Concrete Calculator to estimate slabs, footings, post holes, circular pours, and tube/ring volume. Formula: volume = length × width × depth; bags = total volume ÷ bag yield.
Use this concrete calculator to estimate how much concrete you need for any project. Enter your dimensions and the tool instantly returns the volume in cubic yards, cubic feet, and cubic meters, along with the number of bags required at your chosen bag size. Supported shapes include slabs, patios, sidewalks, rectangular footings, round post holes, columns, circular slabs, and hollow tube or ring pours.
The calculator uses two core formulas. For flat or rectangular projects: Volume = Length × Width × Thickness. For round shapes: Volume = π × radius² × depth. Bag count is calculated as total volume divided by the yield per bag, then rounded up to the next whole bag so you never run short. A waste factor — defaulting to 5% — is added automatically to cover spillage, uneven subgrade, and minor measurement variation.
How the Concrete Calculator Works
The calculator converts the dimensions you enter into a concrete volume estimate, then translates that volume into bag counts or ready-mix quantities.
For rectangular projects — slabs, patios, sidewalks, and rectangular footings — the tool multiplies length by width by depth to get the base volume in cubic feet. For round shapes, including post holes, columns, and circular slabs, it uses the cylinder formula: π times the radius squared times the depth. For tube and ring pours, it calculates the volume of the outer cylinder and subtracts the volume of the inner cylinder, giving only the material that actually fills the hollow wall.
Once the base volume is calculated, the waste factor is applied. If you enter 5% waste, the tool multiplies the base volume by 1.05 to get the total volume you should plan to order or purchase. Bag count is then derived by dividing that total volume by the yield per bag and rounding up to the next whole bag — because a partial bag still needs to be purchased.
Concrete Calculator Formula
The following formulas power every calculation in this tool.
Rectangular slab, patio, sidewalk, or footing:
Volume = Length × Width × Thickness
All three dimensions must be in the same unit before multiplying. The result is in cubic feet if you use feet, or cubic meters if you use meters.
Round hole, column, or circular slab:
Volume = π × radius² × depth
Radius is half the diameter. For a 12-inch diameter post hole, the radius is 0.5 ft. Pi (π) is approximately 3.14159.
Tube or ring (hollow):
Volume = (π × outer radius² × depth) − (π × inner radius² × depth)
This subtracts the hollow interior so only the concrete-filled wall is counted.
Waste adjustment:
Total volume = base volume × (1 + waste ÷ 100)
A 5% waste factor means multiplying by 1.05.
Converting cubic feet to cubic yards:
Cubic yards = cubic feet ÷ 27
There are 27 cubic feet in one cubic yard. This is the standard unit used when ordering ready-mix concrete.
Bags needed:
Bags = total volume ÷ bag yield, rounded up
Bag yield varies by product weight and brand. Always check the printed yield on the bag you are using.
How to Calculate Concrete for a Slab
For a flat slab, patio, or sidewalk, select the Slab / Patio / Sidewalk shape. Enter the length, width, and thickness. Thickness is typically 4 inches for a standard residential slab and 6 inches for driveways or heavier loads.
Because thickness is often measured in inches while length and width are measured in feet, the calculator handles unit conversion automatically. To do it manually, divide inches by 12 to convert to feet before multiplying.
Worked example — 10 ft × 10 ft × 4 in slab:
- Convert thickness: 4 in ÷ 12 = 0.333 ft
- Base volume: 10 × 10 × 0.333 = 33.33 cu ft
- With 5% waste: 33.33 × 1.05 = 35.00 cu ft
- Convert to cubic yards: 35.00 ÷ 27 = 1.30 cu yd
- 80 lb bags at 0.60 cu ft yield: 35.00 ÷ 0.60 = 58.3 → rounded up to 59 bags
This is the default example pre-loaded in the calculator. Change any dimension to instantly see updated results for your own slab, driveway, or sidewalk pour.
Concrete Bags vs Concrete Yards
The right unit depends on the size of your project and how you are sourcing the material.
Bags of concrete are the practical choice for smaller DIY jobs: fence post holes, small slabs, steps, repairs, and footings for sheds or mailboxes. You buy only what you need and mix on site. This concrete calculator returns a bag count for every supported bag size so you can shop accurately.
Cubic yards are the standard unit for ordering ready-mix concrete delivered by truck. Any pour larger than roughly a cubic yard or two is typically more cost-effective and faster to complete with ready-mix rather than bagged product. Use the Concrete Volume Needed result in cubic yards when calling a ready-mix supplier.
