A 10 ft x 10 ft concrete slab at 4 inches thick takes about 56 bags of 80 lb premixed concrete, 75 bags of 60 lb, or 112 bags of 40 lb. The simple conversion behind every project is the bags-per-yard rule: one cubic yard of concrete is the equivalent of 45 bags of 80 lb, 60 bags of 60 lb, or 90 bags of 40 lb. Once you know your project volume, the bag count is a straight division.
For custom dimensions or to factor in waste automatically, the Concrete Calculator does the math instantly. This guide walks through the reference numbers for the most common home projects so you can order without spreadsheet work.
Bags Per Cubic Yard
Every bag of premixed concrete advertises a wet-volume yield on the back of the bag. The three standard sizes have these yields:
| Bag size | Yield (cubic feet) | Bags per cubic yard | |---|---|---| | 40 lb | 0.30 | 90 | | 60 lb | 0.45 | 60 | | 80 lb | 0.60 | 45 |
The numbers come straight from the math: a cubic yard is 27 cubic feet, so divide 27 by the bag's yield. Treat these as accurate within about 1 to 2 percent. Actual yield varies a little with how much water you mix in, but for ordering purposes the values above are what most stores and suppliers use.
The 80 lb bag is the standard choice for any project over a couple of cubic feet because you carry and mix fewer bags. The 40 lb bag is easier on the back and better for small repairs or anchoring single posts. The 60 lb sits in the middle and is common at home centers.
Bags by Project Type
The table below covers the most common home and DIY concrete projects. Volumes are computed at the listed dimensions and rounded up to whole bags. Add roughly 10 percent for spillage and uneven excavation.
| Project | Dimensions | Volume (cu ft) | 80 lb bags | 60 lb bags | 40 lb bags | |---|---|---|---|---|---| | Standard fence post | 8 in diameter x 30 in deep | 0.9 | 2 | 2 | 3 | | Heavy fence post | 10 in x 36 in deep | 1.6 | 3 | 4 | 6 | | Mailbox post | 10 in x 24 in deep | 1.1 | 2 | 3 | 4 | | Deck footing (sono-tube) | 8 in x 48 in deep | 1.4 | 3 | 4 | 5 | | 4 x 4 ft pad | 4 in thick | 5.3 | 9 | 12 | 18 | | 6 x 6 ft pad | 4 in thick | 12.0 | 20 | 27 | 40 | | 10 x 10 ft slab | 4 in thick | 33.3 | 56 | 75 | 112 | | 12 x 12 ft slab | 4 in thick | 48.0 | 80 | 107 | 160 | | 10 x 20 ft driveway | 4 in thick | 66.7 | 112 | 149 | 223 | | 20 x 20 ft slab | 4 in thick | 133.3 | 223 | 297 | 445 |
For example, a typical privacy-fence run with 12 posts at 8 inch by 30 inch holes needs 24 bags of 80 lb concrete (12 posts x 2 bags). A small concrete patio at 6 ft by 6 ft for grilling takes 20 bags of 80 lb plus the waste allowance.
If your project mixes a slab and footings (a deck with concrete piers under a slab landing, for example), calculate each piece separately and add them up. Footings and slabs cure differently, so a single estimate that lumps them together usually under-orders for one or the other.
How to Calculate for Custom Sizes
For sizes the table does not cover, the formula is straightforward.
- Volume in cubic feet = length (ft) x width (ft) x thickness (ft). Convert thickness in inches to feet by dividing by 12.
- Bags needed = volume divided by the bag's yield (0.60, 0.45, or 0.30 cubic feet).
- Round up to whole bags and add 10 percent for waste.
For example, a 12 ft by 12 ft patio at 6 inches thick has a volume of 12 x 12 x 0.5 = 72 cubic feet, or 2.67 cubic yards. That is 120 bags of 80 lb, or 132 with 10 percent waste. For circular footings, use pi times the radius squared, times the depth in feet. The Square Footage Calculator helps if your shape is non-rectangular before you compute the volume.
If you need a finer breakdown of a curved or irregular pour, drop the dimensions into the Concrete Calculator directly and let it handle the conversion to bags.
When to Switch from Bags to Ready-Mix
There is a price point where buying bagged concrete stops being practical. The break-even depends on your area, but the general numbers look like this:
- An 80 lb bag at a home center runs roughly $5 to $7. At $5.50 per bag, one cubic yard (45 bags) costs about $250 in concrete alone, not counting your time or the rented mixer.
- Ready-mix concrete delivered is roughly $130 to $180 per cubic yard, plus a delivery fee of around $100 to $150 and often a 1 to 3 yard truck minimum.
For 1 cubic yard, bags often come out cheaper after delivery fees and minimums. For 2 cubic yards (around 90 bags of 80 lb), ready-mix usually wins on both price and labor. Mixing 90 bags by hand or with a small rented mixer is a long, hot day of work.
A practical rule: if your project is over about 60 bags of 80 lb (roughly 1.3 cubic yards), call two local ready-mix suppliers for a quote. Mention the project size and ask about their minimums. For larger pours like full driveways and 20 x 20 ft slabs in the table above, ready-mix is the only realistic choice unless you have a crew and a real mixer on site.
Common Mistakes
Forgetting waste percentage
Spillage from the wheelbarrow, slightly over-deep holes, and a few cracked bags at the store will all eat into your supply. Always order 10 percent extra. Unopened bags can usually be returned within the receipt window.
Treating bag sizes as interchangeable on big jobs
A 40 lb bag costs the same per pound as an 80 lb bag, but you carry, mix, and finish twice as many of them for the same volume. On a 6 x 6 ft pad, that is 40 forty-pound bags versus 20 eighty-pound bags. The lower price per bag is not actually a saving once the labor is counted.
Ignoring slab thickness changes
A 4 inch slab needs about 0.333 cubic feet per square foot of area. A 6 inch slab needs 0.5 cubic feet per square foot. Going from 4 to 6 inches raises your bag count by 50 percent. For driveways and any slab that will see vehicle weight, double-check thickness against your code.
Confusing dry weight with mixed volume
A 60 lb bag does not yield 60 lb of mixed concrete by volume. It yields the listed cubic-feet volume after water is added. Always work from cubic feet of wet concrete, never from dry pounds.
How This Compares With Other Materials
Concrete is one of several poured materials you might use on a project. For tile setting beds, see How to Calculate Deck Mud and Sand Quantities (the deck-mud ratio is very different from concrete). For wood-frame material counts, the Board Foot Calculator handles lumber. Concrete itself goes through the calculator above, and this guide's chart covers the project sizes that come up most often.
For background on measuring odd-shaped slabs before you compute volume, How to Calculate Square Footage Correctly walks through the breaking-into-rectangles method that prevents most ordering mistakes. The Construction & Home hub lists the rest of the project tools.
Important Note
This article is educational and based on standard premixed concrete yields. Actual bag yields vary by brand and water ratio, and prices vary by region. For structural slabs, footings supporting load, or any project subject to a building permit, follow your local code and the engineering on your plans.