How many 60/80lb bags for your slab, footer, or post holes?
How to Use the Concrete Bags Calculator
Bagged concrete mix is perfect for slabs, footers, and post holes — but underbuying means a cold joint where fresh concrete meets set concrete, weakening the slab. This calculator converts your project’s dimensions to cubic feet, then to the exact number of 60 lb or 80 lb bags, with a waste buffer built in.
- Enter length and width. Measure the slab or footer area in feet (for post holes, use diameter and depth instead).
- Enter the thickness (depth). Use inches — 4 inches is standard for patios and walkways, 6 for driveways.
- Choose your bag size. 80 lb bags yield 0.6 cubic feet each; 60 lb bags yield 0.45 cubic feet.
- Calculate. The tool shows bags needed plus a 5–10% buffer recommendation so you never come up short mid-pour.
Concrete Bags Calculator Formula
The math is volume divided by yield: volume (cu ft) = length (ft) × width (ft) × (thickness in inches ÷ 12), then bags = volume ÷ yield per bag, where an 80 lb bag yields 0.6 cu ft and a 60 lb bag yields 0.45 cu ft. Always round up — you cannot buy a fraction of a bag — and then add 5–10% extra for spillage, uneven subgrade, and form bulging.
Why the buffer matters: concrete begins setting within about 90 minutes of mixing (faster in heat). If you run short, the second batch creates a cold joint — a visible seam and a structural weak plane. An extra two or three bags ($10–$15) is the cheapest insurance on the project. Any surplus can fill a small footer, a stepping stone mold, or post-hole top-ups.
For cylindrical post holes, volume is π × radius² × depth. A 12-inch-diameter hole 36 inches deep is about 2.36 cu ft — four 80 lb bags (2.4 cu ft). Professionals often pour dry mix directly into post holes and soak it, but for structural posts, mixing properly in a wheelbarrow gives stronger, more consistent results.
Bag Yields and Bags per Cubic Yard
Bagged concrete is sold by weight, but what matters is each bag’s yield — the finished concrete it produces. The calculator above covers the two standard DIY sizes; other sizes are listed for reference:
| Bag size | Yield per bag | Bags per cubic yard |
|---|---|---|
| 40 lb | 0.30 cu ft | 90 |
| 50 lb | 0.375 cu ft | 72 |
| 60 lb | 0.45 cu ft | 60 |
| 80 lb | 0.60 cu ft | 45 |
| 90 lb | 0.675 cu ft | 40 |
These are typical published yields (e.g., Quikrete No. 1101). Actual yield varies between mixes, so check the number printed on your bag before buying. For any other bag size, divide your volume in cubic feet by the printed yield — same math the calculator runs.
Bags for Common Projects
Before waste allowance, at 4 inches thick unless noted:
| Project | Volume | 80-lb bags | 60-lb bags |
|---|---|---|---|
| 4 × 8 ft walkway, 4 in | 10.7 cu ft | 18 | 24 |
| 10 × 10 ft patio, 4 in | 33.3 cu ft | 56 | 75 |
| 12 × 12 ft shed pad, 4 in | 48.0 cu ft | 80 | 107 |
| 10 × 10 ft driveway section, 6 in | 50.0 cu ft | 84 | 112 |
Notice how quickly bags multiply: a modest patio is a full pallet of mix. Remember 56 bags at 80 lb each is 4,480 lb — over 2 tons — so plan delivery or several truck trips, and add the 5–10% waste buffer the calculator applies.
Example Calculation
A 10×10-foot patio slab, 4 inches thick. Volume = 10 × 10 × (4 ÷ 12) = 100 × 0.333 ≈ 33.3 cu ft. With 80 lb bags: 33.3 ÷ 0.6 = 55.6 → 56 bags. Add 7% buffer: 56 × 1.07 ≈ 60 bags. At roughly $5 per bag, materials run about $300 — plus a mixer rental if you’re wise (hand-mixing 60 bags is brutal).
Smaller job: setting 6 fence posts in 10-inch-diameter holes, 30 inches deep. Each hole: π × (5/12)² × 2.5 ≈ 1.36 cu ft; six holes ≈ 8.2 cu ft. With 60 lb bags: 8.2 ÷ 0.45 ≈ 18.2 → 19 bags, or 20 with buffer. Two trips in a pickup, one afternoon’s work.
Frequently Asked Questions
Should I buy 60 lb or 80 lb bags?
80 lb bags are cheaper per cubic foot and mean fewer bags to haul and mix — better for slabs. 60 lb bags are kinder to your back and better for small jobs where an 80 lb bag is overkill. The concrete itself is identical.
How much extra should I order?
5–10% over the calculated amount. Uneven excavation, form flex, and spillage always consume more than the math says. Running short mid-pour creates a weak cold joint — the extra bags are cheap insurance.
How much water do I add per bag?
About 3 quarts per 80 lb bag for a workable mix (about 2½ quarts per 60 lb bag) — but add gradually. Too much water is the #1 DIY mistake: it weakens concrete significantly. Aim for a thick oatmeal consistency, not soup.
How long does bagged concrete take to cure?
It sets enough to walk on in 24–48 hours, but full strength takes 28 days. Keep it moist (mist or cover with plastic) for the first week — proper curing can nearly double final strength versus concrete left to dry out.
When should I order ready-mix instead of bags?
Around 1 cubic yard (27 cu ft, ~45 eighty-pound bags) is the crossover. Beyond that, a ready-mix truck is cheaper per yard, far less labor, and gives a single monolithic pour with no cold joints. Get quotes at 30+ bags.
How many bags of concrete are in a cubic yard?
It depends on bag size: 45 bags of 80-lb concrete, 60 bags of 60-lb, 72 bags of 50-lb, or 90 bags of 40-lb make one cubic yard (27 cubic feet). That is why a truckload — a typical mixer holds about 8 yards, roughly 360 eighty-pound bags — is cheaper and easier once your pour approaches a full yard.
How many bags do I need for a 10×10 slab?
A 10 × 10 foot slab at 4 inches thick is 33.3 cubic feet, which needs 56 bags of 80-lb concrete (or 75 bags of 60-lb) before waste. With the standard 10% waste buffer, plan on about 62 bags of 80-lb. Driveways and garage floors should be 5–6 inches thick instead — the 6-inch version of the same slab needs 84 bags of 80-lb.
Related Calculators
Estimating a slab? Our Concrete Slab Calculator works the full job, and the Fence Cost Calculator prices post holes like the ones in the example.