How to Use the Concrete Slab & Post Hole Calculator
Step 1: How to measure
For a slab, measure the inside of your forms in feet and decide on thickness. Standard thickness is 4 inches for patios, sidewalks and shed floors, 5–6 inches for driveways and garage slabs that carry vehicles. Check the grade with a string line — low spots add volume quickly, and a slab that is 1 inch deeper than planned uses 25% more concrete.
For post holes, measure the hole diameter (a rule of thumb is three times the post width, so a 4×4 post gets a 10–12 inch hole) and the depth. Building codes usually require holes to extend below the frost line — often 36 to 48 inches in northern states. The calculator ignores the volume the post itself displaces, which works as a built-in safety margin.
Step 2: The exact formula
Slab volume is length × width × (thickness ÷ 12) in cubic feet. Post hole volume is π × radius² × depth, multiplied by the number of holes. Divide cubic feet by 27 to get cubic yards — the unit ready-mix trucks deliver.
Bagged concrete yields roughly 0.30 cu ft per 40 lb bag, 0.45 cu ft per 60 lb bag and 0.60 cu ft per 80 lb bag. Most suppliers have a 1 yard minimum, and ready-mix generally beats bags on price and labor above about 1 cubic yard (45 × 80 lb bags).
Slab ft³ = L × W × (T in ÷ 12) Post ft³ = π × (D in ÷ 24)² × (Depth in ÷ 12) × posts Yd³ = ft³ ÷ 27 · 80 lb bags = ft³ ÷ 0.60
Pro DIY tips
- Compact 4 inches of gravel under slabs for drainage and to prevent cracking from settling.
- Add rebar or wire mesh to slabs and cut control joints every 8–10 feet at 1/4 of slab depth.
- Pour on a mild, overcast day; keep the slab damp for 7 days so it cures instead of drying out.
- For fence posts, put 4–6 inches of gravel in the hole bottom so water drains away from the post.
- Order ready-mix with 10% extra — running short mid-pour creates a weak cold joint.
Frequently asked questions
How many bags of concrete for a 10×10 slab?
A 10×10 ft slab at 4 inches needs about 1.36 cubic yards with 10% waste — roughly 62 × 80 lb bags. Ordering ready-mix is usually easier at this size.
How much concrete per fence post?
A 10 in diameter hole 36 in deep holds about 1.6 cu ft, or about three 60 lb bags (incl. waste).
How many 80 lb bags make a yard of concrete?
About 45 bags of 80 lb concrete make one cubic yard (27 cu ft ÷ 0.6 cu ft per bag).
How thick should a concrete slab be?
4 inches for patios, walkways and sheds; 5–6 inches for driveways, garages and anything supporting heavy loads.
Is it cheaper to mix bags or order ready-mix?
For under 1 cubic yard, bags are cheaper. Above that, ready-mix typically costs less per yard and saves hours of mixing.