Construction · Materials
Concrete Bag CalculatorHow many bags do you need? Choose your bag size, enter your dimensions, get your count.
Example — 10 × 10 ft slab, 4 in
1.23
cubic yards
A standard 10 × 10 ft slab at 4 inches needs about 57 × 80 lb bags. Add a 5% waste buffer and you should buy 60 — roughly 3 pallets.
How to calculate how many bags of concrete you need
The calculation has two stages. First you work out the volume of your pour in cubic yards. Then you divide that volume by the yield of one bag to get your bag count.
Step 1 — volume
Cubic yards = (Length ft × Width ft × Thickness in ÷ 12) ÷ 27
Step 2 — bag count
Bags = (Volume × waste factor) ÷ bag yield (cu yd)
The waste factor accounts for spillage, slightly uneven ground, and the concrete that gets left in the mixer or on tools. The default is 5% — enough for a well-prepared job. Increase it to 10% if the sub-base is rough or you're working alone.
Worked example: 12 × 10 ft driveway apron at 4 inches
A driveway apron or garage approach is one of the most common small concrete jobs homeowners tackle with bags. Here's how to work through the numbers for a 12 ft × 10 ft area at the standard 4-inch residential thickness:
Convert thickness to feet
4 in ÷ 12 = 0.333 ft
Calculate cubic feet
12 × 10 × 0.333 = 40 cu ft
Convert to cubic yards
40 ÷ 27 = 1.48 cu yd
Add 5% waste
1.48 × 1.05 = 1.55 cu yd
80 lb bag count
1.55 ÷ 0.022 = 71 bags
So you need 71 × 80 lb bags for this job — roughly 1.5–2 pallets depending on the store. That's a manageable hand-mix, though at this size a rented electric mixer saves significant time and produces a more consistent result.
40 lb vs 60 lb vs 80 lb bags — which should you use?
All three bag sizes contain the same mix — the difference is just how much is in each bag. Heavier bags mean fewer trips and less tearing open packaging, but they're harder to lift, especially after a few hours on site. Here's how the yields compare:
40 lb
≈ 0.011 cu yd
Best for confined spaces or when working alone. Easier to handle — you'll need roughly twice as many as 80 lb bags.
60 lb
≈ 0.017 cu yd
A good middle ground. Manageable to lift and reduces the number of bags by about 25% compared to 40 lb.
80 lb
≈ 0.022 cu yd
The most common choice for driveways and slabs. Lowest cost per yard, fewest bags to buy — but weigh around 36 kg each.
Bag yields above are standard manufacturer averages (Quikrete, Sakrete). Always check the label of the specific product you buy — some specialty mixes like fast-setting or crack-resistant concrete have slightly different yields.
How much do bags of concrete cost?
Retail prices vary by brand, store, and region, but these are typical 2025–2026 ranges at big-box home improvement stores:
40 lb bag
$4–6
~$365–545/cu yd
60 lb bag
$5–7
~$295–410/cu yd
80 lb bag
$6–9
~$275–410/cu yd
At roughly $275–545 per equivalent cubic yard, bagged concrete is significantly more expensive than ready-mix ($120–200/cu yd delivered). For anything over 1.5–2 cubic yards, getting a truck is almost always cheaper — even after the delivery fee. The calculator's cost estimate uses a $150/cu yd default; adjust it in the "Adjust waste & price" panel if your local rate is different.
Common mistakes when buying concrete bags
Forgetting to convert inches to feet
The formula needs all dimensions in the same unit. A 4-inch slab is 0.333 feet — not 4 feet. Entering the thickness in inches without dividing by 12 gives a result 3× too high.
Skipping the waste buffer
Buying exactly what the formula says almost always leaves you short. Concrete is lost during mixing, sticks to tools, and fills gaps in the sub-base. A 5–10% buffer costs little but avoids a costly second trip.
Buying the wrong bag size
80 lb bags yield the best value per yard, but if you're mixing by hand for hours the physical toll adds up. 60 lb bags are a practical compromise for solo jobs over 1 cubic yard.
Not checking bag yield on the label
Specialty mixes — rapid-setting, fiber-reinforced, high-strength — often yield slightly less per bag than standard mix. Always confirm the yield printed on the bag before calculating.
When to use this calculator
This tool is best suited for rectangular pours where you plan to use bagged concrete. Common uses:
Driveway aprons
Small approach slabs. Typically 4–6 in thick. 1–3 cu yd range is ideal for bags.
Patio sections
Broken into smaller pours you can mix over a weekend. 3–4 in standard.
Fence posts
Each post uses a fraction of a bag. Use the calculator per post for a precise bag count.
Shed slabs
A 10 × 12 ft shed base at 4 in needs about 1.8 cu yd — roughly 82 × 80 lb bags.
Steps & landings
Irregular shapes may need a structural engineer, but rectangular steps fit this calc well.
Small footings
Poured footings for decks, pergolas, or garden walls are ideal bag jobs.
For pours above 2 cubic yards, the effort and cost of mixing bags makes ready-mix worth considering. At that point, the concrete calculator gives you the volume to quote with a supplier.
Frequently asked questions
How many 80 lb bags of concrete do I need per cubic yard?
An 80 lb bag covers approximately 0.022 cubic yards, so you need about 45–46 bags per yard of plain volume. With a 5% waste buffer that becomes 48 bags.
How do I convert inches to feet for the thickness?
Divide the inch value by 12. A 4-inch slab is 4 ÷ 12 = 0.333 feet. A 6-inch slab is 0.5 feet. The calculator does this conversion automatically — just enter the thickness in inches.
What is the difference between 40 lb, 60 lb, and 80 lb bags?
Only the amount of mix in each bag. A 40 lb bag yields about 0.011 cu yd, a 60 lb bag 0.017 cu yd, and an 80 lb bag 0.022 cu yd. Heavier bags cost less per yard but are harder to handle over long jobs.
Why does this calculator use a different yield than another tool I found?
Bag yields vary slightly by manufacturer and mix type. This calculator uses standard Quikrete/Sakrete averages. If your specific product has a different yield, use the volume output and divide manually.
When should I use bags instead of ordering ready-mix?
Bags make sense for pours under 1.5–2 cubic yards, for jobs where access is tight, or where you want to work in stages. Above that threshold, a ready-mix truck is usually cheaper and faster.
Why add a waste factor if I'm using exact measurements?
Even with perfect measurements, some concrete is always lost — it sticks to mixer blades, fills small voids in the sub-base, and spills during placing. A 5% buffer is minimal insurance against running short.
Disclaimer
This calculator provides estimates only and should be used for planning purposes. Actual quantities and costs may vary based on ground conditions, form accuracy, material yield, and local pricing. Always verify your order with the supplier before purchasing.