Bag count is always rounded up to the next whole bag because a partial bag must still be purchased. The table below shows approximate yields for common bag sizes.
| Bag Size | Approximate Yield |
|---|---|
| 40 lb bag | ~0.30 cu ft |
| 50 lb bag | ~0.375 cu ft |
| 60 lb bag | ~0.45 cu ft |
| 80 lb bag | ~0.60 cu ft |
| 90 lb bag | ~0.68 cu ft |
Actual yield varies by product, water ratio, compaction, and the specific mix formula. Always check the yield printed on your Sakrete, Quikrete, or other concrete mix bag before finalizing your order.
Ready Mix Concrete Calculator Use
Ready-mix concrete is ordered by the cubic yard in the United States and by the cubic meter in metric countries. When using this tool as a ready-mix concrete calculator, use the Concrete Volume Needed figure shown in cubic yards or cubic meters as the basis for your order.
The calculator displays results in cubic yards, cubic feet, and cubic meters simultaneously, so you can communicate clearly with your supplier regardless of which unit they quote in.
Before placing a ready-mix order, confirm the following with your local supplier:
- Minimum load size — many suppliers have a minimum delivery quantity, often around 1 cubic yard.
- Short-load fees — orders below the minimum may carry a surcharge.
- Supplier rounding — some suppliers round to the nearest quarter or half yard. Order slightly above the calculator estimate to account for this.
This tool estimates volume only and does not account for concrete mix design, compressive strength specification, or admixture requirements. Work with your supplier or engineer for structural applications.
Concrete Calculator for Footings, Post Holes, and Sonotubes
This concrete calculator handles footings, fence post holes, columns, and sonotube-style pours directly.
Rectangular footings (such as strip footings or pad footings with a square or rectangular cross-section): select Footing / Rectangular and enter the length, width, and depth of the footing.
Fence post holes and round columns: select Hole / Column / Round Footing. Enter the diameter of the hole and the depth. If you are setting multiple posts, enter the total number in the quantity field — the calculator multiplies volume by quantity automatically.
Sonotube pours: a sonotube is a round cardboard form used for columns and piers. If you are filling the sonotube completely solid, use Hole / Column / Round Footing and enter the tube's outer diameter and depth. If the tube has a hollow core and you are only filling the wall (less common in residential use), select Tube / Ring and enter both the outer diameter and the inner diameter.
For all round shapes, enter the full diameter — not the radius. The calculator divides by two internally to apply the cylinder formula correctly.
What the Calculator Results Mean
The output panel displays several labeled results. Here is what each one means.
Concrete Needed / Concrete Volume Needed — the total estimated volume of concrete required, including the waste factor. This is the primary number to use when ordering.
Cubic Feet — the total volume expressed in cubic feet. Useful for cross-checking against bag yields.
Metric Volume — the total volume in cubic meters, for metric-unit projects or metric supplier quotes.
Volume Breakdown — a summary showing base volume and the waste added separately, so you can see exactly how the total was reached.
Waste Added — the extra volume added by the waste factor as a concrete amount. Displayed separately for transparency.
Quantity — the number of identical pours included in the estimate. If you entered 6 fence post holes, this confirms 6 units were used in the calculation.
Required Bags — the number of bags at your selected bag size, rounded up to the nearest whole bag.
Alternative Bag Sizes — the equivalent bag counts at all other supported bag sizes, so you can compare options at a glance.
Suggested Ordering Check — a reminder to verify the yield on the specific bag product you plan to use, since actual yield can vary.
Common Concrete Estimating Assumptions
This calculator provides volume estimates based on the dimensions you enter. Several real-world factors can affect how much concrete you actually use on site.
Thickness is the actual concrete depth. Enter the thickness of the concrete section itself, not the depth of the excavation or form. If your form is 5 inches deep but you only intend to pour 4 inches of concrete, enter 4 inches.
Ground is assumed to be level. Uneven subgrade, low spots, or soft areas can increase the amount of concrete needed. The waste factor helps offset this, but a significantly uneven base may require more material than the estimate shows.
Forms and excavation tolerance vary. Wood forms can flex slightly. Excavated earth walls are rarely perfectly vertical or dimensionally exact. These variations change the true volume of the pour.
Waste factor covers common overages. The default 5% waste factor accounts for typical spillage, minor depth variation, and rounding. For larger or more complex pours, some contractors increase this to 8–10%.
