How Many Bags of Concrete Do I Need?
Two numbers drive every bag calculation: the volume you need, and what each bag actually yields. The yields are smaller than most people guess.
This article gives you the math, by bag size, for the projects people actually buy bags for: small slabs, fence posts, and pads. It also addresses the questions that send people back to Google a second time. Why does a calculator say 56 bags but the contractor next door says "eh, get 60." Whether dry-pouring fence posts is fine or a YouTube myth. At what point you should stop counting bags and call the ready-mix plant. Want the math done for you? The bags to yards calculator handles every bag size.
Bag Yields, by Size
These are the manufacturer-published yields and they are consistent across the major brands (Quikrete, Sakrete, Bonsal) for standard concrete mix:
| Bag Size | Yield (cu ft) | Yield (cu yd) | Bags per Cubic Yard |
|---|---|---|---|
| 40 lb | 0.30 | 0.011 | 90 |
| 50 lb | 0.375 | 0.014 | 72 |
| 60 lb | 0.45 | 0.017 | 60 |
| 80 lb | 0.60 | 0.022 | 45 |
| 90 lb | 0.675 | 0.025 | 40 |
These yields are for standard concrete mix (Portland cement, sand, gravel). Specialty products (fast-setting, high-early-strength, crack-resistant fiber-reinforced) yield approximately the same volume because the aggregate proportions are similar. Yields are not the same for mortar mix, sand mix, or countertop mix. Those are different products with different math.
The "weight does not equal cost-efficiency" insight
A 60-pound bag does not yield 75 percent of an 80-pound bag's volume. It yields exactly 75 percent (0.45 / 0.60), so on a per-pound basis they are identical in cost-efficiency. Choose between 60-pound and 80-pound bags based on your back and your wheelbarrow, not your wallet.
How Many Bags for a Slab
Walk through the math once and the table makes sense. Example: a 10 x 10 ft pad at 4 inches thick.
10 × 10 × 0.333 = 33.3 cu ft
33.3 / 0.60 = 55.5 bags of 80 lb
Round up + 10% waste: 62 bags of 80 lb
In 60-lb terms: 33.3 / 0.45 = 74 bags + waste = 82 bags of 60 lb
Reference table for common slab sizes at 4 inches thick (waste already included):
| Slab Size | Cubic Feet | 60 lb Bags | 80 lb Bags |
|---|---|---|---|
| 4 x 4 pad | 5.3 | 13 | 10 |
| 6 x 6 pad | 12.0 | 29 | 22 |
| 8 x 8 pad | 21.3 | 52 | 39 |
| 8 x 10 pad | 26.6 | 65 | 49 |
| 10 x 10 pad | 33.3 | 82 | 62 |
| 10 x 12 pad | 40.0 | 98 | 73 |
| 12 x 12 pad | 48.0 | 117 | 88 |
| 4 x 20 walkway | 26.6 | 65 | 49 |
| 4 x 40 walkway | 53.3 | 130 | 98 |
Anything 80 sq ft or larger at 4 inches deep (about one cubic yard) is at the edge of the bag-versus-truck break-even. Once you are mixing more than 50 bags by hand, you are signing up for a long day. For project costs across these sizes, see the concrete slab cost guide.
How Many Bags for a Fence Post
Most articles give one number. The truth is it depends on three things: post size, hole diameter, and hole depth. Standard practice (per Quikrete and most fence contractors):
- - Hole diameter equals roughly 3 times the post width
- - Hole depth equals 1/3 to 1/2 the above-ground post height, and at minimum below the local frost line (see frost line depth by zip code)
Bags per post (50-pound fast-set, the most common size for posts):
| Post Size | Hole | 6 ft Above Ground | 8 ft Above Ground |
|---|---|---|---|
| 4 x 4 | 10-12 in x 24-30 in | 1-2 bags | 2-3 bags |
| 6 x 6 | 16-18 in x 30-36 in | 3-4 bags | 4-6 bags |
| Round 4 in | 12 in x 24-30 in | 1-2 bags | 2-3 bags |
| Round 6 in | 18 in x 30-36 in | 3-4 bags | 4-6 bags |
The dry-pour question
Quikrete officially endorses dry-pouring for fast-set concrete: drop dry mix in the hole, pour about a gallon of water per 50-pound bag on top, let it saturate. Plenty of contractors and concrete subreddits push back hard, citing uneven hydration and weak interior cure.
