Concrete Calculator
How many cubic yards & bags of concrete you need
Pouring a slab, footing, walkway or post holes? Enter your measurements to find out exactly how much concrete to order — in cubic yards for ready-mix, or as a count of 40, 60 and 80 lb bags. Below you'll find a yardage chart that works for any area, a post hole chart, how ready-mix delivery actually gets priced, and which mix strength to buy.
Concrete volume for any shape
Every concrete estimate is the same sum: area × thickness, converted to cubic yards at 27 ft³ per yard. What changes is how you get the area.
- Slabs, patios, walkways, footings: length × width. Set the thickness in inches and the calculator handles the conversion.
- Round columns, tube forms and post holes: switch the shape to Circle and enter the diameter in feet. A 10-inch tube is 0.833 ft, not 0.83 — the third decimal is worth a whole bag on a long fence line.
- Steps and odd shapes: break them into rectangles, run each one, and add the results. Use the "number of areas" field for repeats.
The waste field (10% by default) is applied before the bag count, so the figure on screen is what to order, not the bare theoretical volume. Uneven subgrade quietly eats concrete: a slab formed to 4 inches that sags to 4½ in the middle needs about 12% more than the arithmetic says.
Concrete yardage chart per 100 square feet
This is the table to bookmark. It works as a concrete yardage calculator for any area: find your thickness, then scale the figure to your square footage. A 340 ft² patio at 4 inches is 3.4 × 1.23 = about 4.2 cubic yards of concrete.
| Thickness | Cubic feet per 100 ft² | Cubic yards per 100 ft² | 80 lb bags per 100 ft² |
|---|---|---|---|
| 2 in | 16.7 ft³ | 0.62 yd³ | 28 bags |
| 3 in | 25.0 ft³ | 0.93 yd³ | 42 bags |
| 4 in | 33.3 ft³ | 1.23 yd³ | 56 bags |
| 5 in | 41.7 ft³ | 1.54 yd³ | 70 bags |
| 6 in | 50.0 ft³ | 1.85 yd³ | 84 bags |
| 8 in | 66.7 ft³ | 2.47 yd³ | 112 bags |
| 10 in | 83.3 ft³ | 3.09 yd³ | 139 bags |
| 12 in | 100 ft³ | 3.70 yd³ | 167 bags |
For ordering day, the printable material coverage chart condenses the concrete figures, the bags-per-cubic-yard conversions and the gravel base rates onto one page that survives a job site better than a phone.
Two numbers fall out of this that answer most quick questions about cubic yards of concrete. One cubic yard covers 81 ft² at 4 inches thick, and it takes 45 eighty-pound bags to make one cubic yard (27 ÷ 0.6). If you are pouring a specific slab rather than scaling from a rate, the concrete slab calculator has a size-by-size chart.
Ready-mix delivery: trucks, minimums and short-load fees
Once you know the yardage, the next question is who mixes it. Used as a ready mix concrete calculator, the price field takes your local rate per cubic yard, but the quoted rate is rarely the whole bill.
- A full truck carries 8 to 10 cubic yards. Above that you are booking two loads, and the second one needs to arrive before the first goes off.
- Short-load fees start under about 3 to 4 yards and run roughly $50 to $150. On a 1-yard pour that fee can rival the concrete itself.
- Order in quarter-yard increments and round up. Being half a yard short mid-pour means a cold joint you will see forever.
- Watch the unloading window. Most suppliers allow 5 to 10 minutes per yard, then charge waiting time. Have your forms, tools and helpers ready before the truck arrives.
Ready-mix commonly runs $120 to $220 per cubic yard delivered. Bagged mix works out around $225 to $315 per cubic yard at $5 to $7 an 80 lb bag, so bags are the expensive option per yard. They only win on small pours where a short-load fee would dominate, or where a truck simply cannot reach.
Worked example: fence post concrete calculator
Setting 12 fence posts, each in a hole 10 inches across and 30 inches deep. Switch the shape to Circle, enter a diameter of 0.833 ft, depth 30 in, 12 areas, 10% waste.
| Step | Working | Result |
|---|---|---|
| Volume of one hole | π × (5⁄12 ft)² × 2.5 ft | 1.36 ft³ |
| Twelve holes | 1.36 × 12 | 16.4 ft³ |
| Plus 10% waste | 16.4 × 1.10 | 18 ft³ = 0.67 yd³ |
| In 80 lb bags | 18 ÷ 0.60 | 30 bags |
| Less the posts | 4×4 displaces 0.21 ft³ each | about 25 bags net |
The calculator gives the gross hole volume, because it does not know what you are putting in the hole. A 4×4 post is actually 3.5 inches square and displaces about 0.21 ft³ over a 30-inch depth, so a dozen posts claw back roughly five bags. Buying the gross figure is the sensible call: spare bags cost a few dollars, and a second trip to the store mid-job costs an afternoon.
Concrete per post hole chart
Bare volume for a single hole, with bag counts rounded up. Multiply by your post count, then add 10%.
| Hole size | Concrete | 50 lb bags | 60 lb bags | 80 lb bags |
|---|---|---|---|---|
| 8 in × 24 in deep | 0.70 ft³ | 2 | 2 | 2 |
| 8 in × 30 in deep | 0.87 ft³ | 3 | 2 | 2 |
| 10 in × 24 in deep | 1.09 ft³ | 3 | 3 | 2 |
| 10 in × 30 in deep | 1.36 ft³ | 4 | 4 | 3 |
| 12 in × 30 in deep | 1.96 ft³ | 6 | 5 | 4 |
| 12 in × 36 in deep | 2.36 ft³ | 7 | 6 | 4 |
The rule of thumb for hole width is three times the post width, and for depth one third of the post's above-ground height or below your local frost line, whichever is deeper. Frost depth is the one that gets people: a fence set 18 inches deep in a region with a 42-inch frost line will lift and lean within a couple of winters.
