Concrete Calculator to estimate slabs, footings, post holes, circular pours, and tube/ring volume. Formula: volume = length × width × depth; bags = total volume ÷ bag yield.
The Shape You Pick Changes the Math Entirely
A driveway apron and a round sonotube footing have almost nothing in common geometrically, but plenty of estimating tools treat them identically. Enter length, width, depth — done. That works fine for a rectangular slab. Run it on a 12-inch diameter column and you’ll order somewhere between 21% and 27% more concrete than you need, because you’ve been computing a square around the circle instead of the circle itself. This calculator uses the correct formula for each shape, and which one it applies depends entirely on the shape you select at the top.
What Each Shape Mode Actually Computes
Slab / Patio / Sidewalk and Footing / Rectangular use the same formula: Length × Width × Depth, all converted to feet before multiplication. They’re listed separately because footings and slabs typically involve different depth ranges and ordering contexts, but the math is identical.
Hole / Column / Round Footing and Circular Slab are also treated identically internally — both use the cylinder formula: π × (Diameter ÷ 2)² × Depth. For these shapes the Width field disappears entirely, because a circle only needs one horizontal dimension. Enter the full diameter, not the radius.
The Tube / Ring (Hollow) shape is where it gets more specific. This one is for structures like annular piers, pipe surrounds, or hollow cylindrical forms. It computes the outer cylinder volume and subtracts the inner void: (π × outerRadius² × Depth) − (π × innerRadius² × Depth). The Length field becomes Outer Diameter, Width becomes Inner Diameter. One hard constraint: the inner diameter must be strictly less than the outer diameter. If you switch to Ring mode with an inner diameter that’s already equal to or larger than the outer, the calculator automatically sets the inner to half the outer so you’re not starting with an invalid configuration.
All shapes multiply the single-unit volume by the Quantity input before applying the waste factor. That’s for runs of identical footings or multiple columns — enter the dimensions of one, set the count, and you get the total pour in a single calculation.
Depth Is in Inches by Default — Not Feet
This trips up more experienced estimators than beginners, because they’re used to working in feet for everything. The depth field defaults to inches. A 4-inch slab should be entered as 4, not 0.333. If you switch the depth unit to feet and type 4, you’ll get a result for a 4-foot-thick slab — roughly twelve times what you actually need.
The unit selectors for each dimension are independent, so you can mix feet for length and width with inches for depth and the calculator handles the conversion internally before computing volume. All three dimensions resolve to feet before any multiplication happens.
Worked Example: Deck Post Footings
Twelve 10-inch diameter sonotube footings, each 36 inches deep, for a ground-level deck. One column at a time in a standard tool would mean running the calculation twelve times and adding up the results. Using the Quantity field here:
- Shape: Hole / Column / Round Footing
- Diameter: 10 inches (switching Length unit to inches)
- Depth: 36 inches
- Waste: 10% (tube pours waste more than slabs — concrete doesn’t all make it in cleanly)
- Quantity: 12
- Bag size: 80 lb
Single column volume: π × (5/12 ft)² × (36/12 ft) = π × 0.1736 × 3 = 1.636 cu ft. Twelve of them: 19.63 cu ft base. Add 10% waste: 21.6 cu ft total — just under 0.80 cu yards. The calculator shows 36 bags of 80 lb mix. On the job that worked out to 34 used plus two partial bags for leveling around the bases. The 10% waste factor was the right call.
How the Bag Sizes Work — and Why the Alternatives Column Matters
When you select a bag size, the calculator does two things with it. First, it uses that bag’s yield to compute your primary bag count — ceiling of (total cubic feet ÷ yield per bag). Second, it displays alternative counts for three other bag sizes in the same weight family, so you can compare before buying.
Selecting a metric bag (20 kg, 25 kg, 30 kg, 40 kg) shifts the entire output to cubic meters — both the hero volume and the breakdown card — even if you entered your dimensions in feet. The inverse is also true: switching your dimension units to meters or centimeters triggers metric output even with a US bag selected. The calculator isn’t mixing systems; it’s reading your intent from whichever signal comes first.
Bag yields used in the calculation match the standard figures stated on manufacturer packaging in North America — 80 lb bags at 0.60 cu ft, 60 lb bags at 0.45 cu ft, and so on down the line. These are mixed-yield figures, not wet-volume figures. Actual yield varies slightly with water content and mix type; the numbers here are conservative enough to land you short of a full bag rather than short of a pour.
Where Bagged Concrete Stops Making Sense
Ready-mix trucks typically require a minimum order of 3 to 4 cubic yards, and most suppliers charge a short-load fee below that. For small projects — under 1 yard — bagged mix is almost always the right call logistically. Between 1 and 3 yards, it depends on site access, labor, and whether you can rent a mixer. Above 3 yards, hand-mixing bag by bag becomes impractical and often more expensive per yard than ready-mix delivery.
At 80 lb bags and 0.60 cu ft per bag, one cubic yard requires 45 bags. Three cubic yards — the typical ready-mix minimum — is 135 bags. That’s over 10,000 lbs of dry mix to move, open, mix, and pour.
The calculator doesn’t make a cost or feasibility recommendation because that threshold shifts with local ready-mix pricing, bag cost at your supplier, and how many hands are on site. But the bag count in the output gives you a fast way to sense-check whether you’re in “buy bags” territory or “call the plant” territory.
Frequently Asked Questions
Why does the Ring shape auto-adjust my Inner Diameter when I switch to it?
When you switch to Tube / Ring mode, the calculator checks whether the current Inner Diameter value is valid — meaning strictly less than the Outer Diameter. If it’s equal or larger, the result would be zero or negative volume, which isn’t a usable output. Rather than showing an error immediately, it sets Inner Diameter to half of Outer Diameter as a reasonable starting point. You can adjust it from there. The same validation runs on every calculation — if you manually type an inner value larger than the outer, the tool will stop and flag it.
I entered dimensions in feet and switched to a metric bag — why did the output change to cubic meters?
Selecting a kilogram-based bag size is treated as a signal that you want metric output, regardless of your dimension units. The volume calculations always happen in cubic feet internally, then convert as needed. If you’re working in a mixed environment — feet for dimensions, kilograms for bags because that’s what your supplier stocks — the output in cubic meters is still accurate. The conversion factor used is 1 cu ft = 0.0283168 cu m.
What happens if I enter a non-whole number for Quantity?
Quantity must be a positive whole number. The code explicitly checks whether the quantity has a decimal remainder, and if it does, it treats the input as invalid and stops the calculation. There’s no rounding — 1.5 footings isn’t a valid job input, so it isn’t accepted. If you have a mix of different-sized footings, calculate each set separately and add the bag counts yourself.
Does the waste factor apply before or after the quantity multiplier?
After. The calculator first multiplies the single-unit volume by quantity to get total base volume, then adds waste on top of that combined figure. So a 5% waste factor on 12 footings applies to the full 12-footing volume, not to each individual footing. The Volume Breakdown card shows base volume and waste added as separate line items so you can verify this directly.
Column and Circular Slab use the same formula — when would I pick one over the other?
They compute identically: both use π × radius² × depth. The distinction is purely contextual. Column / Hole mode is intended for vertical pours into tube forms or drilled holes where depth is measured vertically. Circular Slab is for horizontal pours where depth is the slab thickness. The label on the depth field stays “Depth / Thickness” in both cases, but choosing the shape that matches your project keeps your printed or copied results readable when you’re on the phone with your supplier or reviewing a takeoff.