Concrete Calculator
0 yd³
Enter the length, width and thickness in the concrete calculator to see how much concrete you need: cubic yards for a ready-mix order, or 80, 60, 50 or 40 lb bags. The math is length (ft) × width (ft) × thickness (in) ÷ 12 ÷ 27 = cubic yards, so a 10' × 10' × 4" slab takes 1.23 yd³.
The tool above is a concrete volume calculator with four shapes: Slab, Footing / Wall, Round and Steps. Most mistakes with a concrete online calculator come from units, so thickness and depth here are read in inches unless you type the foot mark. You can enter 12' 6 1/2" the way it reads on the tape, down to 1/16.
How to calculate concrete
How many yards of concrete do I need? Multiply length by width by thickness, then convert to cubic yards. The formula in one line:
length (ft) × width (ft) × thickness (in) ÷ 12 ÷ 27 = cubic yards
Dividing by 12 turns inches of thickness into feet. Dividing by 27 turns cubic feet into cubic yards.
For a 10' × 10' × 4" slab: 10 × 10 × 4 ÷ 12 = 33.33 ft³, and 33.33 ÷ 27 = 1.23 yd³. That is 0.94 m³ if you switch the site to metric. The display also shows the weight: 5,000 lb at 150 lb/ft³, the top of the 140–150 range Iowa DOT gives for normal-weight concrete and the figure WSDOT uses in design. For tons and kilograms, use the concrete weight calculator.
We add 10% by default, because running short with the truck on site costs more than a few spare bags. With 10% the same slab is 36.67 ft³, or 1.36 yd³. Change Extra % to anything from 0 to 50; 0 means no extra. The result is labeled incl. 10% extra, so you always see what you're ordering.
Why divide by 27
A cubic yard is 3 ft × 3 ft × 3 ft, which is 27 ft³. Ready-mix plants sell by the yard, so a concrete yard calculator has to end with ÷ 27. Measuring gravel, topsoil or mulch in yards? The cubic yard calculator does the same math for them.
From square feet to cubic yards
Already know the area? Skip the length and width step: square feet × thickness (in) ÷ 12 ÷ 27 = cubic yards. The 10' × 10' slab is 100 ft². At 4" it comes to the same 1.23 yd³.
Turned around, one yard of concrete covers 81 ft² at 4": 27 ÷ (4 ÷ 12) = 81. That's the quick check for any concrete calculator square footage question. For L-shapes, circles or a room with a cutout, get the area first with the square footage calculator.
Footings and foundations
Use the Footing / Wall tab. For a continuous footing, enter the total length of the run, the width and the depth; for separate pad footings of the same size, enter one and set Quantity. It works as a concrete footing calculator or a foundation calculator: concrete for the whole perimeter comes out as one number, in yards or bags.
Deck footings
Deck piers are usually poured in round forms. Switch to Round: diameter in inches, depth, and the number of footings in Quantity. Spacing and size of footings come from your plans or local code. This calculator gives the concrete volume.
Concrete walls
Same Footing / Wall tab; read the fields as length, wall thickness and height. A concrete wall calculator is the same box formula stood on end. Type the unit with each number (8', 8") and the field reads it as written.
Sonotubes, columns and post holes
Round covers a sonotube, a column or a post hole: diameter (in), depth or height, and quantity. The math is π × radius² × depth, then ÷ 27 for yards. Got twelve tubes of one size? Enter one and set Quantity to 12 rather than adding them up by hand.
That's the whole sonotube concrete calculator. For a concrete column calculator, enter the height instead of the depth; for a post hole concrete calculator, see the next section.
Concrete for fence posts
A post takes up room in the hole, so Round has an optional Post diameter field. Fill it in and the post is subtracted from the hole volume. Enter the hole diameter, the depth and the number of posts, then set Buy as to the bag size you plan to carry. Used as a fence post concrete calculator, it gives bags for the whole fence, rounded up to whole bags.
Concrete steps
The Steps tab takes the number of steps, rise (in), run (in) and width, plus an optional landing depth. The calculator treats each step as a solid block down to the ground:
width × run × rise × (1 + 2 + … + n)
A landing adds width × landing depth × (n × rise).
