Free concrete calculator
Find out exactly how much concrete to order. Enter your slab's length, width and thickness in feet-inches or metres. The calculator returns the cubic yards (and cubic metres) you need, the bag count for 60lb and 80lb bags, and the cost of bags versus a ready-mix truck — updated live, as you type.
On this page14 sections
| Option | Quantity | Est. cost |
|---|---|---|
| 80lb bags | 62 bags | $403 |
| 60lb bags | 82 bags | $410 |
| Ready-mix (delivered) | 1.50 cu yd | $240 |
Prices are regional estimates: 80lb bags ≈ $6.50, 60lb bags ≈ $5.00, ready-mix ≈ $160/yd. Ready-mix excludes a short-load fee that applies below the truck minimum (often 1–3 cu yd).
Bag yields and prices vary by mix and supplier. How accurate is this?
Results are estimates. Consult a professional.
How the concrete calculator works
Concrete is sold by volume, not by area, so the only thing that matters is how much three-dimensional space you need to fill. The calculator multiplies your slab's length, width and thickness to get cubic feet, then converts that to cubic yards — the unit ready-mix suppliers quote — and to cubic metres for metric users. From the same volume it works out how many bags of dry mix you would need instead.
What the result actually means
Two numbers do the work. The cubic-yard figure is what you tell the ready-mix supplier when you order a truck. The bag count is what you carry out of the home-improvement store if you are mixing on site. The calculator shows both so you can compare the two ways of buying the same pour — and it adds a waste margin on top, because the volume that reaches the forms is always a little less than the volume you bought.
What goes into your concrete estimate
A concrete estimate is built from three pieces. Get each one right and the order is right; miss one and you are either short a few bags mid-pour or staring at leftover concrete going off in the wheelbarrow.
Slab volume — the base number
Length times width times thickness. This is the geometry, and it is the part the calculator nails exactly. The single biggest lever is thickness: going from a 4-inch slab to a 6-inch slab is a 50% jump in concrete for the same footprint, because you are adding depth to every square foot at once.
Bag yield — how much each bag actually makes
A bag of dry mix weighs more than the volume it produces, because water is added on site. Published yields are about 0.60 cubic feet for an 80lb bag, 0.45 for a 60lb bag and 0.30 for a 40lb bag. Those numbers set the bag count: 27 cubic feet in a yard divided by 0.60 is roughly 45 eighty-pound bags per cubic yard.
Waste factor — the safety margin
Real pours lose a little concrete to spillage, to forms that bow outward, to a subgrade that is lower in spots than you measured, and to the material left clinging to the mixer and the wheelbarrow. A 5–10% overage covers it. The calculator adds 10% by default and lets you dial it between 0 and 20%.
A worked example using the concrete calculator
Dave is pouring a 10 ft × 10 ft floor for a backyard shed at the standard 4-inch thickness. He wants to know whether to buy bags or order a small ready-mix load, so he needs the volume, the bag count and a 10% waste margin.
Step 1 — Find the volume in cubic feet
Thickness in feet first: 4 in ÷ 12 = 0.333 ft. Then 10 × 10 × 0.333 = 33.3 cu ft.
Step 2 — Convert to cubic yards
33.3 ÷ 27 = 1.23 cu yd (about 0.94 m³). That is the bare volume of the slab, before any waste.
Step 3 — Add the waste margin
With 10% added, 1.23 × 1.10 = 1.36 cu yd. Dave would order 1.5 yd of ready-mix, or buy enough bags to cover 1.36 yd.
Step 4 — Work out the bag count
Including the 10% margin, 33.3 ft³ × 1.10 = 36.6 ft³. Divided by the 0.60 ft³ yield of an 80lb bag and rounded up, that is 62 bags; at the 0.45 ft³ yield of a 60lb bag it is 82 bags.
How much concrete do I need?
If you just want a ballpark before you measure, this table gives the volume for common slab sizes at the standard 4-inch thickness. These are bare volumes — add 5–10% for waste before you order.
| Slab size (4 in thick) | Cubic feet | Cubic yards | Cubic metres |
|---|---|---|---|
| 4 ft × 4 ft | 5.3 | 0.20 | 0.15 |
| 8 ft × 8 ft | 21.3 | 0.79 | 0.60 |
| 10 ft × 10 ft | 33.3 | 1.23 | 0.94 |
| 12 ft × 12 ft | 48.0 | 1.78 | 1.36 |
| 20 ft × 20 ft | 133.3 | 4.94 | 3.78 |
| 24 ft × 24 ft | 192.0 | 7.11 | 5.44 |
All figures assume a 4-inch (0.333 ft) thickness. A 6-inch slab uses 50% more; an 8-inch slab uses twice as much.