Which concrete mix strength to buy
Bagged products are sold by compressive strength in psi, and the differences matter more than the price gap between them.
| Job | Typical strength | Product type |
|---|---|---|
| Footings and foundation walls | 2,500 to 3,000 psi | Standard concrete mix |
| Patios, walkways, shed floors | 3,000 to 3,500 psi | Standard or crack-resistant |
| Driveways and garage floors | 3,500 to 4,000 psi | High-strength mix |
| Fence and mailbox posts | 2,500 to 4,000 psi | Fast-setting post mix |
| Thin overlays and repairs | 4,000 psi and up | Sand mix or topping mix |
Fast-setting post mix is the exception to everything above: you pour it into the hole dry, add water on top, and skip mixing entirely. It sets in 20 to 40 minutes, which is a genuine advantage on a fence line and a genuine hazard anywhere you need time to level and finish.
How much gravel under concrete slab and footing pours
A slab wants 4 to 6 inches of compacted gravel underneath for drainage and a stable, frost-resistant base. Footings are different and this trips people up: a footing is normally poured on undisturbed soil at the bottom of the trench, with no gravel at all, because the footing's whole job is to transfer load to ground that has never been dug.
For a 12 × 20 ft slab, a 4-inch base is 88 ft³ with waste, which is 3.26 cubic yards or about 4.6 tons of crushed stone. Price the stone separately in the gravel calculator, since it sells by the ton rather than the yard. Trench and footing dimensions have their own rules, covered in the concrete footing calculator.
Where concrete estimates go wrong
- Ordering the exact calculated volume. Subgrade is never flat. Always carry 10%.
- Measuring form depth instead of pour depth. Concrete fills to where the ground actually is, not to where you set the form.
- Forgetting the short-load fee when comparing bags with ready-mix on a small pour.
- Mixing bags by hand past about one cubic yard. That is 45 eighty-pound bags, roughly a full day, and the batches will not match.
- Adding water to loosen a stiff mix. Every extra litre cuts the finished strength.
- Ignoring the frost line on post holes and footings. The most expensive mistake on this page, and the least visible on the day.
How this concrete calculator works out its numbers
Volume is area × thickness converted at 27 ft³ per cubic yard, with the waste percentage applied before the bag count so the displayed figure is an order quantity. Circular shapes use πr² from the diameter you enter.
- Bag yields are the manufacturer-printed figures: 80 lb = 0.60 ft³, 60 lb = 0.45 ft³, 50 lb = 0.375 ft³, 40 lb = 0.30 ft³. Bags per cubic yard follow directly (45, 60, 72 and 90).
- Density is 150 lb/ft³ for normal-weight concrete. Lightweight mixes run 90 to 120 lb/ft³.
- Gravel base tonnage uses 105 lb/ft³ for compacted crushed stone, the same constant as the gravel calculator. Real densities range 95 to 125 lb/ft³ depending on stone type and moisture.
- Cost ranges ($120 to $220 per yard delivered, $5 to $7 per 80 lb bag, $50 to $150 short-load fee) are US planning figures and vary widely by region. Get a local quote before ordering.
- Strength and depth guidance reflects common residential practice, not any single code. Frost depth and footing requirements are set locally, so check your building department before pouring anything structural.
Reviewed September 2026.
Figures reviewed . Every worked example on this page is checked against the calculator above.
Concrete Calculator: frequently asked questions
How many 80 lb bags of concrete are in a cubic yard?
45. One 80 lb bag yields about 0.60 cubic feet and a cubic yard is 27 cubic feet, so 27 ÷ 0.60 = 45. A 60 lb bag yields 0.45 ft³ (60 per yard) and a 40 lb bag yields 0.30 ft³ (90 per yard).
How much extra concrete should I order?
Add about 10% for spillage and uneven subgrade, or 10 to 15% on large or critical pours. Ready-mix is sold in quarter-yard increments, so round up to the next quarter rather than risk stopping mid-pour.
How much does a cubic yard of concrete cover?
About 81 square feet at 4 inches thick, 65 ft² at 5 inches, or 54 ft² at 6 inches. One cubic yard is 27 cubic feet, so divide 27 by your thickness in feet.
How many bags of concrete for a fence post?
Two to four 50 lb bags per hole is typical. A 10-inch hole 30 inches deep holds 1.36 cubic feet, which is three 80 lb bags or four 50 lb bags before you subtract the post itself.
Is it cheaper to buy bags or ready-mix concrete?
Ready-mix is cheaper per yard above about one cubic yard: roughly $120 to $220 delivered versus $225 to $315 in bags. Below that a short-load fee of $50 to $150 usually tips the balance back to bags.
How much concrete does a truck hold?
A standard ready-mix truck carries 8 to 10 cubic yards. Anything larger needs multiple loads scheduled close together so each one goes in before the previous pour starts to set.
How deep should a fence post hole be?
One third of the post's above-ground height, or below your local frost line, whichever is deeper. Hole width should be about three times the post width, so a 4×4 post wants a 10 to 12 inch hole.
What concrete mix strength do I need for a driveway?
3,500 to 4,000 psi for driveways and garage floors. Patios and walkways are fine at 3,000 to 3,500 psi, and footings are typically 2,500 to 3,000 psi.