Concrete estimator
Concrete Calculator
Estimate concrete bags, cubic yards, waste, material cost, and optional labor for rectangular slabs and patios.
Country
Units
Dimensions
Enter the project measurements for this shape.
Measure finished length and width, then use depth for the installed material thickness.
Report Mode
Homeowner reports stay simple. Contractor reports include job details.
Materials
Buy what you need
Product suggestions are matched to this calculator material so you can compare package sizes and accessories before purchasing.
Concrete project supplies
Budget80 lb concrete mix bags
General-purpose bagged mix for slabs, patios, footings, and small repairs.
Search term: 80 lb concrete mix bag
Concrete project supplies
PremiumPortable concrete mixer
Rental-friendly mixer for larger bagged concrete pours.
Search term: portable concrete mixer
Add-ons
Common measuring and safety add-ons
Useful across measuring, setup, and cleanup.
Laser distance measure
Fast room and project measurements for length, width, height, and area.
Work gloves
General hand protection for mixing, cutting, carrying, and cleanup.
Safety glasses
Eye protection for mixing, cutting, sanding, and installation work.
How this concrete calculator works
This rectangular concrete calculator estimates the volume for a slab, pad, walkway panel, shed base, hot-tub pad, or small patio from length, width, and depth. In imperial mode, enter length and width in feet and depth in inches. In metric mode, enter length and width in meters and depth in centimeters. The calculator converts the measurements to square feet and cubic feet, applies the waste percentage, converts the total to cubic yards, and divides by the selected bag yield.
The default example is a 12 ft by 10 ft slab at 4 in deep with 10 percent waste and an 80 lb bag yield of 0.6 cubic feet. That is 120 square feet, 40 cubic feet before waste, 44 cubic feet after waste, and 74 bags after rounding up. Rounding up is intentional because partial bags are not useful on a job site and a short pour can leave a cold joint or uneven edge.
Use the output as a planning estimate, not a substitute for local code, soil, drainage, reinforcement, or ready-mix advice. Exterior slabs often need base preparation, control joints, rebar or mesh, forms, curing time, and access planning. Bagged concrete is practical for smaller projects; ready-mix delivery is usually more efficient when the required cubic yards climbs past what your crew can mix and place consistently.
Bag yield reference table
| Package | Approximate yield | Typical use |
|---|---|---|
| 40 lb concrete bag | 0.30 cu ft | Fence posts, patch work, very small pads |
| 50 lb concrete bag | 0.375 cu ft | Small repairs and narrow forms |
| 60 lb concrete bag | 0.45 cu ft | Small slabs where lifting weight matters |
| 80 lb concrete bag | 0.60 cu ft | Common patio, walkway, and pad estimate |
| Ready-mix concrete | 27 cu ft per cu yd | Larger pours with delivery access |
Bag yields vary by product, moisture, mix design, and measuring practice. Always compare this table with the yield printed on the bag or supplier quote you plan to buy. If your product says an 80 lb bag yields 0.60 cubic feet, the default package is appropriate. If it lists a different yield, update the package assumption before relying on the bag count.
Common mistakes
The most common mistake is entering slab depth in feet instead of inches. A 4 inch slab is 0.333 feet, not 4 feet. Another frequent issue is omitting waste. Small pours lose material to form irregularities, uneven subgrade, over-excavation, spillage, and cleanup. Ten percent is a practical starting point for many residential estimates, but rough forms or hand excavation may justify more.
Measure the finished concrete footprint, not the lumber outside edge, unless your forms are exactly aligned to the target dimensions. Check that the base is compacted before measuring depth; soft pockets can swallow more concrete than expected. For vehicle slabs, frost areas, thickened edges, and structural footings, calculate each depth zone separately or ask a local pro to review the section.
Regional pricing notes
Bag prices differ by country, province, state, season, and store format. Urban areas may have higher shelf prices but better delivery options. Rural projects may pay more for delivery or need larger safety margins because a second store trip is expensive. Canadian estimates should use Canadian bag pricing and local tax assumptions; U.S. estimates should use the local store price and any pallet discount.
Labor ranges are even more regional. A simple ground-level patio with easy access is very different from a tight backyard pour that needs wheelbarrow runs, demolition, regrading, drainage correction, or hand finishing around obstructions. Use the labor toggle for budget planning, then update the range with a contractor quote when the job scope is known.
FAQ
How many bags do I need for a 12 by 10 slab?
At 4 inches deep, a 12 ft by 10 ft slab is 120 square feet and 40 cubic feet before waste. With 10 percent waste and 0.6 cubic feet per 80 lb bag, plan on 74 bags.
Should I estimate concrete in bags or cubic yards?
Use bags for small projects where you will mix on site. Use cubic yards when comparing against ready-mix delivery. The calculator shows both because bag count is useful for shopping and cubic yards are useful for delivery quotes.
Is 10 percent waste enough?
Ten percent is a reasonable default for clean rectangular forms and a prepared base. Increase waste for uneven excavation, thickened edges, complicated formwork, or long handling paths.
Why does the calculator round up bags?
Concrete is sold in whole bags. Rounding down can leave a pour short, so the package count always rounds up after waste is added.
What depth should I use for a patio?
Many residential patios are estimated at 4 inches, but soil, load, climate, reinforcement, and local practice matter. Use the depth specified by your plan, contractor, or local requirement.
Does this include rebar, mesh, gravel, or forms?
No. This page estimates concrete volume, bags, material cost, and optional labor only. Add base gravel, reinforcement, form lumber, stakes, tools, and curing supplies separately.
Sources and assumptions
Last updated 2026-05-03. The calculator uses the cited reference above, common retail package labels, and editable default assumptions for planning quantities. Confirm product coverage, package yield, price, and local requirements before purchasing materials or scheduling work.