Concrete Bag Calculator: How Many 40, 60 or 80 lb Bags Do You Need?
This calculator turns your project's length, width, and thickness into the exact number of 40, 60, or 80 lb concrete bags to buy. It compares all three bag sizes side by side so you can pick the most efficient option for your slab, footing, or post.
Estimated concrete
Estimates only. Add ~5–10% for waste. Bag yields: 40 lb ≈ 0.30, 60 lb ≈ 0.45, 80 lb ≈ 0.60 cu ft. Concrete ≈ 150 lb per cu ft.
How to Use the Concrete Bag Calculator
- 1
Enter your dimensions
Type the length, width and thickness of your slab, footing or pad and choose your units, feet, inches or meters.
- 2
Get instant volume
See the result in cubic yards, cubic feet and cubic meters, plus weight, the moment you type, no page reloads.
- 3
Buy the right amount
We convert volume into the exact number of 40, 60 and 80 lb bags, truck loads and cost so you order without waste or shortfalls.
The formula
bags = cubic feet ÷ bag yield, rounded up; cubic feet = Length_ft × Width_ft × (Thickness_in ÷ 12)
- ·cubic feet = total concrete volume needed
- ·bag yield = ft³ one bag makes: 0.30 (40 lb), 0.45 (60 lb), 0.60 (80 lb)
- ·Length_ft × Width_ft = footprint area (ft²)
- ·Thickness_in ÷ 12 = depth converted to feet
- ·result is always rounded up to whole bags
Each bag makes a fixed volume of mixed concrete, so the count is simply your volume divided by that yield. Always round up, and buy a few spare bags because you cannot pour a partial bag mid-project.
Worked examples
Example 1: 10 × 10 ft slab, 4 inches, 80 lb bags
- Thickness 4 in ÷ 12 = 0.333 ft
- Volume = 10 × 10 × 0.333 = 33.3 ft³
- 80 lb yield = 0.60 ft³ per bag
- Bags = 33.3 ÷ 0.60 = 55.6, round up to 56
Result: 56 bags of 80 lb
Example 2: 3 × 3 ft pad, 6 inches, 60 lb bags
- Thickness 6 in ÷ 12 = 0.5 ft
- Volume = 3 × 3 × 0.5 = 4.5 ft³
- 60 lb yield = 0.45 ft³ per bag
- Bags = 4.5 ÷ 0.45 = 10
Result: 10 bags of 60 lb
Concrete calculators by project type
Concrete bag yield & coverage
How much mixed concrete a standard bag yields, and how far a cubic yard goes at common slab depths.
| Bag size | Yield (cu ft) | Bags per cubic yard |
|---|---|---|
| 40 lb | 0.30 | 90 |
| 60 lb | 0.45 | 60 |
| 80 lb | 0.60 | 45 |
- 40 lb
- 60 lb
- 80 lb
Related guides
Concrete Bag Calculator FAQs
How many 80 lb bags of concrete are in a yard?
An 80 lb bag yields about 0.60 cubic feet, so it takes roughly 45 bags to make one cubic yard (27 cubic feet).
How many 60 lb bags of concrete do I need per yard?
A 60 lb bag yields about 0.45 cubic feet, so you need around 60 bags to fill one cubic yard.
How much does an 80 lb bag of concrete cover?
An 80 lb bag makes about 0.60 cubic feet. At 4 inches thick that covers roughly 1.8 square feet, and about 1.35 square feet at 5 inches thick.
Is it cheaper to buy bags or ready-mix concrete?
Bags are usually cheaper for small jobs under about 1 cubic yard, but ready-mix delivery becomes more economical and far less labor once you pass roughly 1 to 2 cubic yards.
How many 40 lb bags make a cubic yard?
A 40 lb bag yields about 0.30 cubic feet, so it takes roughly 90 bags to make a full cubic yard of concrete.
Should I buy extra bags of concrete?
Yes. Order about 5 to 10 percent extra to cover spillage, uneven subgrade, and over-excavation so you don't run short mid-pour.
How many cubic feet does each bag size actually make?
A 40 lb bag yields about 0.30 ft³, a 60 lb bag about 0.45 ft³, and an 80 lb bag about 0.60 ft³ of mixed concrete. The calculator divides your total volume by the yield of the size you pick. Larger bags mean fewer to mix but each is heavier to lift.
Why does the calculator always round bags up?
You cannot mix and place a fraction of a bag, so any leftover volume forces one more full bag. Rounding up guarantees you have enough to finish the pour. It is also why the raw division rarely lands on a whole number.
How much water does each bag of concrete need?
Follow the bag label, but it is roughly 3 quarts for a 60 lb bag and about 4 quarts for an 80 lb bag. Add water gradually until the mix holds its shape without being soupy. Too much water weakens the finished concrete.
How many 80 lb bags can I realistically mix by hand in a day?
Mixing in a wheelbarrow or small mixer, one person can handle roughly 15 to 30 bags before fatigue and set time become limiting. Beyond about a cubic yard, which is around 60 bags, ready-mix delivery usually makes more sense. Plan help or a mixer for larger counts.
Do bag counts include a waste allowance?
The core bag count is your exact volume divided by yield, then rounded up. Because spillage and slightly overfilled forms are common, buy 5 to 10% extra bags on top. Unopened bags can typically be returned, so erring high costs little.
Can I mix different bag sizes in the same pour?
Yes, as long as they are the same product and strength, the mixed concrete is identical once combined. People often top up an 80 lb order with a few 60 lb bags to hit the exact volume. Just keep the water-to-mix ratio consistent for each batch.
How do I keep a multi-bag pour from showing cold joints?
Mix and place fast enough that each fresh batch blends into the previous one before it starts to set. For a slab, have all bags and water staged in advance and pour continuously. Working in small sections with control joints also hides any minor batch-to-batch seams.