Footings & piers
Concrete Calculator & Deck Footing Calculator
This concrete calculator works out exactly how much concrete a set of round deck footings needs — cubic feet, cubic yards, 60 or 80 lb bag counts and total cost. Every number shows the formula behind it, so it can go straight into a quote.
How to calculate concrete, step by step
1. Step 1 — Measure the hole
Concrete footings are cylinders. Measure the hole diameter in inches (a 12 in tube is the most common for deck posts) and the depth in feet, usually to your local frost line.
2. Step 2 — Convert to volume
Volume per hole = π × radius² × depth, where radius is half the diameter converted to feet. A 12 in × 3 ft hole is π × 0.5² × 3 = 2.36 cubic feet.
3. Step 3 — Multiply by hole count
Total cubic feet = volume per hole × number of footings. Divide by 27 to get cubic yards if you are ordering ready-mix instead of bags.
4. Step 4 — Convert to bags
A 60 lb bag yields about 0.45 cu ft and an 80 lb bag about 0.60 cu ft. Bags = total cubic feet ÷ bag yield, always rounded up — you cannot buy a partial bag.
5. Step 5 — Price it
Cost = bags × your price per bag. Use your real supplier price so the number can go straight onto an estimate.
Bag yield reference
| Bag size | Yield | Bags per cubic yard |
|---|---|---|
| 60 lb | 0.45 cu ft | 60 |
| 80 lb | 0.6 cu ft | 45 |
Concrete calculator questions
How much concrete do I need for a deck footing?
A standard 12 inch diameter footing poured 3 feet deep takes about 2.36 cubic feet of concrete, which is 6 bags at 60 lb or 4 bags at 80 lb. Multiply that by your number of footings and round bags up.
How many 60 lb bags of concrete are in a cubic yard?
A cubic yard is 27 cubic feet. At roughly 0.45 cubic feet per 60 lb bag that is 60 bags per yard, or 45 bags per yard using 80 lb bags at 0.60 cubic feet each.
Should I use bags or order ready-mix?
Bags are practical up to about 1 cubic yard — roughly 60 bags. Past that, mixing by hand costs more in labor than a short-load ready-mix delivery, so use the cubic yard figure below when you call the plant.
How deep should deck footings be?
Deep enough to sit below your local frost line, which ranges from 12 inches in warm climates to 48 inches or more in northern states. Check your local building department before pouring — the depth drives both your concrete volume and your inspection.
Does this calculator add waste?
No, it returns the exact geometric volume and rounds bags up. Most crews add one extra bag per five holes for spillage and slightly oversized holes.
A one-page checklist so you never forget footing depth, waste factor, or permit costs on a deck quote.
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