How much concrete do you need from these dimensions?
The order volume below starts with the exact geometric volume and then adds only the contingency percentage you selected.
Change a dimension, shape, quantity or waste allowance and this interpretation updates automatically.
Continue your construction estimate
Use the concrete quantity as the starting point, then continue with the materials that sit below, above or beside it.
Concrete calculator for slabs, footings, walls and columns
A useful concrete calculator should do more than multiply three dimensions. On a real job you normally need to know the geometric volume, how much extra to allow, whether the supplier quotes in cubic metres or cubic yards, how many bags a small project may take and roughly how much the material weighs.
This calculator handles rectangular concrete such as slabs, strip footings, pads and walls, plus cylindrical concrete such as round post holes, piers and columns. Set the number of identical elements when several footings or posts share the same dimensions. The result then separates net volume from order volume including your allowance, so the contingency is visible instead of being hidden inside the answer.
For bagged concrete, enter the mixed yield shown for the product you intend to buy. For ready-mix, enter your supplier's local price only if you want a cost estimate. Leaving price at zero keeps the calculator focused on quantity.
Where to get the input values for a concrete estimate
The formula is simple, but the quality of a concrete quantity takeoff depends on the inputs. Use dimensions from the latest drawing, formwork layout or a careful site measurement. For excavated footings and post holes, measure the space that concrete will actually fill rather than relying only on a nominal drawing dimension if the excavation has changed.
- Length, width, thickness, diameter and depth: take these from drawings, formwork or measured excavation. Keep units consistent.
- Quantity: count only genuinely identical elements. Separate footings of different sizes into separate calculations.
- Waste / contingency: choose it from project conditions rather than treating the default as a rule. The Construction Material Waste Allowance Table gives planning ranges and reasons to adjust them.
- Mixed yield per bag: use the yield printed on the bag or in the manufacturer data sheet. Bag weight alone is not the same as finished mixed volume.
- Concrete density: use the specified mix or supplier value when weight matters. The Building Material Density Table is only a planning reference.
- Ready-mix and bag prices: use current supplier quotations. Delivery, pump hire, short-load charges and taxes are not added unless they are already included in the price you enter.
Rectangular slab, wall or footing
For a rectangular element, volume is length × width × thickness or height. In metric mode, length and width are entered in metres while thickness is entered in centimetres. In US mode, length and width use feet and thickness uses inches.
This mode works for patios, floor slabs, shed bases, concrete walls, strip footings and rectangular pads. Measure the actual space that concrete will occupy, not just the nominal outside dimensions of a project.
Round post hole, pier or column
For a round element, the calculator treats the concrete as a cylinder. Enter the diameter rather than the radius and use depth or height for the cylinder length.
This is useful for round fence-post holes, deck piers, sonotube-style forms and columns. If a hole is wider at the bottom, irregular or bell-shaped, a simple cylinder can underestimate the real volume.
What affects the concrete quantity most?
For a rectangular slab, footing or wall, length, width and thickness all affect volume directly. A 10% increase in thickness produces a 10% increase in net concrete volume when the other dimensions stay unchanged. Quantity also scales the result directly.
For a round hole or column, diameter is especially influential because the formula contains diameter squared. Increasing diameter by 10% increases the circular cross-sectional area by about 21%, before depth and quantity are considered. The waste percentage changes the order volume, but it does not change the measured net geometry.
Does your concrete result look realistic?
Run a quick reasonableness check before ordering. For a rectangular element, multiply the dimensions manually once. A 6 m × 4 m slab at 12 cm thick is 6 × 4 × 0.12 = 2.88 m³ before allowance. If the calculator shows roughly ten times that amount, thickness units are the first thing to check.
The order volume should be the net volume multiplied by 1 + allowance ÷ 100. Bag count should also be consistent with the entered mixed yield. If either relationship looks wrong, verify units and product data before using the result for purchasing.
How much extra concrete should you allow?
The calculator does not force a universal waste percentage because site conditions vary. A formed slab with carefully controlled dimensions can be very different from excavated footings in uneven soil. Extra concrete may be needed for over-excavation, variations in thickness, spillage, form movement or measurement error. If you need a starting reference rather than a fixed rule, compare the reasons and ranges in the construction material waste allowance table.
The result therefore shows both numbers. If the net requirement is 2.88 m³ and you choose 7%, the order calculation becomes 3.0816 m³ before any supplier-specific rounding or minimum-order rule. You can change the percentage and immediately see how much volume that decision adds.
