How to use the concrete calculator
Pick the shape you're pouring, type in the measurements and the yardage updates as you go. You can use feet, inches, yards or metric for each measurement, since tapes and plans rarely agree. The result already includes the waste allowance you set, and the bag counts are rounded up, because a shortfall of half a bag halfway through a pour is a real problem.
Use How many of these? for repeats: eight fence-post holes, four pier footings, or two matching pads. If a job mixes shapes, such as a slab with a thickened edge, run each part separately and add the yards together.
The formula
Concrete is ordered by volume, so every mode comes down to length × width × depth, with all three in feet:
cubic feet = length × width × thickness
cubic yards = cubic feet ÷ 27
bags = cubic feet ÷ bag yield (rounded up)
A round column or post hole uses the area of a circle instead: π × (diameter ÷ 2)² × depth. Steps are treated as a stack of solid blocks. The bottom step is one rise tall, the next is two rises tall, and so on, and a landing at the top adds its own block.
Worked example: a 10 × 10 ft patio, 4 inches thick
- Convert the thickness to feet: 4 ÷ 12 = 0.333 ft.
- Volume: 10 × 10 × 0.333 = 33.3 ft³.
- Add 10% for waste: 33.3 × 1.10 = 36.7 ft³.
- In cubic yards: 36.7 ÷ 27 = 1.36 yd³.
- In 80 lb bags: 36.7 ÷ 0.60 = 61.1, so buy 62 bags.
Those are the numbers the calculator shows when the page first loads. That's 62 bags at 80 lb each, about two and a half tons to carry, mix and place. Most people would order ready-mix at that size.
Bag yields
Bag weight isn't bag volume. Premixed concrete yields about 0.0075 cubic feet per pound of dry mix. These are the figures QUIKRETE publishes for its standard Concrete Mix; Sakrete lists the same yields for its high-strength mix.
| Bag | Yield | Bags per cubic yard |
|---|---|---|
| 40 lb | 0.30 ft³ | 90 |
| 50 lb | 0.375 ft³ | 72 |
| 60 lb | 0.45 ft³ | 60 |
| 80 lb | 0.60 ft³ | 45 |
Fast-setting and high-strength mixes can yield slightly differently, so check the bag before you buy a large quantity.
How thick should it be?
| Job | Typical thickness |
|---|---|
| Walkway, patio, shed floor | 4 in |
| Hot-tub pad, heavier shed | 4-6 in |
| Driveway, garage floor | 5-6 in |
| Fence and deck post holes | Common guidance: hole about 3× the post width, bottom below your frost line |
These are common practice for residential work, not engineering. Footings for anything structural, like a house addition, a deck ledger or a retaining wall, should follow your local building code and any stamped drawings.
Common mistakes
- Forgetting the base isn't flat. A dip of half an inch across a slab adds up fast. On a 10 × 10 pad, an average of ½ in extra depth is another 4 ft³, about seven 80 lb bags. That's why the waste allowance exists.
- Using nominal lumber sizes for form depth. A 2×4 form board is 3½ in tall, not 4 in. If your forms are 2×4s, measure the thickness at 3.5 in.
- Mixing units. Feet times inches gives nonsense. The calculator converts each field for you; if you work it by hand, convert everything to feet first.
- Ordering exactly the calculated yardage. Ready-mix plants batch in fractions of a yard, often quarter yards. Round up, and agree with the driver in advance where any leftover goes.
Bags or a truck?
A person with a wheelbarrow and a hoe can mix and place maybe 20 to 30 bags an hour before the first batch starts to set at the edges. A mixer helps, but past about a cubic yard (45 bags of 80 lb) consistency and cold joints become the real risk. Enter both prices above and the calculator shows the cost each way. Remember that ready-mix suppliers often add a short-load fee for small orders, so ask for the delivered total.
Questions people ask
How many 80 lb bags of concrete make a yard?
45. A cubic yard is 27 cubic feet and QUIKRETE rates an 80 lb bag of Concrete Mix at about 0.60 cubic feet, so 27 ÷ 0.60 = 45 bags. With 60 lb bags (0.45 ft³) it takes 60, and with 40 lb bags (0.30 ft³) it takes 90.
How many bags of concrete do I need for a 10x10 slab?
At 4 inches thick a 10 × 10 ft slab is 33.3 cubic feet. Add 10% for spillage and an uneven base and you need about 36.7 cubic feet: 62 bags of 80 lb mix or 82 bags of 60 lb mix.
How thick should a concrete slab be?
4 inches is the usual minimum for patios, walkways and shed floors. Driveways and garage floors that carry cars are normally poured 5 to 6 inches thick, often with a thickened edge. Check your local code and any engineering plans for structural work.
Is it cheaper to mix bags or order ready-mix concrete?
For small jobs bags win, because ready-mix suppliers charge short-load fees on small orders. Somewhere around one to two cubic yards (45 to 90 bags of 80 lb mix) the truck usually becomes cheaper and much less work. Get a delivered price per yard from a local supplier and compare it with the bag total above.
How much concrete do I need for a fence post?
Use the column mode with the hole diameter and depth, then subtract nothing for the post: the difference is small and the extra covers waste. A 10 in hole 3 ft deep holds about 1.6 cubic feet, which is four 60 lb bags or three 80 lb bags.
How much does a yard of concrete weigh?
Roughly 3,900 to 4,000 lb once mixed and placed. Normal-weight concrete weighs about 140 to 150 lb per cubic foot, and this calculator uses 145 lb/ft³.
Sources and assumptions
- QUIKRETE Concrete Mix (No. 1101) data sheet: bag yields 0.30 / 0.375 / 0.45 / 0.60 ft³
- Portland Cement Association: normal-weight concrete weighs about 140-150 lb/ft³
- Waste allowance of 10% is a common contractor rule of thumb, not a standard. Change it to suit your job.
Last reviewed 26 October 2026. Every formula on this page is checked against the worked example above in our automated tests. Found a problem? Tell us.