Example: 3 steps, rise 7", run 11", width 4', no landing. That's 12.83 ft³, or 0.48 yd³: 22 bags of 80 lb, 24 with 10% extra. The same tab works as a concrete stairs calculator for any short run of steps poured solid.
Driveways and patios
A driveway or patio is a slab. Use the default Slab tab. Quikrete's patio and sidewalk projects use 4". For a driveway, Concrete Network gives 4" minimum for passenger cars and 5–6" for heavy trucks or RVs. Set the thickness before you compare numbers: going from 4" to 5" adds a quarter more concrete.
Typical slab sizes and thickness tips are on the concrete slab calculator page; use it as a concrete driveway calculator or concrete patio calculator with nothing else on screen.
Ready-mix or bags
Buy as switches the big number between Ready-mix yd³ and 80, 60, 50 or 40 lb bags, with no need to press Calculate again. Bags are counted by yield and rounded up. Quikrete lists 0.60 ft³ per 80 lb bag, 0.45 for 60 lb, 0.375 for 50 lb and 0.30 for 40 lb.
One cubic yard is 45 bags of 80 lb. The 10' × 10' × 4" slab is 1.23 yd³, or 56 bags of 80 lb, 75 of 60 lb, 89 of 50 lb or 112 of 40 lb. Whether a truck makes sense at that size depends on your plant: ask about its minimum order and delivery terms. Quikrete rates its bagged Concrete Mix at 4,000 psi at 28 days.
All sizes side by side are on the concrete bag calculator.
Concrete mix ratio
Mixing your own? A common rule of thumb is 1 part cement, 2 parts sand and 3 parts gravel by volume, as a University of Illinois materials science page puts it.
| Cement | Sand | Gravel |
|---|---|---|
| 1 part | 2 parts | 3 parts |
Add water a little at a time until the mix is workable. For bagged mix, Quikrete lists about 6 pt of water per 80 lb bag.
Came looking for a concrete mix calculator? The tool above gives the volume of concrete, not the amount of each part.
Concrete cost
There's no price list on this page, because concrete prices are local. Asking how much is a yard of concrete? Your ready-mix plant quotes it. Enter that price in Price, and the result shows Cost $…: yards × your price, or bags × your price per bag. That makes it a concrete cost calculator with your own numbers, and the concrete calculator price line updates with Extra % and Buy as.
What's next
Press Add to project under the result. The concrete goes to My project as a line with what you entered, for example Back patio · 10' × 10' × 4" · 56 bags 80 lb, plus the cost if you entered a price. Tap the name to rename the line. Lines from different calculators sit in one list, with a total per material at the bottom, and you can print it. The numbers you enter stay in your browser; no account needed.
Pouring a slab on a base? Work out the stone with the gravel calculator, and the weight with the concrete weight calculator.
FAQ
How much concrete do I need?
Multiply length (ft) × width (ft) × thickness (in), divide by 12, then by 27 for cubic yards. A 10' × 10' × 4" slab needs 1.23 yd³, or 1.36 yd³ with 10% extra.
How much does 1 yard of concrete cover at 4 inches?
81 ft². One yard is 27 ft³, and 27 ÷ (4 ÷ 12) = 81.
Why do you divide by 27 for cubic yards?
A cubic yard is 3 ft × 3 ft × 3 ft, which is 27 ft³. Dividing cubic feet by 27 gives yards, the unit ready-mix is sold in.
How much is a yard of concrete?
It depends on your local plant, so ask it for a quote. Enter that price in Price and the calculator shows the cost as yards × your price.
How many 80 lb bags of concrete do I need for 3 yards?
135 bags with no extra: 3 × 27 ÷ 0.60, using Quikrete's 0.60 ft³ per 80 lb bag. With 10% extra it's 149. Other sizes are on the concrete bag calculator.
How much concrete do I need for stairs?
Use the Steps tab. Three steps with a 7" rise, 11" run and 4' width take 12.83 ft³, or 0.48 yd³: 22 bags of 80 lb, 24 with 10% extra.
Results are estimates for ordering material. Check the quantity with your supplier before you order.