Concrete Calculator
Slab volume in cubic yards and bags — with the order margin that prevents a short pour.
Updated
You need
1.36cubic yards
the unit ready-mix suppliers quote and deliver
- Volume
- 36.7 cu ft
- Bags equivalent
- 62 bags
In short
How much concrete do I need for a slab?
Multiply length by width by thickness in inches, divide by 12 for cubic feet, then by 27 for cubic yards. A 10 by 10 ft slab at 4 inches is 33.3 cubic feet, or 1.23 cubic yards, and 1.36 yards once you add the 10 percent order margin. In 80 lb bags that is 62 bags, because each yields 0.60 cubic feet.
NRMCA advises ordering 4 to 10 percent more than the plan-dimension estimate, which is exactly what the recommended margin does.
How to use the concrete calculator
Measure the inside of the forms in feet and set the thickness for the job. Residential practice puts patios, walkways, and shed slabs at four inches and thickens to five or six where vehicles drive or park, but local code, frost depth, and your subgrade are the real authority — check them before you accept any national rule of thumb, including this one.
For irregular pours, break the shape into rectangles and add the volumes. Round piers are a different calculation entirely; size those off the tube label rather than trying to force a rectangular model to describe a cylinder. And probe the excavation in several spots, because the depth you dug is rarely the depth you planned.
33.3 cu ft
The default slab
10 by 10 ft at 4 inches
1.36 yd³
To order with the 10% margin
1.23 exact volume
62 bags
Same slab in 80 lb bags
nearly 2.5 tons of dry mix
Leave the order margin at the recommended plus 10 percent unless you have a measured, formed, dead-level excavation. This is not padding invented for comfort: NRMCA’s own guidance on ordering ready mixed concrete is that the quantity ordered should be four to ten percent more than an estimate calculated from plan dimensions, to cover contingencies.
Real subgrades are never flat, and a quarter-inch of extra average depth across a 10 by 10 slab swallows two cubic feet by itself, before spillage, over-dig, and form deflection eat in the same direction.
The result leads with cubic yards because that is the unit ready-mix is quoted, batched, and sold in — NRMCA defines the sale as the volume of fresh concrete as discharged from the truck mixer.
The bag count converts the same volume into sacks of pre-mixed concrete using the yields printed on the bags themselves: 0.60 cubic feet from an 80 lb bag, 0.45 from a 60 lb, and 0.30 from a 40 lb, figures Sakrete publishes on its product data and QUIKRETE matches on data sheet 1101. Switch sizes under Advanced to match what your store stocks and what you can actually lift repeatedly for an hour.
Do
- Measure the inside of the forms and add irregular pours up as rectangles.
- Probe the excavation in several spots and enter the true average depth.
- Keep the order margin at plus 10 percent on any real subgrade.
- Have help, tools and a screed board on site before the drum turns.
Don't
- Order on the exact-volume figure, which exists for comparing quotes.
- Force a rectangular model onto a round pier; size that off the tube label.
- Take four inches as code, since frost depth and subgrade soil decide it.
- Expect the margin to rescue a thickness that was typed in wrong.
Bag counts and dry weight for every common pour volume, so you can see exactly where bagged concrete stops being sensible. The weight column is the number that usually settles the bags-versus-truck argument on its own.
| Volume needed | Cubic yards | 80 lb bags | 60 lb bags | 40 lb bags | Dry weight in 80 lb bags |
|---|---|---|---|---|---|
| 1 cu ft — one post footing | 0.04 | 2 | 3 | 4 | 160 lb |
| 2 cu ft — two small footings | 0.07 | 4 | 5 | 7 | 320 lb |
| 3 cu ft — a step repair | 0.11 | 5 | 7 | 10 | 400 lb |
| 5 cu ft — small equipment pad | 0.19 | 9 | 12 | 17 | 720 lb |
| 10 cu ft — compact landing | 0.37 | 17 | 23 | 34 | 1,360 lb |
| 15 cu ft — short walkway run | 0.56 | 25 | 34 | 50 | 2,000 lb |
| 20 cu ft — small patio | 0.74 | 34 | 45 | 67 | 2,720 lb |
| 27 cu ft — one cubic yard | 1.00 | 45 | 60 | 90 | 3,600 lb |
| 40 cu ft — 10 by 12 slab at 4 in | 1.48 | 67 | 89 | 134 | 5,360 lb |
| 54 cu ft — two cubic yards | 2.00 | 90 | 120 | 180 | 7,200 lb |
| 81 cu ft — three cubic yards | 3.00 | 135 | 180 | 270 | 10,800 lb |
| 108 cu ft — four cubic yards | 4.00 | 180 | 240 | 360 | 14,400 lb |
Bagged mix or the ready-mix truck?
