How Many Bags of Concrete Do I Need?
An 80 lb bag yields about 0.60 cu ft; you need roughly 45 bags per cubic yard before waste. Divide your project volume in cubic feet by the bag yield (about 0.30 / 0.45 / 0.60 for 40 / 60 / 80 lb), then round up. For pours larger than about one yard, ready-mix is usually more practical.
Bag yield table
YardTally uses the same approximate yields published for many QUIKRETE and Sakrete general-purpose mixes. Specialty high-strength or lightweight products can differ—read the label.
| Bag | Yield (approx.) | Bags / cu yd |
|---|---|---|
| 40 lb | 0.30 cu ft | ~90 |
| 60 lb | 0.45 cu ft | ~60 |
| 80 lb | 0.60 cu ft | ~45 |
How it's calculated
Bags = ceil(volume_cu_ft ÷ yield). Volume depends on shape:
- Slab / footing: length × width × (thickness_inches ÷ 12)
- Post hole / Sonotube: π × r² × height × count
Divide cubic feet by 27 for cubic yards. Add 5–10% waste for uneven subgrade and spillage—Sakrete’s calculator guidance commonly reminds users to add about 10% for safety. Convert after waste: for example, 27 cu ft × 1.10 = 29.7 cu ft → ceil(29.7 ÷ 0.60) = 50 bags of 80 lb mix for one yard with 10% waste (versus ~45 bags with zero waste).
Pick the right tool: concrete calculator for slabs and steps, post hole calculator for fence posts, and Sonotube calculator for cardboard forms.
Weight also matters for logistics. Forty-five 80 lb bags are about 3,600 lb of dry mix before water—roughly the payload of a small trailer trip or several SUV loads. Sixty-pound bags trade more bag count for easier lifting. If you are working alone, factor mixing time: even an efficient helper rarely places a full yard of bagged concrete as fast as a chute. That labor math is why the “about one cubic yard” ready-mix threshold is a rule of thumb, not a hard industry law.
When comparing store prices, normalize to cost per cubic foot of yield, not cost per bag. An 80 lb bag at 0.60 cu ft and a 60 lb bag at 0.45 cu ft are different products; the cheaper bag on the shelf is not always the cheaper cubic foot in the form.
Write the yield on your shopping list before you leave home. If the store only stocks a high-strength mix with a different coverage note, re-run the division rather than forcing the 0.60 cu ft assumption.
Worked example: 10×10×4" slab
10 × 10 × (4÷12) = 33.33 cu ft ≈ 1.23 cu yd. With 10% waste → 36.7 cu ft. That is ceil(36.7 ÷ 0.60) = 62 bags of 80 lb or ceil(36.7 ÷ 0.45) = 82 bags of 60 lb. See the dedicated 10×10 slab guide for thickness charts and patio sizes.
Worked example: eight Sonotube piers
Eight 12-inch-diameter tubes, 4 ft tall: one tube is π × 0.5² × 4 ≈ 3.14 cu ft. Eight tubes ≈ 25.1 cu ft before waste. With 5% waste (typical for formed cylinders) ≈ 26.4 cu ft → about 44 bags of 80 lb mix. That is near one cubic yard—worth pricing ready-mix if the truck can reach the pour.
Worked example: fence posts vs slab bags
People often search “how many bags of concrete” when they mean fence posts. A single 8" × 2 ft hole (~0.70 cu ft) needs a couple of 80 lb bags after rounding—not the sixty-plus bags of a patio. Always identify the shape first. The fence post guide and post-hole calculator keep cylinder jobs from inheriting slab charts.
Tips
- Buy all bags from the same product line so color and strength stay consistent on visible slabs.
- Stage water, a mixing tub or mixer, and wheelbarrow routes before you open bags—cement waits for no one in hot weather.
- For fence posts, jump to the fence post concrete guide so you do not apply slab thickness math to round holes.
- Cold weather and hot weather change set times; follow the bag’s temperature notes.
- Ready-mix minimum loads vary by plant—call before you assume a short load is available.
Common mistakes
- Mixing inches and feet (treating a 4-inch slab as 4 feet thick).
- Using 45 bags per yard for an 80 lb mix, then forgetting waste.
- Ordering exact yardage on rough soil with no contingency.
- Applying slab bag charts to post holes or Sonotubes.
- Ignoring that gravel base under a slab is a separate material order.
Choosing a mix and mixing method
Bag yield is only useful if the product matches the job. General-purpose concrete mix suits slabs, steps, and footings. High-early-strength and fast-setting products shorten wait times but may have a shorter working window. Mortar and sand mixes are different products and should not be substituted for concrete. Always confirm the label says the product is intended for your application, and read the stated yield and water quantity, because those numbers can differ between brands and between bag sizes.
For bagged jobs of a few bags, a wheelbarrow or mixing tub and a hoe are common. Above about a dozen bags, many homeowners rent a mixer or price delivered ready-mix, because consistent water-to-mix ratios are hard to maintain by hand over dozens of batches. Too much water makes concrete easier to place but weaker and more prone to cracking, so measure water per bag rather than judging by eye.
Curing and weather
Placing the concrete is half the job. Many residential references recommend keeping fresh concrete moist for several days after finishing, often about a week, so it can gain strength; the bag and the ACI residential guidance linked below describe curing practices. Hot, dry, or windy days dry the surface too fast, and freezing temperatures can damage concrete that has not yet set. If the forecast is poor, move the pour date rather than gambling with a full order of bags you cannot return once they are opened.
When to call a pro / safety / codes
Call a concrete contractor for structural footings, large driveways, stamped or colored finishes, or sites with poor access. Cement products are alkaline—wear gloves, eye protection, and long sleeves; wash skin promptly. Reinforcing steel, control-joint spacing, and thickness for vehicle loads are code- and engineer-driven. YardTally estimates volume and bags only; it does not replace ACI guidance or a local building permit review. For residential concrete overview documents, see ACI resources linked below.
Sources
- QUIKRETE calculator & yields
- Sakrete calculators (includes ~10% safety note)
- ACI 332.1R-18 preview – residential concrete
- American Cement Association
FAQ
How many 80 lb bags of concrete are in a cubic yard?
About 45 bags before waste, using a typical 0.60 cu ft yield per 80 lb bag (27 ÷ 0.60). Always confirm the yield printed on the bag you buy—lightweight and specialty mixes can differ.
How many 60 lb bags per cubic yard?
About 60 bags before waste at a typical 0.45 cu ft yield (27 ÷ 0.45). After a 10% waste factor, plan closer to 66 bags per cubic yard of placed concrete.
When should I order ready-mix instead of bags?
As a practical rule of thumb, when the pour approaches or exceeds about one cubic yard, ready-mix often wins on labor and consistency if a truck can access the site. Smaller pads, repairs, and fence posts usually stay on bags.
Do bag yields include waste?
No. Manufacturer yields assume you place that volume of mixed concrete. Uneven subgrade, form bulge, and spillage routinely add 5–10%. Round up after you divide volume by bag yield.
Which calculator should I use for bags?
Use the slab/footing concrete calculator for rectangles, the post hole calculator for fence posts, and the Sonotube calculator for cardboard forms. Each shape needs a different volume formula before you convert to bags.
Related calculators
- Concrete Calculator — Slabs, footings, and solid steps with cubic yards and bag counts.
- Concrete Calculator for Post Holes — Fence and sign post hole concrete by diameter, depth, and count.
- Sonotube Concrete Calculator — Round column and pier forms by diameter and height.