How Many Bags of Concrete Do I Need? (60 lb vs 80 lb Bags)

Walk into any Home Depot and you'll see the wall of concrete bags — 40 lb, 60 lb, 80 lb. The question everyone stands there asking: how many do I actually need? Buy too few and your project stalls mid-pour. Buy too many and you've got $150 of dead weight in the garage.

Here's the math, no guessing.

The bag formula

Bags = Total cubic feet ÷ cubic feet per bag

Each bag size yields a fixed amount of mixed concrete:

| Bag size | Yields | Bags per cubic foot | |---|---|---| | 40 lb | 0.30 cu ft | ~3.3 bags | | 60 lb | 0.45 cu ft | ~2.2 bags | | 80 lb | 0.60 cu ft | ~1.7 bags |

Always buy 80 lb bags if you can lift them. They're the cheapest per cubic foot by a mile — usually 30-40% cheaper than buying the equivalent in 60 lb bags. Your back will complain, your wallet won't.

Real examples

Example 1: 4×4 ft pad, 4 inches thick (shed base, AC unit)

1. Cubic feet: 4 × 4 × (4/12) = 5.33 cu ft
2. 80 lb bags: 5.33 ÷ 0.6 = 8.9 → 9 bags
3. With 10% waste: 10 bags

Example 2: Fence posts — 6 posts, 10" diameter holes, 2 ft deep

1. Per hole: π × (5/12)² × 2 = 1.09 cu ft
2. 6 holes: 6.5 cu ft
3. 80 lb bags: 6.5 ÷ 0.6 = 10.8 → 11 bags
4. Round to 12 (holes are never perfect cylinders)

Example 3: 10×10 ft patio, 4 inches thick

1. Cubic feet: 10 × 10 × (4/12) = 33.3 cu ft
2. 80 lb bags: 33.3 ÷ 0.6 = 55.5 → 56 bags
3. 🛑 Stop. At 56 bags, call the ready-mix plant. Mixing 56 bags by hand will destroy your weekend and your mixer.

The bags-vs-truck line

Here's my honest rule after doing both:

• Under 20 bags: Bags are fine. Rent a mixer ($50/day), invite a friend, knock it out.
• 20–40 bags: Grey zone. Doable but miserable. Get quotes for a short load — sometimes it's barely more expensive.
• Over 40 bags: Truck. No debate. I mixed 45 bags once for a walkway. Never again. My shoulders still remember.

Don't forget the short-load fee — most plants charge extra under 3-4 yards. Even with the fee, it's usually cheaper than 50+ bags once you factor in your time and the mixer rental.

Mixing tips that actually matter

• Water first, then powder. Pour about 3/4 of the water in the mixer, add the bag, then adjust. Way less dust, way better mix.
• Don't eyeball water. Too wet = weak concrete (it'll crack and crumble in a year). Follow the bag — usually about 1 gallon per 80 lb bag.
• Work fast in heat. Above 85°F, you've got maybe 45 minutes before it starts setting. Mix small batches.
• Cold weather: below 40°F, concrete basically stops curing. Cover with blankets or wait for a warmer day.

Quick cheat sheet (80 lb bags, 4" thick)

| Project | Cubic feet | Bags (+10%) | |---|---|---| | 4×4 ft pad | 5.3 | 10 | | 8×8 ft pad | 21.3 | 40 (truck?) | | 10×10 ft patio | 33.3 | 61 (truck!) | | 6 fence post holes | 6.5 | 12 | | 10×3 ft walkway | 10 | 19 |

Skip the arithmetic — our free concrete calculator does cubic yards AND bag counts instantly.