sibling page's job.
bags = ceil( volume_ft³ × (1 + waste) ÷ bag_yield_ft³ )
order_yd³ = max( supplier_minimum ,
roundUpTo( supplier_increment ,
volume_yd³ × (1 + waste) ) )
bags per cubic yard : 27 ÷ 0.60 = 45 (80 lb)
27 ÷ 0.45 = 60 (60 lb)
27 ÷ 0.30 = 90 (40 lb)The waste allowance, shown as its own line
The default waste allowance is 10%. It is a planning margin for spillage, over-excavation, an uneven subgrade and the concrete left in the chute or the wheelbarrow — not a measured quantity, and adjustable from 0 to 25%. It is applied once, to the in-place volume, before the bag and order figures are worked out.
Waste appears as a separate line in the bill of materials rather than being silently folded into the totals, so the parts still add up to the order: in-place volume plus the waste allowance equals what you order. A pour with no spare is a pour that stops halfway, but the margin should be visible, not hidden inside a rounded-up number.
How thick a slab should be, and what the mix is
A concrete slab is 4 inches thick for patios, shed floors and walkways, and 5 to 6 inches for driveways and garage floors that carry vehicles. Edges that carry a wall or a post are often thickened to 8 to 12 inches. Thickness sets both the volume and the coverage per yard, so changing it on the page recomputes everything.
A standard site-batched concrete mix is 1 part cement to 2 parts sand to 3 parts gravel by volume, which gives roughly 3,000 psi. Bagged concrete mix is pre-blended to a set strength — the QUIKRETE No. 1101 data sheet lists 4,000 psi at 28 days — so the ratio only matters when you are batching from raw materials. Mix ratio changes how you make the concrete, never how much of it you need. Reinforcing bar and mesh are a separate quantity, not covered on this page.
The five shapes: slab, footing, pier, post-hole, stairs
Slab and footing are the same rectangular prism — length × width × thickness. A footing is just a slab that happens to be narrow and deep, and it can be repeated with the count field for a run of identical pads.
A round column or pier is π × radius² × height. Enter the diameter and the calculator halves it. A 12-inch tube 4 feet tall holds π × 0.5² × 4 = 3.14 cubic feet; three of them is 9.42 cubic feet, about 0.35 cubic yards in the tubes.
A post-hole is the hole's volume minus the post that sits in it. A 10-inch-wide hole 2 feet deep is π × 0.417² × 2 = 1.09 cubic feet; a nominal 4×4 post is actually 3.5 inches on a side, so it displaces 0.292² × 2 = 0.17 cubic feet, leaving 0.92 cubic feet of concrete per hole — a little over 1.5 × 80 lb bags. A post-hole is bagged by nature, so the calculator does not show a ready-mix line for it.
A flight of steps is a staircase cross-section — run × rise × (1 + 2 + … + n) for n uniform steps — times the width. A 3-step flight 4 feet wide with a 7-inch rise and an 11-inch run is 0.917 × 0.583 × 6 × 4 = 12.83 cubic feet, or 0.48 cubic yards in the steps. The same figure comes out of stacking three prisms of increasing height and adding them, which is the check the golden fixture runs.
slab / footing : length_ft × width_ft × thickness_ft
column / pier : π × (diameter_ft ÷ 2)² × height_ft
post-hole : (π × hole_radius_ft² × depth_ft) − (post_side_ft² × depth_ft)
stairs : run_ft × rise_ft × (1 + 2 + … + n) × width_ftConstants register
| Constant | Value | Source | As of |
|---|---|---|---|
| Cubic yard | 27 cu ft per cu yd | Definition — 1 yd = 3 ft, and 3³ = 27 | n/a |
| Volume identity | area × depth = L × W × D | Geometry — computed both ways on every input; a mismatch means a dimension is in the wrong unit | n/a |
| 80 lb concrete mix bag yield | 0.60 cu ft per bag (→ 45 bags per cu yd) | QUIKRETE Concrete Mix No. 1101 Product Data Sheet, "YIELD" table ("An 80 lb (36.2 kg) bag yields approximately 0.60 ft³ (17 L)"), rev. October 2022 | 2026-09-02 |
| 60 lb concrete mix bag yield | 0.45 cu ft per bag (→ 60 bags per cu yd) | Same data sheet, "YIELD" table ("A 60 lb (27.2 kg) bag yields approximately 0.45 ft³ (12.7 L)") | 2026-09-02 |
| 40 lb concrete mix bag yield | 0.30 cu ft per bag (→ 90 bags per cu yd) | Same data sheet, "YIELD" table ("A 40 lb (18.1 kg) bag yields approximately 0.30 ft³ (8.5 L)") | 2026-09-02 |
| Bagged mix compressive strength | 4,000 psi at 28 days (2,500 psi at 7 days) | Same data sheet, Table 1 — context only, not fed into any quantity | 2026-09-02 |
| Ready-mix order increment | ¼ yd³ (some suppliers ½ yd³) | Supplier ordering convention — a regional packaging fact resolved in lib/packaging/us.ts, not an engine constant; confirmed per region at build | n/a |
| Ready-mix minimum viable order | ≈ 1 yd³ (regional; 0 where a supplier sets none) | Supplier short-load convention — a regional packaging fact; below it, suppliers apply short-load pricing rather than refusing the order | n/a |
| Coverage at 4 in | 81 sq ft per cu yd | Derived — 27 ÷ (4 ÷ 12); 81 × 0.3333 ft = 27 cu ft = 1 cu yd | n/a |
| Default waste allowance | 10% (adjustable 0–25%) | Planning convention for spillage, over-excavation and chute loss — an adjustable input, not a measured value | n/a |
| Nominal 4×4 post actual size | 3.5 in × 3.5 in | US softwood lumber dressing convention — a nominal 4×4 finishes at 3½ in; used for post-hole displacement | n/a |
Rounding rule
Concrete is a poured volume, not a unit count, so the "floor the in-place count" rule used for blocks does not apply here. Instead:
Slab, footing and pier volume carries full precision through the whole chain — length, width and thickness in feet; volume in cubic feet is area × depth; volume in cubic yards is that ÷ 27. Rounding happens once, at the point the number is displayed.