When to use this concrete calculator
Reach for it any time a pour turns on how much concrete to buy — which is every time, because over-ordering wastes money and under-ordering can ruin a slab with a cold joint.
- Pouring a slab — patios, shed and garage floors, walkways, basement floors and equipment pads.
- Setting footings — for decks, fences, retaining walls and porch piers, where each footing is a small box of concrete.
- Filling columns or post holes — round or square, where depth and diameter set the volume.
- Deciding bags vs. ready-mix — to see at what size a truck delivery beats carrying and mixing bags by hand.
Types of concrete mix by project
The volume math is the same for every mix, but which bag you reach for depends on the job. The four most common residential options trade speed, strength and crack control against price.
| Mix type | Best for | Note |
|---|---|---|
| Standard concrete mix | Slabs, footings, general flatwork | Typically rated around 3,000–4,000 PSI at 28 days |
| High-early-strength | Cold weather, fast turnaround | Reaches handling strength sooner; useful when you need the form back |
| Fiber-reinforced | Thin slabs, patios, walkways | Synthetic fibers reduce surface cracking; can replace wire mesh in light flatwork |
| Fast-setting | Posts and small footings | Sets in 20–40 minutes; can be poured dry into a wet hole for fence posts |
PSI ratings and set times follow typical QUIKRETE and Sakrete product specifications; confirm the rating on the bag for your project.
For a structural slab, footing or anything carrying significant load, follow the strength your local building code or engineer specifies rather than picking by convenience.
Bags vs. ready-mix: which to buy
The choice comes down to volume. Below a cubic yard or so, bags are simpler and cheaper because you skip delivery fees. Above a couple of yards, ready-mix wins decisively — mixing dozens of bags by hand is slow, and the seams between batches can weaken the slab.
| Your volume | 80lb bags | Better choice |
|---|---|---|
| Under 0.5 cu yd | up to ~24 | Bags — small, no delivery fee |
| 0.5 – 1 cu yd | ~24–48 | Either — compare bag total vs. short-load fee |
| 1 – 2 cu yd | ~48–95 | Ready-mix usually, despite the short-load fee |
| Over 2 cu yd | 95+ | Ready-mix — bags become impractical |
Bag counts assume 0.60 ft³ per 80lb bag and exclude waste. The truck minimum varies by supplier; below it you pay a short-load or minimum-delivery fee.
One practical limit: a single batch of hand-mixed concrete should be placed continuously, so a pour that needs more bags than you and your crew can mix and place before the first batch starts to set is a pour that wants ready-mix.
Waste factor: how much extra to order
Order the exact calculated volume and you will come up short. Some concrete spills, some clings to the mixer and wheelbarrow, forms flex outward under the weight, and the subgrade is almost never perfectly level — the low spots quietly swallow extra material. A modest overage covers all of it.
The 5–10% rule
For most slabs, add 10%. On a clean, well-formed pour over a flat, compacted subgrade you can trim that to 5%. On rough ground, hand-dug footings or anything with an uneven base, 10% is the floor and some estimators go to 15%. The calculator's waste field defaults to 10% and adjusts up to 20%.
Why running short is worse than running over
Leftover concrete is wasted money, but running out mid-pour is worse: the concrete already in the forms begins to set, and the joint where fresh concrete meets it — a cold joint — is a permanent weak line in the slab. The overage exists to make sure that never happens.
Concrete definitions
How accurate is this concrete calculator?
The volume math is exact. Length times width times thickness, divided by 27, is the precise cubic-yard volume of your slab, and the cubic-metre conversion uses the exact 0.7646 factor. If your measurements are right, the geometry is right to the decimal.
The bag counts and costs are estimates, and on purpose. Published bag yields — about 0.60 cubic feet for an 80lb bag — vary slightly by manufacturer, by mix formulation and by how much water is added on site, so the real count can drift a bag or two either way. Prices move with your region and the day. Treat the bag count and the cost table as planning figures, confirm the yield printed on your specific bag, and confirm the price and truck minimum with your supplier before you order. When in doubt, order to the high side: the waste factor exists precisely because running short mid-pour is far more costly than a little left over.
Frequently asked questions about the free concrete calculator
About this concrete calculator
This concrete calculator runs entirely in your browser. Every dimension you type stays on your device — nothing is sent to a server, logged, or shared. Volume is computed as length × width × thickness, converted to cubic yards and cubic metres, and bag counts use published 60 lb and 80 lb yields with your waste factor applied — all updating instantly.
Calculators Cloud offers 400+ free tools with no sign-up. The whole Construction calculators shelf includes Cubic yards, Stair, and material-estimator tools alongside this one. Or browse the full calculator directory.