Here is the decision the bag count is really for. Around one cubic yard, bags stop making sense. The default slab in this tool needs 62 eighty-pound bags — very nearly 2.5 tons of material to buy, load, haul, lift, mix a wheelbarrow at a time, and place before the first batches begin to stiffen.
Ready-mix costs more per yard on paper, but past a yard the truck wins on cost per finished slab, on batch-to-batch consistency, and on your spine. A typical mixer carries nine to eleven cubic yards, so a small order is a short load and most plants price it accordingly.
Bags are for post footings, small pads, repairs, and pours a truck simply cannot reach. None of this replaces a contractor’s takeoff on a structural pour.
Footings for a fence run?
Post footings are the classic bagged pour, and the post count is what decides how many of them you mix. Settle the run length and the post spacing first.
Open the fence calculator →Read it: Every one of those piles gets lifted twice, once into the vehicle and once into the mixer, which settles the bags-versus-truck question more reliably than any price comparison.
Computed from the 0.60 cu ft yield of an 80 lb bag: 45 bags make one cubic yard.
Full guide
How Much Concrete Do I Need? Slab Math, Bag Counts, and When to Call the Truck
Yards and bag counts for real projects — patios, shed slabs, and deck piers — with the ordering math nobody tells you.
Read the full guide →The formula, worked line by line
A slab is a rectangular box, so its volume is length times width times thickness. The complication is that nobody works in one unit: the slab is measured in feet, the thickness is chosen in inches, ready-mix is sold by the cubic yard, and bagged mix is sold by a yield printed in cubic feet. Four units, one box.
Divide the thickness by 12 to convert inches to feet so the product is cubic feet. Multiply by the order margin, because the hole you dug is never exactly the hole you drew. Divide by 27 for cubic yards, since a cubic yard is three feet cubed. Divide the cubic feet by the bag yield for a bag count, and round that up, because half a bag of concrete is not a thing you can buy.
cubic feet = length ft × width ft × (thickness in ÷ 12) × margin
margin = 1.10 recommended · 1.00 exact volume
cubic yards = cubic feet ÷ 27
bags = cubic feet ÷ bag yield [80 lb 0.60 · 60 lb 0.45 · 40 lb 0.30]- Volume
- 10 × 10 × (4 ÷ 12) = 33.33 cu ft
- With the 10 percent margin
- 33.33 × 1.10 = 36.67 cu ft
- In cubic yards
- 36.67 ÷ 27 = 1.36
- In 80 lb bags
- 36.67 ÷ 0.60 = 61.1
- Order
- 1.36 yd³, or 62 bags
Select exact volume instead and the same slab reads 33.33 cubic feet and 1.23 cubic yards, the figure to quote when you are comparing two suppliers, not the figure to order.
Sixty-two 80 lb bags is 4,960 lb of dry mix, close to two and a half tons. That single number decides the bags-versus-truck question more reliably than any price comparison, because it is also two and a half tons you personally lift twice: once into the vehicle and once into the mixer.
The conversion that makes it obvious is that a cubic yard is 27 cubic feet, so at 0.60 cubic feet a bag it takes exactly 45 eighty-pound bags to make one yard — and 45 × 80 lb is 3,600 lb of dry product per yard, the same 3,600 lb whether you buy 45 large bags, 60 medium, or 90 small.
Two scheduling notes that save pours. Order ready-mix on the cubic-yard figure rounded up to whatever increment your supplier delivers, which is commonly a quarter yard, and expect a short-load charge on small orders — NRMCA puts typical truck mixer capacity at nine to eleven cubic yards, so anything under a yard or two is a very short load indeed.
And have help, tools, and a plan on site before the drum starts turning, because concrete does not pause while you look for a screed board. For a structural slab, footing, or anything carrying a building, treat this estimate as a sanity check on a professional takeoff rather than a replacement for one.
Questions people ask
Sources
Where the constants and formulas on this page come from. Each line names the figure it backs.
The bag yields the estimate rests on: an 80 lb bag gives about 0.60 cubic feet, a 60 lb bag 0.45 and a 40 lb bag 0.30.
Concrete Mix, Product No. 1101 — Product Data Sheet — QUIKRETE, Data sheet 1101
The industry guidance to order 4 to 10 percent more than the figure calculated from plan dimensions.
CIP 31 — Ordering Ready Mixed Concrete — National Ready Mixed Concrete Association, Concrete in Practice series
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