Measurement & project guide
From a measured pour to an orderable volume
Use the shape that matches the actual pour. A slab, continuous footing and group of circular piers need different measurements, even when the supplier prices all three by the cubic yard.
- A Pour length
- B Pour width
- C Concrete thickness
Before you calculate
- Slab or footing
- Record length, width and thickness in the selected units. Keep footing width separate from wall thickness.
- Round piers
- Use the full diameter, concrete height and number of identical piers. Enter dissimilar sizes as separate takeoffs.
- Bags or ready mix
- Obtain the mixed yield per bag or the supplier's minimum delivery quantity. Bag weight alone does not establish yield.
Read the result before using it
- Net volume
- The geometric space occupied by the entered concrete. It excludes the explicit ordering allowance.
- Order volume
- The net volume after your allowance. Check the supplier's sales increments and short-load terms before booking.
- Bag count
- A whole-package quantity using the entered mixed yield. It does not include water or choose a concrete mix.
Check before ordering or handing over
- Record the mix designation and required delivery time separately from volume.
- Confirm access, discharge method, placing sequence and washout arrangements.
- Ask the designer to resolve reinforcement, exposure, strength and footing details before ordering.
Detailed results and planning tools
View calculations, report and comparisons
Calculation breakdown
These values reflect the editable inputs and assumptions above.
| Quantity | Value |
|---|
Calculation report
Includes formulas, entered assumptions, limitations and sources.
Compare scenarios
Keep up to three alternatives to compare measured values and assumptions.
Quick answer#
Choose the concrete shape, measure its dimensions and keep the geometric volume separate from the amount you intend to order. The calculator handles slabs, rectangular footings, round columns and a volume you have already measured.
Ready-mix volume and bagged-mix count are alternative purchasing views of the same quantity. Do not add the two results together.
Formula and calculation method#
For a slab or rectangular footing, volume = length × width × depth with all dimensions converted to the same unit. For a round column, use π × (diameter ÷ 2)² × height × column count.
In US units, divide cubic feet by 27 to obtain cubic yards. An allowance of 5% multiplies the base volume by 1.05; it is not another geometric dimension. Bag count divides the allowance-adjusted volume by the yield entered for one bag and rounds up.
Worked example#
A 20 ft × 10 ft slab at 4 in thickness contains 66.667 ft³, or 2.469 yd³. Adding a chosen 5% allowance gives 70 ft³, or 2.593 yd³.
If the selected bag product yields 0.6 ft³ per bag, 70 ÷ 0.6 = 116.667, so the purchase estimate is 117 bags. A different mix or bag size may have a different yield. Replace 0.6 with the exact product label rather than assuming all bags of a given weight produce the same volume.
Selecting inputs and project assumptions#
Measure thickness at representative locations. A nominal 4 in slab over an uneven excavation can require more concrete than a perfect rectangular volume suggests. Reflect measured changes in the geometry or use a documented allowance; do not count the same extra twice.
For multiple round posts, confirm that the input is diameter rather than radius. Doubling a circle's diameter multiplies its area by four. Known volume is useful when a plan or takeoff already supplies the quantity; entering that value does not need another length-and-width calculation.
For ready mix, ask the supplier about minimum orders, delivery, waiting time and any short-load charges. For bags, check yield and the mixing-water instructions for the actual product.
Assumptions and limitations#
This page estimates volume, not structural capacity or a concrete mix design. It does not select footing dimensions, slab thickness, reinforcement, strength, exposure class or curing procedure. Optional prices use entered figures and may need delivery or other project charges added separately.
Sources and methodology#
- ACI guide to ordering ready-mixed concrete — product yield and volume context.
- NIST conversion factors — U.S. customary and SI unit conversions.
Frequently asked questions
How many cubic yards do I need for a slab?
Multiply slab length, width and thickness in consistent units, divide cubic feet by 27, then apply an explicit project allowance.
Can I calculate round columns?
Yes. Select Round columns, enter actual diameter, height and count.
Does the calculator design my footing?
No. It calculates volume from the dimensions you enter; the engineering design belongs to the project professional.
Is the bag count based on 80 lb bags?
No weight is assumed. It divides volume by an editable cubic-foot yield per bag; enter the yield printed on the exact product.
Can I enter a volume I already measured?
Yes. Choose Known volume, then add an order allowance if appropriate.
Are ready mix and bags added together?
No. They are alternative ways to purchase the same estimated volume.
Does the price include delivery?
Only if you enter delivery/fixed costs and any applicable tax; no current supplier quote is fetched.