Concrete planning · Updated November 3, 2026
Concrete slab and round form volume calculator
Estimate rectangular slab or round form tube concrete volume, then calculate bag-equivalents from the exact mix yield and allowance you enter.
Enter one rectangular slab section per line as length × width × thickness, using feet for length and width and inches for thickness. Use separate lines for sections with different dimensions. Add the finished yield printed on the exact concrete mix you plan to use. The calculation runs in your browser and is a material estimate, not a structural design or quote.
Enter values and calculate.
Read yield from the current label or product data sheet. As one product-specific reference, the QUIKRETE Concrete Mix No. 1101 data sheet lists approximate yields of 0.30, 0.375, 0.45, 0.60 and 0.675 cu ft for its 40, 50, 60, 80 and 90 lb bags, respectively; the 90 lb size is regional. These values are not universal: use the exact mix and package you selected. NRMCA says ready-mixed concrete orders should account for 4%–10% above plan-dimension estimates; conditions vary, so this tool leaves the allowance to you and does not set a default.
Round form tube volume and bag estimate
Enter one round form or column group per line as Label: inside diameter in × fill height ft × quantity. Enter the measured inside diameter when it matters; manufacturer tube sizes may be nominal. Use the finished yield from the exact mix package you selected. This estimates concrete volume only and does not size or approve a footing, post, column or form.
Enter values and calculate.
The cylinder calculation uses your entered inside diameter and fill height. It does not include bells, enlarged bases, over-excavation, embedded posts or uneven holes.
Sources and method
- QUIKRETE concrete calculator (reviewed November 3, 2026): its bag counts round up; the manufacturer notes its yield estimates do not allow for uneven substrate or waste.
- QUIKRETE: Mixing and Placing Concrete (reviewed November 3, 2026): calculate rectangular slab volume by multiplying length, width and thickness; divide volume by the selected bag's yield.
- NRMCA, Concrete in Practice 31: Ordering Ready Mixed Concrete (reviewed November 3, 2026): ready-mix quantity is sold by volume and the guide calls for 4%–10% over plan-dimension estimates to account for contingencies.
- QUIKRETE Concrete Mix No. 1101 data sheet: check the exact product information before using a yield value.
- Sonoco Sonotube Builder's Tube guide (accessed November 3, 2026): lists concrete volume per foot for nominal 6, 8, 10 and 12 inch forms, and says to measure if diameter is critical.
- QUIKRETE QUIK-TUBE pillar foundations (accessed November 3, 2026): explains the cylinder area × height volume method and lists product-specific bag estimates by diameter.
Slab formula: section volume (cu ft) = length × width × (thickness ÷ 12); add section volumes for the total. Round form formula: volume per form (cu ft) = π × (measured inside diameter in ÷ 24)² × fill height in ft; multiply by the entered quantity and add form groups. Cubic yards = cubic feet ÷ 27. Estimated whole bags = ceil(total volume × (1 + allowance ÷ 100) ÷ user-entered bag yield). The NRMCA range applies to its ready-mixed ordering guidance; choose an allowance that fits the product and site. This tool does not choose mix, thickness, reinforcement or structural requirements.
How it works
Enter one rectangular slab section per line as length × width × thickness, in feet × feet × inches. Add the finished volume yield per bag from your selected concrete mix label or data sheet and any allowance you choose. The estimate adds all sections, reports cubic feet and cubic yards, shows the unrounded theoretical bag-equivalent count, then rounds up to whole bags. For round forms, enter measured inside diameter, fill height and whole quantity per group; the calculator uses cylinder volume and the selected mix yield without sizing the footing or form. For round forms, enter measured inside diameter, fill height and whole quantity per group; the calculator uses cylinder volume and the selected mix yield without sizing the footing or form.
Good to know
Quick answer: how much concrete do I need?
For each rectangular slab section, multiply length by width by thickness in feet to get cubic feet; add the sections and divide by 27 for cubic yards. To estimate bags, apply your chosen allowance, divide by the selected mix's finished yield per bag to see the theoretical bag-equivalent count, then round up to whole bags. Yield varies by product, so use the current label or data sheet. Reviewed November 3, 2026.
Where do I find bag yield?
Use the finished yield shown on the exact product's current label or data sheet. QUIKRETE Concrete Mix No. 1101 lists approximate yields by bag size; other products, including specialty mixes, can differ.
What does the allowance cover?
The allowance is an optional value you choose for measurement variation, uneven subgrade or placement loss. It is not a recommendation. NRMCA's 4%–10% guidance concerns ordering ready-mixed concrete; select an allowance appropriate to the product and site. QUIKRETE says its calculator yields are approximate and do not account for uneven substrate or waste.
Does this tool size a footing or structural slab?
No. It estimates material volume for a rectangular pour only. It does not determine thickness, reinforcement, mix strength, structural design or code compliance; follow project plans, product instructions and local requirements.
How do I estimate concrete for round form tubes?
For each round form, volume in cubic feet = π × (inside diameter in inches ÷ 24)² × fill height in feet. Multiply by the number of identical forms and add groups. For bag estimates, use the finished yield on the exact selected mix package. Tube diameters may be nominal; measure the inside diameter when it matters. This is a volume estimate only, not footing or form design. Reviewed November 3, 2026.
How do I estimate concrete for round form tubes?
For each round form, volume in cubic feet = π × (inside diameter in inches ÷ 24)² × fill height in feet. Multiply by the number of identical forms and add groups. For bag estimates, use the finished yield on the exact selected mix package. Tube diameters may be nominal; measure the inside diameter when it matters. This is a volume estimate only, not footing or form design. Reviewed November 3, 2026.
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