βοΈ Project Type
π Slab Dimensions
π Standard residential slab thickness: 4 inches. Driveways: 4β6 inches. Garage floors: 4 inches minimum. Sidewalks: 4 inches. Heavy equipment pads: 6+ inches.
βοΈ Bag Options
Bags to Buy
βΓ 80 lb bags
Cubic Feet
β
Cubic Yards
β
Est. Bag Cost
β
Ready-Mix Cost*
β
*if available in your area
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How Many Bags of Concrete Do You Actually Need?
The formula is straightforward: calculate the volume of your project in cubic feet, then divide by the coverage per bag. A standard 80 lb bag of concrete covers roughly 0.60 cubic feet of finished concrete.
// Volume formula
Slab: Length (ft) Γ Width (ft) Γ Thickness (ft)
Post hole: Ο Γ radiusΒ² Γ depth
Bags = (cubic feet Γ· cu ft per bag) Γ waste factor
// Always round UP to the nearest whole bag
Slab: Length (ft) Γ Width (ft) Γ Thickness (ft)
Post hole: Ο Γ radiusΒ² Γ depth
Bags = (cubic feet Γ· cu ft per bag) Γ waste factor
// Always round UP to the nearest whole bag
Bag Coverage at a Glance
| Bag Size | Cubic Feet | Cost (approx) | Best For |
|---|---|---|---|
| 80 lb | 0.60 cu ft | ~$6.50 | Most projects β best value per cu ft |
| 60 lb | 0.45 cu ft | ~$5.50 | Smaller work or when weight is a concern |
| 40 lb | 0.30 cu ft | ~$4.00 | Small post holes, repairs, one person mixing |
When to order ready-mix instead of bags
If your project exceeds 1 cubic yard (~45 bags of 80 lb), the math changes fast. Ready-mix concrete costs around $150β200 per cubic yard delivered, versus $6.50+ per 80 lb bag β above 1β1.5 cubic yards, ready-mix is almost always cheaper and far less labor-intensive. Ask your local batch plant about minimum order size (usually 1 yard).
Concrete Mixing Tips
- Water ratio matters most. An 80 lb bag needs 3 quarts of water. Too much water weakens the finished concrete β it won't gain its rated PSI strength.
- Mix in a wheelbarrow for small jobs. Use a rented mixer for anything over 10 bags at once.
- Work in the right temperature. Don't pour when temperatures are below 40Β°F or above 90Β°F without special additives. Cold concrete won't cure properly; hot concrete sets too fast.
- Let it cure, not just dry. Concrete reaches ~70% strength in 7 days but full cure takes 28 days. Don't load a slab for at least 7 days.
- Dampen the subgrade before pouring. Dry soil sucks moisture out of your mix and weakens the pour.
Common Project Estimates
| Project | Dimensions | Cu Ft | 80 lb Bags |
|---|---|---|---|
| Stepping stone (2") | 2Γ2 ft Γ 2 in | 0.67 | 2 bags |
| Post hole (8" dia, 3 ft deep) | β | 1.05 | 2 bags per hole |
| Sidewalk section (4" thick) | 4Γ4 ft | 5.3 | 9 bags |
| Small patio slab (4" thick) | 8Γ10 ft | 26.7 | 45 bags |
| Driveway apron (6" thick) | 10Γ12 ft | 60 | 100 bags β consider ready-mix |
| 10Γ10 shed pad (4" thick) | 10Γ10 ft | 33.3 | 56 bags β near ready-mix threshold |
Frequently Asked Questions
How many 80 lb bags of concrete for a 10Γ10 slab?
A 10Γ10 foot slab at 4 inches thick = 33.3 cubic feet. An 80 lb bag covers 0.60 cu ft, so you'd need 33.3 Γ· 0.60 = 55.6, rounded up to 56 bags. Add 10% for waste: buy 62 bags. At this size, you're near the threshold where ready-mix concrete becomes cost-competitive and much less labor-intensive.
How deep should fence post holes be?
The general rule is 1/3 of the total post length. A 9-foot post should be buried 3 feet deep. In areas with hard frost, go at least 6 inches below the local frost line. This calculator automatically applies that logic when you enter your fence post dimensions.
What's the difference between Quikrete and Sakrete?
Both are reliable pre-mixed concrete products. Quikrete is slightly more widely distributed and comes in more formulations (including fast-setting varieties). Sakrete is also a quality product. For most DIY projects, coverage rates are nearly identical β the brand rarely matters. Fast-setting concrete (like Quikrete Fast-Setting) is useful for fence posts as it can set in 20β40 minutes without mixing.
Can I use fast-setting concrete for slabs?
Fast-setting concrete is best for post holes and repair work, not large slabs. For slabs, standard concrete gives you more working time to screed and finish the surface before it sets. Fast-set can begin hardening in 20β30 minutes, which is too quick for slab work.