How the calculator works
Every tab does the same three things: it converts your measurements to feet, multiplies them to get cubic feet, then divides by 27 to get cubic yards. The waste percentage is added last, and the bag count is rounded up to a whole bag.
| Project | Formula (feet) |
|---|---|
| Slab, footing, wall | length × width (or height) × thickness |
| Round slab, column | π × (diameter ÷ 2)² × thickness (or height) |
| Post holes | (π × (hole diameter ÷ 2)² − post width²) × depth × number of holes |
| Steps | rise × run × width × n(n + 1) ÷ 2, plus the top platform if there is one |
Two details that most calculators skip: the post-hole formula subtracts the post itself, because a 4×4 standing in a 10 in hole takes up about 15% of it, and the steps formula treats a stair as a stack of blocks, which is how a solid-poured stair is actually formed. Thickness is entered in inches by default, but every field accepts inches, feet, yards, centimetres or metres, so you can type what is on your tape measure.
cubic yards = length (ft) × width (ft) × thickness (in) ÷ 12 ÷ 27
Three worked examples
10 × 12 ft patio, 4 in thick. 10 × 12 × 0.333 = 40 ft³ = 1.48 yd³. Add 10%: 1.63 yd³, which is 44 ft³. That is 74 bags of 80 lb (44 ÷ 0.60) or a short-load delivery of 2 yd³ from a ready-mix plant.
Eight fence post holes, 10 in wide, 30 in deep, 4×4 posts. One hole holds π × 0.417² × 2.5 = 1.36 ft³; the post (3.5 in = 0.29 ft square) takes 0.21 ft³ of that, leaving 1.15 ft³. Eight holes: 9.2 ft³, plus 10% = 10.1 ft³, about 17 bags of 80 lb or 27 bags of 50 lb fast-setting mix.
Four steps, 7 in rise, 11 in run, 4 ft wide, 3 ft platform. Steps: 0.583 × 0.917 × 4 × (4 × 5 ÷ 2) = 21.4 ft³. Platform: 3 × 4 × 0.583 × 4 = 28 ft³. Total 49.4 ft³, plus 10% = 54.3 ft³ = 2.0 yd³. Order ready-mix; this is 91 bags otherwise.
Bag yields by product
Yields come from the manufacturers' data sheets. Premix is sold by weight, but what you need is volume, and the ratio is nearly the same across brands: about 133 lb of dry mix per cubic foot of placed concrete, or 3,600 lb per cubic yard.
| Product | Bag | Yield | Bags per yd³ |
|---|---|---|---|
| Quikrete Concrete Mix (1101) | 80 lb | 0.60 ft³ | 45 |
| Quikrete Concrete Mix | 60 lb | 0.45 ft³ | 60 |
| Quikrete Concrete Mix | 40 lb | 0.30 ft³ | 90 |
| Quikrete 5000 (high early strength) | 80 lb | 0.60 ft³ | 45 |
| Quikrete Fast-Setting (posts) | 50 lb | 0.375 ft³ | 72 |
| Sakrete High Strength | 80 lb | 0.60 ft³ | 45 |
| Sakrete High Strength | 60 lb | 0.45 ft³ | 60 |
Source: Quikrete and Sakrete product data sheets, checked October 2026. Yields assume the water quantity printed on the bag; a wetter mix does not give you more concrete, only weaker concrete.
How thick, and how strong
A patio or walkway is poured at 4 in. A driveway for cars is 4 in in mild climates and 5 to 6 in where it freezes, or where you park a truck or RV. The International Residential Code (IRC R506.1) sets the floor at 3.5 in for interior slabs; in practice nobody pours less than 4 in outside, because a thinner slab cracks at the first frost or the first delivery truck.
| Use | Thickness | Minimum strength (IRC R402.2) |
|---|---|---|
| Walkway, patio, shed base | 4 in | 2,500 psi (3,000 psi in a freezing climate) |
| Driveway, cars | 4–5 in | 3,000 psi; 3,500 psi air-entrained where de-icing salt is used |
| Driveway, trucks or RV; garage floor | 5–6 in | 3,500 psi |
| Footings, foundation walls | per plan | 2,500–3,000 psi, sized by the engineer or the IRC footing tables (R403) |
Bagged concrete mix is rated 4,000 psi at 28 days, so strength is rarely the problem with bags; thickness, sub-base and curing are. For footings, the width and depth come from your soil bearing capacity and frost line, both of which your local building department publishes. Frost depth ranges from zero in Florida to 60 in or more in Minnesota, and a footing that stops above it will heave.
Reinforcement and joints
Rebar and mesh do not stop concrete from cracking; they hold the cracks tight so the slab stays flat. For a patio, 6×6 in welded wire mesh (W1.4 × W1.4) set in the middle of the slab is standard. For a driveway, #3 or #4 rebar on a 12 to 18 in grid, held up on chairs so it sits in the middle third of the slab, is better than mesh, because mesh that is walked into the bottom of the pour does nothing.
Control joints give the slab a place to crack on purpose. Space them at 2 to 3 times the thickness in inches, read as feet: a 4 in slab gets joints every 8 to 12 ft, cut to a quarter of the slab depth (1 in) within 6 to 18 hours of finishing. The Portland Cement Association recommends not exceeding 24 to 30 times the thickness between joints. Keep the panels roughly square; long narrow panels crack in the middle.
