Concrete Calculator
Estimate concrete volume in cubic yards, bag quantity, waste allowance and ready-mix material cost for a rectangular slab or pour.
Project details
Estimate
Bag yields are approximate planning values. Verify the yield printed on the product you plan to buy.
How to calculate concrete volume
For a rectangular slab or pad, start with the inside dimensions of the forms. Convert the thickness from inches to feet, multiply length × width × thickness to get cubic feet, then divide by 27 because one cubic yard contains 27 cubic feet. Add a waste allowance only after calculating the base volume.
Worked example: 20 ft × 10 ft × 4 in
First convert 4 inches to feet: 4 ÷ 12 = 0.333 ft. Then multiply 20 × 10 × 0.333 = about 66.67 ft³. Divide by 27 to get about 2.47 yd³ before waste. With a 10% planning allowance, 2.47 × 1.10 = about 2.72 yd³.
Concrete calculation examples
These examples use the same rectangular-volume formula as the calculator. The final column adds 10% as a planning allowance; your project may justify a different allowance.
| Project | Cubic feet | Before waste | With 10% |
|---|---|---|---|
| 10 ft × 10 ft × 4 in | 33.33 ft³ | 1.23 yd³ | 1.36 yd³ |
| 20 ft × 20 ft × 4 in | 133.33 ft³ | 4.94 yd³ | 5.43 yd³ |
| 12 ft × 24 ft × 6 in | 144.00 ft³ | 5.33 yd³ | 5.87 yd³ |
These are estimating examples, not project specifications. Re-measure your actual forms and depth before purchasing material.
How much extra concrete should you plan for?
A waste allowance helps account for small measurement differences, uneven subgrade, spillage and placement losses. It is a planning margin—not a building-code requirement and not a substitute for measuring carefully.
Concrete bag sizes and yield
Bagged concrete is sold in several package sizes, commonly including 40 lb, 50 lb, 60 lb and 80 lb bags depending on the product and retailer. The important number for estimating is the yield in cubic feet printed by the manufacturer, not the bag weight by itself.
This calculator currently offers planning assumptions of approximately 0.30 ft³ for a 40 lb bag, 0.45 ft³ for a 60 lb bag and 0.60 ft³ for an 80 lb bag. Those values are estimates only; the actual product label or technical data sheet should control your purchase calculation.
Because stores do not sell partial bags, round the calculated bag count up. If your chosen product has a different stated yield, use that product-specific yield rather than relying on a generic bag-size assumption.
Concrete bags vs. ready-mix
Bagged concrete and ready-mix can both work well, but the practical choice changes with project size, access and labor. Avoid choosing only by the advertised material price.
Compare the whole job
Consider the number and weight of bags, vehicle capacity, lifting, mixing time, mixer capacity, available helpers, placement speed, truck access and any delivery or short-load charges. There is no universal break-even quantity because local material prices and supplier policies vary.
For a larger project, use this calculator to estimate the bag count and cubic yards, then compare that workload and material estimate with a local ready-mix quote.
Common concrete estimating mistakes
How to estimate an irregular slab
When a project is not one clean rectangle, divide the footprint into smaller rectangles whenever practical. Calculate the concrete volume for each section using its own length, width and thickness, then add the cubic yards together before applying your chosen waste allowance.
Curves, changing thicknesses and highly irregular excavations create more uncertainty, so careful field measurements matter more than forcing the project into a single simplified dimension.
Before you buy or order concrete
Use this quick check after calculating and before spending money:
- Re-measure the inside length and width of the forms.
- Check the planned depth or thickness at several locations.
- Confirm that feet and inches were entered in the correct fields.
- Choose a waste allowance that reflects the actual uncertainty of the job.
- Verify the cubic-foot yield printed on the exact bagged product.
- Round bag quantities up to whole bags.
- For large bag counts, compare the workload and cost with a local ready-mix quote.
- If using ready-mix, confirm truck access, delivery rules, minimums and order increments.
- Check applicable local project requirements before construction.
Concrete calculator FAQ
How many cubic feet are in one cubic yard?
There are 27 cubic feet in one cubic yard. Divide cubic feet by 27 to convert a volume estimate to cubic yards.
How many cubic yards do I need for a 10 × 10 slab?
A 10 ft × 10 ft slab at 4 inches thick is about 1.23 yd³ before waste. With a 10% planning allowance, it is about 1.36 yd³. A different thickness changes the result.
How much concrete do I need for a 4-inch slab?
Thickness alone is not enough to calculate volume. Enter the slab's actual length and width along with 4 inches of thickness; the calculator converts the thickness and returns cubic yards.
Should I round ready-mix concrete up?
Use the calculated quantity as a planning starting point, then confirm supplier minimums, order increments and delivery policies. Do not assume every supplier accepts the exact decimal quantity shown by a calculator.
Why can bag counts differ from a store calculator?
Different concrete mixes and manufacturers can state different yields. Check the yield on the exact bag or technical data sheet and use product-specific information when available.
Is 10% waste always required?
No. A waste allowance is a planning choice, not a universal requirement. Simple, accurately measured work may need less margin, while irregular or uncertain conditions may justify more.
Can I use this calculator for an irregular patio?
Yes, if you divide the patio into simpler rectangular sections, calculate each section separately and add the volumes together. Curves or varying depth can require additional judgment.
When should I consider ready-mix instead of bags?
There is no single cutoff. As the required bag count grows, compare transportation, lifting, mixing time, labor, placement speed and local ready-mix delivery charges—not just material price.
How to use this concrete estimate in a real project
Estimate slab, footing and general concrete volume, then translate the result into orderable ready-mix or bag quantities.
Worked planning check: 12 ft × 16 ft patio at 4 in thick
12 × 16 × (4/12) = 64 ft³. Divide by 27 = 2.37 yd³ before waste. A 7% planning allowance gives about 2.54 yd³. Round to the supplier’s order increment only after confirming dimensions.
What to verify before you buy
- Measure the project after the layout is fixed, not from an early sketch.
- Replace every default value with the specification, package coverage or local quote for the exact material you will use.
- Round to whole bags, boards, sheets, bundles or cartons only at the final purchasing step.
- Keep delivery, tax, labor, equipment, permits and structural/code requirements separate unless a calculator field explicitly includes them.
Reference sources and cross-checks
- QUIKRETE Concrete Calculator — Manufacturer calculator notes that yields are approximate and do not include waste.
- QUIKRETE Mixing & Placing Concrete training — Manufacturer training material shows common slab-volume math and approximate bag yields.
- Inch Calculator — Concrete Calculator — Useful benchmark for presenting ready-mix quantity, premix bags, cost assumptions and worked explanations together.
BuildCalc Guide is an independent planning tool. External references are used to check terminology, typical estimating methods and manufacturer-specific cautions; they do not sponsor or endorse this site. Product labels, project documents and locally adopted codes take priority.