Bag yields are approximate. The yields shown in this tool are general industry averages. The actual yield of a specific bag depends on the product formulation, how much water is added, and compaction during placement. Check the yield printed on your bag before finalizing quantities.
This tool estimates volume only. It does not design structural concrete, specify mix strength, calculate rebar or reinforcement requirements, or account for load conditions. Consult a licensed engineer for structural applications.
Concrete Calculator FAQs
How do I calculate how much concrete I need?
Multiply length × width × thickness for rectangular shapes (all in the same unit), or use π × radius² × depth for round shapes. Add a waste factor of 5–10% to the result. This concrete calculator handles all of that automatically — just enter your dimensions and select a shape.
How many bags of concrete do I need?
Divide the total volume (including waste) by the yield per bag, then round up. For an 80 lb bag at 0.60 cu ft yield, a 35 cu ft pour needs 59 bags. Use the calculator above and select your bag size to get an instant count.
How many 80 lb bags of concrete are in a cubic yard?
One cubic yard equals 27 cubic feet. At approximately 0.60 cu ft per 80 lb bag, you need about 45 bags per cubic yard. At 0.45 cu ft per 60 lb bag, that rises to about 60 bags. Exact counts depend on the specific product yield.
How much concrete do I need for a 10×10 slab?
A 10 ft × 10 ft slab at 4 inches thick requires approximately 1.30 cubic yards or 59 bags of 80 lb concrete (with 5% waste). At 6 inches thick, that increases to roughly 1.93 cubic yards. Use the calculator to adjust for your exact thickness.
How do I calculate concrete yards?
Calculate the volume in cubic feet first, then divide by 27 to convert to cubic yards. This concrete calculator in yards does the conversion automatically and displays cubic yards alongside cubic feet and cubic meters in the results.
Should I order concrete by bags or by ready-mix yards?
For small pours — post holes, small slabs, repairs — bagged concrete is convenient and cost-effective. For larger pours above roughly 1–2 cubic yards, ready-mix delivered by truck is usually faster and more economical. Use the cubic yards result from this calculator when calling a ready-mix supplier.
How much waste should I add to a concrete estimate?
5% is a reasonable default for straightforward pours with level ground and well-built forms. Increase to 8–10% for irregular shapes, uneven subgrade, complex forms, or when the consequences of running short are high.
Can this calculator be used for fence post holes?
Yes. Select Hole / Column / Round Footing, enter the hole diameter, depth, and number of posts in the quantity field. The calculator multiplies the single-hole volume by the quantity and returns total bags needed.
Can this calculator be used as a sonotube concrete calculator?
Yes. For a standard solid sonotube pour, select Hole / Column / Round Footing and enter the tube's inner diameter and depth. For a hollow-wall sonotube, select Tube / Ring and enter both the outer and inner diameters.
Is this the same as a Quikrete or Sakrete concrete calculator?
No. This is an independent concrete volume and bag estimator. It can estimate bags for any bagged concrete product — including Quikrete and Sakrete mixes — if you select the bag size that matches the product. Always check the yield printed on your Sakrete, Quikrete, or other concrete mix bag, as yields vary by product.
Why does the calculator round bag counts up?
Concrete bags are sold as whole units. If your pour requires 58.3 bags, you must purchase 59 bags — you cannot buy a fraction of a bag. Rounding up ensures you have enough material to complete the pour without running short.
Does this calculator estimate concrete cost?
Not directly, because material prices vary by location, supplier, and product. To estimate cost, multiply the bag count result by the price per bag at your local store. For ready-mix, multiply the cubic yards result by the price per yard quoted by your supplier.
References
- QUIKRETE® Concrete Mix Data Sheet — Provides approximate bag yields for common concrete mix bag sizes, including 40 lb, 50 lb, 60 lb, 80 lb, and 90 lb bags. These yield values support the bag-count logic used by this calculator.
- QUIKRETE® Concrete Mix Product Information — Manufacturer reference for common concrete mix applications such as sidewalks, floors, steps, patios, curbs, and general concrete work.
- Khan Academy — Volume Formulas Review — Reference for standard solid-volume formulas, including rectangular prism and cylinder volume calculations used for slabs, footings, columns, and round holes.
- NIST — SI Units and Measurement Reference — General measurement reference supporting unit consistency and conversion practice when comparing cubic feet, cubic yards, and cubic meters.