What concrete costs in 2026
Prices move with cement and diesel, so treat these as planning numbers and get two local quotes. The figures below are the ranges we saw in supplier and retailer pricing in October 2026 across the US; coastal metros run higher, rural plants lower.
| Item | Typical price | Notes |
|---|---|---|
| Ready-mix, 3,000–4,000 psi | $150–$200 per yd³ | Delivered, 2026 quotes; add $10–$25 for air entrainment or fibre |
| Short-load fee | $50–$150 | Charged below the plant's minimum, usually 3–5 yd³ |
| Waiting time | $1–$3 per minute | After the first 30–60 minutes on site |
| Concrete pump | $300–$900 per day | Only if the truck cannot reach the forms |
| 80 lb bag, standard mix | $6–$9 | Big-box retail; 5000 psi and fast-setting cost $1–$3 more |
| Finished slab, contractor | $6–$12 per ft² | Forms, base, pour, finish; stamped or coloured $12–$20 |
Break-even: one cubic yard is 45 bags at roughly $7, so about $315 in material, plus a day of mixing. The same yard delivered is $150–$200 plus a $75–$150 short-load fee, so $225–$350 and no mixing. Below about half a yard, bags win; above one yard, the truck wins on both price and quality, because every batch is identical.
Mixing, placing and curing
An 80 lb bag takes about 3 quarts (2.8 litres) of water. Pour most of it into the mixer first, add the mix, then add the rest until the concrete holds its shape when sliced with a shovel but still flows into the corners. Too much water is the most common DIY mistake: an extra quart per bag can cut strength by a quarter and doubles surface dusting later.
Place within 60 to 90 minutes of mixing, screed level, let the bleed water disappear, then float and finish. After finishing, the slab needs to stay moist for 7 days (ACI 308R): cover it with plastic sheeting, wet burlap or a spray-on curing compound. Keep foot traffic off for 24 to 48 hours, cars off for 7 days, and expect the design strength at 28 days.
Do not pour below 40 °F (4 °C) unless you can keep the slab warm for three days, and in hot weather above 90 °F (32 °C) pour early in the morning, dampen the sub-base and shade the slab, or it will flash-set and crack (ACI 305 and 306 cover the details).
Weight and density
| Concrete type | Density | Weight per yd³ |
|---|---|---|
| Normal-weight (what bags and ready-mix give you) | 145–150 lb/ft³ | 3,900–4,050 lb |
| Lightweight structural | 90–115 lb/ft³ | 2,400–3,100 lb |
| Dry premix, as bagged | 133 lb per ft³ of concrete produced | 3,600 lb of bags per yd³ |
The calculator uses 150 lb/ft³ for the weight line. That matters for two things: whether your trailer can carry the bags (45 bags is 1.8 tons), and whether a deck or balcony can carry a planter or hot tub pad.
Frequently asked questions
How many 80 lb bags of concrete make a yard?
45 bags. An 80 lb bag yields 0.60 ft³ and a cubic yard is 27 ft³, so 27 ÷ 0.60 = 45. It takes 60 bags of 60 lb or 90 bags of 40 lb.
How much concrete is in a 10 × 10 slab, 4 inches thick?
1.23 cubic yards before waste (10 × 10 × 0.333 ÷ 27). With 10% extra, order 1.36 yd³, which is 62 bags of 80 lb or a 1.5 yd³ short load.
How much does a cubic yard of concrete weigh?
About 4,000 lb, or two US tons. Normal-weight concrete is 145–150 lb per cubic foot.
How much concrete do I need for a 4×4 fence post?
A 10 in wide, 30 in deep hole with a 4×4 post in it needs about 1.15 ft³ of concrete, which is two 80 lb bags or four 50 lb bags of fast-setting mix. Dig the hole three times the post width and a third of the post's height, and below the frost line.
Does the calculator subtract the post from the hole?
Yes. Enter the post width in the optional field and the post's volume is removed from the hole. Leave it empty if the hole will be filled solid, for example for a mailbox sleeve or a bollard.
How much extra concrete should I order?
5 to 10%. Spillage, an uneven sub-base and forms that bow outward all eat concrete. For a hand-dug footing on rough ground use 15%; running out mid-pour creates a cold joint that you cannot fix.
What is the difference between concrete mix and fast-setting mix?
Standard mix (Quikrete 1101, Sakrete High Strength) is mixed with water and reaches 4,000 psi in 28 days. Fast-setting mix is poured dry into a post hole and then wetted; it sets in 20 to 40 minutes and is only meant for posts, not slabs.
How thick should a concrete slab be for a shed?
4 inches for a garden shed or workshop. If a car, tractor or heavy machine will sit on it, pour 5 to 6 inches with rebar.
Can I pour a slab in two batches on different days?
You can, but the joint between them is a cold joint and will crack. Plan the pour so each panel between control joints is finished in one go, and if you must stop, stop at a joint.
How long before I can walk or drive on new concrete?
Walk after 24 to 48 hours, park a car after 7 days, and treat 28 days as full strength. Keep it damp for the first 7 days; concrete that dries out early never reaches its rated strength.
How we built this calculator
The formulas are the plain geometric ones, shown on the result board for every calculation. Bag yields are taken from the manufacturers' data sheets and the dry-mix ratio of 2,150 kg per m³ (133 lb per ft³) is what those sheets imply; density is 2,400 kg/m³ (150 lb/ft³), the Portland Cement Association's figure for normal-weight concrete. Prices are ranges we collected from supplier and retailer price lists in the month shown and are not quotes.
References
- International Residential Code 2024, R402.2 (concrete strength), R403 (footings), R506 (slabs)
- ACI 332-20, Residential Code Requirements for Structural Concrete
- ACI 308R-16, Guide to External Curing of Concrete
- Portland Cement Association, Design and Control of Concrete Mixtures (joint spacing, density)
- Quikrete Concrete Mix 1101, Quikrete 5000, Fast-Setting Concrete Mix: product data sheets
- Sakrete High Strength Concrete Mix: product data sheet
Estimates for planning purposes. Always check the yield printed on your bags and local building rules.