For a large pour, also ask the ready-mix supplier how they accept orders and whether they recommend rounding to a practical delivery increment. For structural work, quantity planning does not replace engineering or site supervision.
Concrete bags calculator: use mixed yield, not just bag weight
When you are working out how many bags of concrete you need, bag weight alone is not enough; volume planning is more direct when you know the product's mixed yield. The calculator asks for yield per bag in litres or cubic feet, depending on the selected unit system.
The bag count is calculated from the order volume including your selected allowance and is rounded up to a whole bag. If your project requires 246.2 bags by volume, the shopping quantity is 247 bags. Always replace the default yield with the value for the actual mix you plan to purchase.
For larger projects, bag count can become a useful warning that ready-mix may be operationally easier. The calculator shows both paths without assuming which option is cheaper in your area.
m³, cubic feet or cubic yards?
Metric ready-mix is commonly planned in cubic metres, while US projects often use cubic yards. One cubic yard equals 27 cubic feet. Switch unit systems and the calculator converts the same physical volume, so you can check a supplier quote without doing a separate conversion. For other construction measurements, use the Construction Unit Conversion Table.
Concrete weight estimate
The weight result is volume × density. The default density is only a planning value and can be changed. In metric mode the input is kg/m³; in US mode it is lb/ft³. Density does not change the quantity of concrete required to fill the form—it only changes the estimated mass represented by that volume.
This can help with rough logistics, but it is not a transport or structural load calculation. Fresh concrete properties vary with the mix, aggregate, moisture and specification. Use project and supplier data when weight matters operationally.
For construction students: concrete quantity takeoff from a drawing
In a construction technology or estimating class, the important skill is not memorising one calculator result. It is recognising the measurement type, converting units and keeping net quantity separate from order quantity. A slab is a volume calculation, so a thickness given in millimetres or centimetres must be converted to the same length basis used by the plan dimensions.
Exercise: a training drawing shows a slab 5.5 m long, 3.8 m wide and 100 mm thick. Use an 8% ordering allowance and a bagged-mix yield of 12.5 L per bag. Calculate the net concrete volume, the order volume and the number of whole bags.
On a real project, the drawing also controls concrete grade, reinforcement, cover, joints and structural dimensions. This calculator is a quantity-takeoff tool, not a substitute for those design requirements. Continue with the Construction Estimating Academy for the underlying length-area-volume workflow.
Practical concrete quantity scenarios
Use these scenarios to test the calculator. “Load values” places the example directly into the form. “Show answer” calculates the solution with the same formula used by the main calculator.
Patio slab
6 m × 4 m × 12 cm, one slab, 7% allowance.
Eight round footings
8 holes, 35 cm diameter × 80 cm deep, 5% allowance.
US walkway
24 ft × 4 ft × 4 in, one pour, 10% allowance.
Common mistakes when estimating concrete
- Mixing units: centimetres, metres, inches and feet must be converted before multiplying dimensions.
- Using area instead of volume: a 24 m² slab still needs a thickness before concrete volume can be calculated.
- Forgetting quantity: one pier volume must be multiplied by the number of identical piers.
- Using nominal excavation dimensions: actual holes and trenches can be larger than the planned drawing.
- Treating bag weight as mixed volume: use the stated mixed yield for the specific product.
- Hiding waste inside dimensions: it is usually clearer to enter measured dimensions and apply an explicit allowance.
How to use the result before ordering concrete
First verify every dimension and the number of repeated elements. Then decide whether the selected contingency matches the way the concrete space will actually be formed or excavated. For bagged mix, confirm the bag yield. For ready-mix, confirm whether the supplier wants cubic metres or cubic yards and whether delivery has minimum quantities or rounding rules.
Print the report if you are comparing alternatives. It includes the active shape dimensions, unit system, allowance, density, bag yield, optional prices, all calculated outputs and the current interpretation. That makes it easier to compare a 100 mm slab with a 120 mm slab or 5% with 10% allowance without losing the assumptions behind each estimate.
If the concrete is part of a footing or foundation, do not stop at the pour volume. Use the Foundation Material Planning guide to keep excavation, sub-base, concrete, reinforcement and backfill as separate quantities instead of mixing them into one number.
Learn the estimating method
Go deeper into the method or check reference values without losing the context of this calculation.