The dimensions used
| Run | 40 ft |
|---|---|
| Width | 16 in |
| Depth | 8 in |
Footing
Concrete for a 40 ft footing, 16×8 in
A 40 ft run of 16 in × 8 in strip footing. The volume is simply the trench cross-section times the run; here it is worked out with the bag counts.
Concrete needed
1.32cubic yards· 1.007 m³· 35.56 ft³
| Bag size | Bags needed |
|---|---|
| 40 lb | 119 |
| 60 lb | 80 |
| 80 lb | 60 |
| 25 kg | 87 |
| 30 kg | 72 |
Planning estimate, with no waste allowance included. Add 5–10% for spillage and over-excavation, and check the yield printed on your bag. Structural work must follow your local building code and an engineer's specification.
How it is worked out
A strip footing is a long rectangular prism under a wall; a spread footing is a pad under a column. Either way the volume is length × width × depth. Enter the total run of the footing as the length, and the trench cross-section as width × depth.
That volume — 35.56 ft³ — is divided by 27 to give 1.32 cubic yards, and by each bag's yield to give the bag count. An 80 lb bag of premix yields about 0.60 cubic feet, so this job takes 60 of them; bag counts always round up, because you cannot buy part of a bag.
Ordering ready-mix instead? Suppliers sell by the cubic yard (or cubic metre) and usually have a minimum load and a short-load surcharge — worth a call before you choose between bags and a truck.
Formwork
What is being measured
Nearby
More footing examples
Concrete for a 10 ft footing, 16×8 in0.33 yd³Concrete for a 10 ft footing, 24×12 in0.74 yd³Concrete for a 15 ft footing, 16×8 in0.49 yd³Concrete for a 15 ft footing, 24×12 in1.11 yd³Concrete for a 20 ft footing, 16×8 in0.66 yd³Concrete for a 20 ft footing, 24×12 in1.48 yd³Concrete for a 24 ft footing, 16×8 in0.79 yd³Concrete for a 24 ft footing, 24×12 in1.78 yd³