Concrete bag calculator
Bag counts are easy. What most calculators leave out is how long it will take to mix them, which is the number that decides whether your pour works.
Concrete bag calculator
Work out how many bags a pour needs, and how long each mixing method will take to get through them. The example loaded is a 12 × 10 ft slab at 4 inches.
What you need
Includes 8% for waste, spillage and an uneven subgrade — a real allowance rather than a rounding.
| Method | Rate | Time for this pour | Cost |
|---|---|---|---|
| Wheelbarrow and hoe | 8 bags/hr | 9 hr | $40 in tools |
| Bucket and paddle mixer | 14 bags/hr | 5.1 hr | $150 |
| Drum mixer, 3.5 cu ft | 20 bags/hr | 3.6 hr | ~$100/day to rent |
| Mud Mixer MMXR-3225 | 45 bags/hr | 1.6 hr | $3,695 to buy |
At 1.6 cubic yards, order ready-mix instead. Bags for this pour cost roughly $468 in materials against about $280 delivered, before you count your own labour — and a truck pours it in thirty minutes with no cold joints.
On this page
What this concrete bag calculator works out
Three shapes cover almost every residential pour.
Slab or patio. Length × width in feet, thickness in inches. Also correct for a shed base, a walkway or a driveway section.
Footing or trench. Length in feet, width and depth in inches, multiplied by the number of runs. Use it for strip footings, a trench for a garden wall, or a thickened slab edge.
Post holes. Diameter and depth in inches, multiplied by the number of holes. This one uses the cylinder volume properly rather than approximating a hole as a box, which matters because a 12-inch hole is only 79% of the volume of a 12-inch square one.
Every result carries an 8% waste allowance. That is not padding. Subgrades are not level, forms bow slightly under load, material stays in the mixer, and some lands on the ground. Running out mid-pour creates a cold joint you cannot remove; ending up one bag over costs six dollars.
Bag yields
The figures behind the calculation, from the standard manufacturer values:
| Bag size | Yield | 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 |
Eighty-pound bags are the most economical per cubic foot and the heaviest to handle. If you are working alone or lifting from a car boot, 60 lb bags cost a few percent more and are considerably easier on your back.
Why mixing time is in the results
Because it is the number that determines whether your pour comes out as one slab or several, and no other calculator shows it.
Concrete begins its initial set at 30 to 45 minutes in typical conditions, and faster in heat. Place fresh concrete against material that has already begun setting and you get a cold joint — a plane through the slab with no aggregate interlock across it. It is a designed-in crack.
So the practical question is not "how many bags" but "can I place each section before the previous one sets".
Take a 10 × 12 ft patio at 4 inches: 72 bags after waste.
| Method | Rate | Time | Result |
|---|---|---|---|
| Wheelbarrow and hoe | 8 bags/hr | 9 hr | Cold joints throughout |
| Bucket and paddle mixer | 14 bags/hr | 5.1 hr | Cold joints unless sectioned |
| Drum mixer, 3.5 cu ft | 20 bags/hr | 3.6 hr | Needs deliberate sectioning |
| Continuous auger mixer | 45 bags/hr | 1.6 hr | Manageable with a helper |
| Ready-mix truck | — | 30 min | One monolithic pour |
That table is why the calculator reports time alongside bag count. A pour that takes nine hours is not a slower version of a pour that takes ninety minutes — it is a structurally different slab.
If the numbers say you cannot beat the set time, the answer is to section the pour deliberately with proper construction joints, rather than accepting accidental cold joints wherever you happened to be when the previous batch set.
The ready-mix threshold
Above about 1.5 cubic yards — 68 × 80 lb bags — delivered concrete is usually cheaper than bags, and the calculator says so when you cross it.
The arithmetic: bagged mix runs $250 to $300 per cubic yard in materials, and ready-mix runs $150 to $200 per cubic yard delivered, plus a short-load fee below about 3 yards. So at 1.5 yards you are already paying more for bags before counting nine hours of your own labour and a slab full of cold joints.
The legitimate reasons to keep mixing above that volume are access — a truck cannot reach a back garden — and pour geometry a chute cannot serve. Those are real, and they are why concrete pumps and power buggies exist. Price is not one of them.
Getting a better result
Four things that matter more than the bag count.
Compact the subgrade. Most residential slab cracking traces to the ground under it rather than the concrete in it. A plate compactor for an afternoon is the best money you will spend on the project.
Do not add water to make it easier. The water-cement ratio is the dominant variable in concrete strength — a mix at 0.60 can lose 25 to 30% of the strength of the same mix at 0.45. Over three hours of hoe work the temptation is constant and gradual.
Wet the subgrade before pouring. Dry ground pulls water out of the mix from below, accelerating set at the bottom of the slab where you cannot see it.
Cure it for seven days. Concrete gains strength through hydration, not drying. Keep it damp under plastic for a week and you get the strength you paid for; let it dry out in two days and you do not.
For the throughput detail behind the mixing-time column, how many bags an hour you can realistically mix works through each method, and our Mud Mixer MMXR-3225 review covers what the fastest option costs to own.
Questions people actually ask
How many 80 lb bags of concrete do I need for a 10x10 slab?
A 10 × 10 ft slab at 4 inches thick is 33.3 cubic feet, or 1.23 cubic yards. At 0.6 cubic feet per 80 lb bag that is 56 bags, and with an 8% waste allowance, 60. At that volume you are close to the point where ready-mix delivery starts costing less than bags.
How much does a bag of concrete cover?
An 80 lb bag yields about 0.6 cubic feet of mixed concrete — enough for roughly 1.8 square feet at 4 inches thick. A 60 lb bag yields 0.45 cubic feet, a 50 lb bag 0.375, and a 40 lb bag 0.30.
How thick should a concrete slab be?
Four inches for a patio, walkway or shed base. Five to six inches for a driveway or anywhere a vehicle parks. Six inches with reinforcement for anything carrying heavy loads. Going thinner than four inches is where slabs crack, and the material saving is small enough not to be worth it.
Should I add extra for waste?
Yes, and this calculator adds 8% automatically. Real subgrades are not level, forms bow slightly, some material stays in the mixer and some ends up on the ground. Running out of concrete mid-pour creates a cold joint, which is a permanent defect — being one bag over is a minor annoyance by comparison.
When is ready-mix cheaper than bags?
Around 1.5 cubic yards, or roughly 68 × 80 lb bags. Bagged mix costs $250 to $300 per cubic yard in materials against $150 to $200 delivered, so above that volume the truck is cheaper before you value your own labour at all. Short-load fees apply below about 3 yards and rarely change the answer.
Do the mixing times assume one person or two?
The hand and drum figures assume one person working steadily. The continuous mixer figure of 45 bags an hour is the manufacturer's rate and assumes someone is feeding the hopper — working alone, expect about 30.