How to use the concrete calculator
- Choose the shape that matches your pour: slab, footing, wall, round column (sonotube), post hole, steps, or "I already know the cubic yards".
- Measure length and width in feet and thickness or depth in inches. The calculator converts inches for you, so don't divide by 12 first.
- Set the quantity. If you are pouring eight identical sonotubes or ten post holes, enter the count instead of running the calculator ten times.
- Leave the waste at 10% or adjust it. Raise it for uneven ground or hand-dug holes. Lower it only if your forms are perfectly square and the base is level.
- Read the result. For anything around one cubic yard or more, compare ready-mix with bags (see below). For small jobs, use the bag count.
Concrete formulas by shape
Every shape comes down to one idea: volume = area × thickness, with every measurement in feet. Thickness is usually measured in inches, so divide it by 12.
| Shape | Volume in cubic feet |
|---|---|
| Slab, patio, sidewalk, driveway | length × width × (thickness in ÷ 12) |
| Footing | length × (width in ÷ 12) × (depth in ÷ 12) |
| Wall | length × height × (thickness in ÷ 12) |
| Round column or sonotube | π × (diameter in ÷ 24)² × height |
| Post hole (square post set in concrete) | [π × (hole diameter in ÷ 24)² − (post width in ÷ 12)²] × (hole depth in ÷ 12) |
| Solid steps, n steps | width × (run in ÷ 12) × (rise in ÷ 12) × n(n + 1) ÷ 2 |
Then convert:
- Cubic yards = cubic feet ÷ 27, because a yard is 3 ft × 3 ft × 3 ft.
- Cubic meters = cubic feet × 0.0283168, because a foot is exactly 0.3048 m (NIST).
- Bags = cubic feet ÷ bag yield, rounded up to a whole bag.
Bag yields we use
QUIKRETE's Concrete Mix data sheet lists these yields, and SAKRETE's High-Strength Concrete Mix sheet gives the same figures for 40, 60 and 80 lb bags:
| Bag size | Yield per bag | Bags per cubic yard |
|---|---|---|
| 40 lb | 0.30 ft³ | 90 |
| 60 lb | 0.45 ft³ | 60 |
| 80 lb | 0.60 ft³ | 45 |
Specialty products (fast-setting, high-early-strength or crack-resistant mixes) can print different yields, so check the bag you are buying.
Worked examples
A 10 × 10 ft slab, 4 inches thick
- Volume: 10 × 10 × (4 ÷ 12) = 33.33 ft³, which is 33.33 ÷ 27 = 1.23 yd³.
- With 10% waste: 36.67 ft³ = 1.36 yd³.
- Bags: 36.67 ÷ 0.60 = 61.1, so 62 × 80 lb bags, or 82 × 60 lb, or 123 × 40 lb.
That's about 2.5 tons of bags to lift and mix. At this size, ready-mix is usually worth pricing.
A 20 ft footing, 16 in wide and 8 in deep
- 20 × (16 ÷ 12) × (8 ÷ 12) = 17.78 ft³ = 0.66 yd³.
- With 10% waste: 0.72 yd³, or 33 × 80 lb bags.
Eight 12 in sonotubes, 4 ft tall
- One tube: π × (12 ÷ 24)² × 4 = 3.14 ft³.
- Eight tubes plus 10%: 3.14 × 8 × 1.10 = 27.6 ft³ = 1.02 yd³, or 47 × 80 lb bags.
A fence post hole
Take a 10 in hole, 30 in deep, holding a 4×4 post. A "4×4" really measures 3.5 × 3.5 in, so enter 3.5.
- [π × (10 ÷ 24)² − (3.5 ÷ 12)²] × (30 ÷ 12) = 1.15 ft³.
- That's 1.92 × 80 lb bags with no waste, so 2 bags. With 10% waste it is 2.11, which rounds up to 3 bags.
- For a fence line, enter the number of holes and buy for the total. The leftover from one hole covers the shortfall in the next.