Honest answer: dry-pouring fast-set works for non-load-bearing fence posts in most soils, especially when you follow the manufacturer's water-on-top procedure. For gate posts, corner posts, or anything taking lateral load, mix wet. Do not dry-pour standard (non-fast-set) concrete in a post hole. Different formulation, the technique does not work the same way.
Should you subtract the volume of the post itself?
For a 4x4 in a 10 to 12 inch hole, the post displaces only about 0.21 cubic feet over 30 inches. Subtracting changes the answer by less than half a bag. Skip it. For sonotube piers or 6x6 posts in larger holes, do subtract, because you will be off by half a bag or more. The post hole calculator handles both modes. For the full bag-by-post chart, see bags of concrete for post holes.
When Bags Stop Making Sense
The honest, numeric break-even:
- Under 0.5 cubic yard: bags, every time. No short-load fee, no minimum, no truck access concerns.
- 0.5 to 1.0 cubic yard: judgment call. Bags cost a little more in material but no delivery fee, no minimum charge, and you can mix at your pace. A truck saves 6 to 10 hours of labor.
- 1.0 to 1.5 cubic yards: the genuine break-even. At about $5 per 80-pound bag (45 bags per yard), one yard of bags costs roughly $225 in material. A short-load yard delivered with fees runs $200 to $300. You are trading $50 in savings for 8 hours of mixing.
- Above 1.5 cubic yards: ready-mix wins on cost, on consistency, and on your back. About 67 bags is the realistic upper limit of a one-day DIY hand-mix. Beyond that you are in trouble.
Hand-mix reality check: each 80-pound bag takes 5 to 7 minutes to mix properly in a wheelbarrow. Forty-five bags equals 4 to 5 hours of mixing alone, before you have placed a single trowel-full. That is why crews with mortar mixers stop at about one yard and crews without one stop sooner. For the full ready-mix ordering process, see how to estimate concrete yards.
Mixing the Bags: What Actually Weakens Concrete
Three rules, in order of how much damage they do:
1. Water ratio
Manufacturer-printed: about 6 pints (3/4 gallon) per 80-pound bag, 4 pints per 60-pound bag. More water makes mixing easier. It also drops compressive strength by roughly 200 PSI per extra gallon per yard. A 4,000 PSI mix can become 3,000 PSI on the back of one wheelbarrow that got a little extra water. The rule on jobsites: workability comes from technique, not water.
2. Mix consistency across batches
If you mix dry on Monday and wet on Tuesday, you have two slabs glued together. For continuous pours, mix the same way every batch: same water, same mix time, same dump pattern.
3. Hot and cold weather
Below 40°F, bag mix needs cold-weather protection (blankets, accelerator) just like ready-mix. Above 90°F, expect 30 to 40 percent less working time before initial set. Pre-wet the subgrade and shade the bags before they go in. For the standards, ACI 305 covers hot-weather concreting, ACI 306 covers cold-weather.
Specialty Bags: When to Pay for Them
- Fast-setting concrete (Quikrete Q-MAX, Sakrete Fast-Setting): sets in 20 to 40 minutes. Fence posts, mailbox posts, anything where you do not want to brace overnight. Costs about 30 percent more.
- High-early-strength (Quikrete 5000, Sakrete High-Strength): walk on it in 10 to 14 hours, drive on it in 3 days. Worth it for repair patches in driveways or commercial floors.
- Crack-resistant or fiber-reinforced: micro-fibers reduce surface cracking on slabs without rebar. Patios and shed pads benefit. Not a structural substitute for rebar, see the rebar spacing guide.
- Counter-top mix: completely different product. Not interchangeable with standard concrete mix. Do not substitute.
Sanity-Check Rules of Thumb
The numbers crews actually carry in their heads:
- - 45-60-90. 45 bags of 80-pound, 60 bags of 60-pound, 90 bags of 40-pound = 1 cubic yard.
- - 1.66 to a foot. 1.66 bags of 80-pound makes one cubic foot.
- - The 0.6 / 0.45 / 0.3 yields. Tape these to the inside of the truck door if you do this often.
- - One pallet of 80-pound bags. 42 bags = 25.2 cubic feet = 0.93 cubic yard. Round up to the next pallet rather than hand-counting.
The Bottom Line
Round up. Buy one extra bag for every ten you have calculated. Open bags become driveway repair material six months from now, and the home center will take unopened ones back. The actual mistakes that hurt are buying the wrong product (mortar mix when you needed fast-set), adding water to make it easier to mix (you just dropped the strength), and trying to hand-mix more concrete in a day than your back can move.
Use the bags to yards calculator to convert any volume to a bag count, and the slab or post hole calculator for the underlying volume.