Free estimate — verify against local code before building
Concrete Calculator
Audit raw volume, waste, bags, ready-mix increments, and editable costs for common concrete shapes.
What this calculator includes
Choose a concrete shape, enter its measured dimensions, and see the raw cubic feet and yards before waste, the waste allowance, bag rounding for 40, 60, or 80 lb mixes, and ready-mix rounding to your supplier's increment. Rectangular slabs can also include explicit base, straight form-board, remesh, or user-entered rebar-grid planning allowances. The calculator estimates material geometry only: it does not determine concrete strength, member thickness, bearing, reinforcement, form pressure, stairs, foundations, joints, curing, drainage, or code compliance.
How to use this concrete calculator
- 01
Choose the geometry that matches the pour
Use a slab, circle, strip footing, wall, annular round column or post hole, sloped-waist stair flight, or simplified rectangular curb-and-gutter section. Split irregular work into separate calculations.
- 02
Enter measured dimensions
Use the dimensions that will actually be formed or excavated. For stairs, the calculator adds a constant sloped waist, equal triangular step wedges, and an optional landing; confirm that model matches the drawings.
- 03
Add a project-specific allowance
NRMCA ordering guidance notes that concrete is commonly ordered above calculated dimensions. Enter the waste percentage appropriate to subgrade variation, forms, placement, and supplier guidance rather than relying blindly on a default.
- 04
Compare bags with ready-mix
Bag counts always round up by the selected product's published approximate yield. Ready-mix quantity rounds up to the supplier increment you enter; ask about minimums, short-load, delivery, waiting-time, fuel, washout, and cancellation charges.
- 05
Verify design before ordering
Have the drawings, designer, building official, and supplier confirm dimensions, concrete specification, reinforcement, support, joints, embeds, forms, access, placement rate, finishing, curing, weather plan, testing, and locally adopted code requirements.
Calculation sources and review
Primary references and formula assumptions are linked so you can verify them against the selected product, supplier, and adopted local requirements.
Internal formula review completed July 13, 2026. What this review covers
- QUIKRETE Concrete Mix No. 1101 technical data (opens in a new tab)
Published approximate yields of 0.60, 0.45, and 0.30 cubic feet for 80, 60, and 40 lb bags.
- NRMCA CIP 31 - Ordering Ready Mixed Concrete (opens in a new tab)
Ready-mix ordering information, cubic-yard basis of sale, and guidance to order additional quantity for waste and dimensional variation.
- ACI 318 Building Code Portal (opens in a new tab)
Current structural concrete materials, design, detailing, construction, and inspection reference; locally adopted requirements control.
- 2024 International Residential Code - Section R506 (opens in a new tab)
Residential concrete floor provisions and site-preparation context; verify the edition and amendments adopted locally.
Frequently asked questions
How many 80 lb bags of concrete make one cubic yard?
At the manufacturer's approximate 0.60 cubic foot yield, exactly 27 cubic feet converts to 45 bags before any waste. A 60 lb bag at 0.45 cubic foot takes 60 bags, and a 40 lb bag at 0.30 cubic foot takes 90 bags.
Why are ready-mix yards higher than the calculated volume?
The calculator first adds your waste allowance, then rounds upward to the supplier increment you entered. Supplier minimums and billing rules vary and are not added automatically.
How does the post-hole option subtract a wood or steel post?
It subtracts the inner circular cross-section from the outer hole or column cross-section, then multiplies by depth and count. Use zero inner diameter for a solid round column. Irregular holes still need a field allowance.
How is concrete for stairs calculated?
The stair mode adds a constant-thickness sloped waist, equal triangular wedges for the entered step modules, and an optional landing using the same thickness. It is a volume model only and does not design the stair, landing, reinforcement, support, headroom, riser/tread geometry, guard, or formwork.
Does this calculator tell me what rebar or concrete strength to use?
No. The optional rectangular-slab inventory only converts user-entered spacing or remesh coverage into stock quantities. The approved design and locally adopted code must specify concrete properties, reinforcement size and grade, spacing, laps, cover, support, joints, and inspection.