Three solid steps
The steps are 4 ft wide, with an 11 in run and 7 in rise. 4 × (11 ÷ 12) × (7 ÷ 12) × (3 × 4 ÷ 2) = 12.83 ft³ = 0.48 yd³, or 24 × 80 lb bags with 10% waste.
The calculator treats the steps as a solid block. If your steps sit on a fill-compacted base or against a slab, your real volume is smaller.
Bags or ready-mix?
The bag count tells you when mixing by hand stops making sense: 45 × 80 lb bags make one cubic yard. That's 3,600 lb of material, plus the water and mixing time.
- Under roughly half a yard (post holes, a pad for an AC unit, a few steps), bags are usually simplest.
- Around a yard or more, call a local ready-mix supplier. Ask about their minimum order and any short-load fee, because those decide whether a small truck delivery is worth it. They vary by supplier, so we don't quote prices.
- For a mid-sized job, renting a mixer or using a "mini-mix" trailer service can split the difference.
Tips and common mistakes
- Measure thickness at several points. A slab that is "4 inches" but averages 4.5 inches over uneven ground needs 12.5% more concrete. This is the most common reason people run short.
- Use actual lumber sizes. A 2×4 form board is 1.5 × 3.5 in. A 4×4 post is 3.5 in square.
- Don't skip the waste allowance. Spillage, overfilled forms and a soft subgrade all eat concrete. Running out mid-pour leaves a cold joint.
- Round up, never down. Half a bag short is a whole bag short on the day.
- Plan curing before you pour. QUIKRETE's data sheet says curing should start as soon as possible and continue for 5 days in warm weather (70 °F or higher) or 7 days at 50–70 °F. Keep the surface moist or use a curing compound.
- Check local rules. Footing depth, slab thickness and reinforcement for structural work are set by your local building code, not by a calculator.
Frequently asked questions
How many 80 lb bags of concrete make a yard?
- Each 80 lb bag yields about 0.60 ft³, and a cubic yard is 27 ft³, so 27 ÷ 0.60 = 45.
How many bags do I need for a 10 × 10 slab?
At 4 inches thick, you need 33.33 ft³. That's 56 × 80 lb bags exactly, or 62 with a 10% waste allowance.
How thick should a concrete slab be?
For patios and walkways, 4 inches is a common choice, and it is the calculator's default. Driveways and slabs that carry heavier loads are often thicker. For structural or permitted work, the thickness, base and reinforcement are set by your plans and your local building department, not by a calculator.
How much concrete do I need per fence post?
It depends on the hole. A 10 in × 30 in hole around a 4×4 post takes about 1.15 ft³, roughly two 80 lb bags. Use the post hole shape with your own dimensions.
Should I add a waste percentage?
Yes. The calculator defaults to 10%. Add more for hand-dug holes, rough ground or footings dug without forms.
Is the bag yield the same for every brand?
For standard 40, 60 and 80 lb concrete mix from QUIKRETE and SAKRETE, yes: 0.30, 0.45 and 0.60 ft³. Other products can differ, so check the bag.
How long should concrete cure before I use it?
Curing and strength gain are different. QUIKRETE's data sheet lists about 2,500 psi at 7 days and 4,000 psi at 28 days for its Concrete Mix, and asks for 5–7 days of moist curing depending on temperature. Follow the bag's instructions before you load the slab.
Sources
- QUIKRETE Concrete Mix (No. 1101) data sheet: bag yields, strength and curing. https://www.quikrete.com/pdfs/data_sheet-concrete%20mix%201101.pdf
- SAKRETE High-Strength Concrete Mix technical data sheet: bag yields. https://www.sakrete.com/wp-content/uploads/2024/01/Sakrete-High-Strength-Concrete-Mix-TDS.pdf
- American Cement Association (formerly the Portland Cement Association), "Applications of Cement": finishing and curing basics. https://www.cement.org/cement-concrete/applications-of-cement/
- NIST, SI units of length: 1 in = 25.4 mm, 1 yd = 0.9144 m. https://www.nist.gov/pml/owm/si-units-length