Bag count is volume in cubic feet × (1 + waste%) ÷ the bag's cubic-foot yield, then rounded up — you buy whole bags and round the breakage allowance up.
Ready-mix order is volume in cubic yards × (1 + waste%), rounded up to the supplier increment (¼ or ½ yard), then taken as the larger of that and the supplier's minimum viable order (many US suppliers will not deliver below about 1 yd³). Both the increment and the minimum are regional packaging facts, not engine constants; a region with no minimum uses 0. A post-hole emits no ready-mix line at all — the line's absence is the signal, never a zero-quantity line.
Waste is an adjustable input, default 10%, shown as a separate line in the bill of materials so the parts still sum to the order quantity.
An L-shaped slab or a stepped footing decomposes into rectangles and prisms; the parts are allocated so they sum exactly to the total.
FAQ
- How much concrete do I need for a slab?
- A concrete slab's volume is length × width × thickness in feet, divided by 27 for cubic yards. A 10 ft × 10 ft slab at 4 inches (one third of a foot) is 10 × 10 × 0.3333 ÷ 27 = 1.23 cubic yards in the slab. Add waste — 10% is typical — for the quantity to buy or order.
- How many bags of concrete do I need for a 10×10 slab?
- A 10 ft × 10 ft slab at 4 inches holds 33.33 cubic feet in the slab. Add 10% waste to get 36.67 cubic feet, then divide by 0.60 cubic feet per 80 lb bag: 61.1, rounded up to 62 bags to buy. The yard-route cross-check — 33.33 ÷ 27 × 1.10 × 45 — also gives 61.1, because neither route rounds mid-chain.
- How many bags of concrete make a yard?
- Divide 27 cubic feet by the bag's yield. An 80 lb bag yields 0.60 cubic feet, a 60 lb bag 0.45, and a 40 lb bag 0.30 — from the QUIKRETE No. 1101 data sheet. So one cubic yard takes 45 × 80 lb bags, 60 × 60 lb bags, or 90 × 40 lb bags. Apply the waste factor first, then round up to whole bags.
- How many cubic yards of concrete do I need?
- Cubic yards is cubic feet divided by 27, because a yard is 3 feet and 3³ = 27. Get the volume in cubic feet first — area × thickness in feet — then divide by 27 and add waste. Area × thickness and length × width × thickness must match; if they do not, a dimension is in the wrong unit.
- How much does a yard of concrete cover?
- One cubic yard of concrete covers about 81 square feet at 4 inches thick, 65 square feet at 5 inches, and 54 square feet at 6 inches. The rule is 27 ÷ thickness in feet: at 4 inches, 27 ÷ 0.3333 = 81. Bag coverage follows the same rule from the bag's cubic-foot yield.
- How thick should a concrete slab be?
- A concrete slab is 4 inches thick for patios, shed floors and walkways, and 5 to 6 inches for driveways and garage floors that carry vehicles. Load-bearing edges are often thickened to 8–12 inches. Thickness sets both the volume and the coverage per cubic yard, so the calculator recomputes on every change.
- How big should a concrete footing be?
- A rectangular footing's volume is length × width × depth; a round pier's is π × radius² × height. A 12-inch tube (radius 0.5 ft, 4 ft tall) holds π × 0.5² × 4 = 3.14 cubic feet — 0.12 cubic yards in the tube, or with 10% waste about 6 × 80 lb bags to buy.
- How many bags of concrete for a fence post?
- A post-hole takes the hole's volume minus the post's. A 10-inch hole 2 ft deep is π × 0.417² × 2 = 1.09 cubic feet; a nominal 4×4 post — actually 3.5 inches — displaces 0.292² × 2 = 0.17 cubic feet, leaving 0.92 cubic feet per hole, about 1.5 × 80 lb bags.
- How many bags of concrete are on a pallet?
- Bags per pallet varies by brand and bag size, and the retailer's product listing is the place to confirm it for the mix you are buying. It is a delivery and stacking detail, not part of the volume maths — the calculator gives you the total bag count, and how many pallets that is depends on your supplier's pallet.