Calculate Concrete for Any Project — Bags, Yards & Cost

Use our free concrete calculator to input your project dimensions and instantly get the volume in cubic feet and cubic yards, plus exact bag counts for both 60lb and 80lb bags. The built-in waste factor (adjustable from 5–20%) ensures you always order enough material without expensive over-ordering. Simply select your project type — slab, footing, post hole, wall, column, stairs, or curb — enter your measurements, and the calculator does the rest.

Our concrete bag calculator converts your total project volume into exact bag counts. One 80lb bag (Quikrete or Sakrete) yields 0.6 cubic feet; one 60lb bag yields 0.45 cubic feet. Enter your dimensions and the calculator shows both bag sizes simultaneously with waste factor included — so you never run short at the hardware store.

Our concrete calculator displays your total volume in cubic yards — the standard ordering unit for ready-mix truck delivery. Enter your dimensions and the tool shows cubic yardage, along with a recommendation for when truck delivery ($125–$175 per yard) makes more sense than buying bags. One cubic yard equals 27 cubic feet or 45 bags of 80lb concrete.

To calculate cubic yards manually: multiply Length (ft) × Width (ft) × Thickness (inches ÷ 12), then divide by 27. For example, a 12×20 slab at 4 inches thick = 12 × 20 × 0.333 = 80 cubic feet ÷ 27 = 2.96 cubic yards. Add 10% for waste, so order about 3.25 yards. Or simply enter your dimensions in our calculator and it handles all the conversions automatically.

Our fence post concrete calculator handles standard round post holes. A typical 4×4 fence post needs a 10-inch diameter hole at 30 inches deep, requiring 2–3 bags of 80lb mix per post. Use the quantity multiplier to calculate all posts at once — a 100-foot fence with posts every 8 feet (13 posts) needs about 28–39 bags of 80lb concrete total.

The amount of concrete mix depends on your project's total volume. Our calculator determines the exact cubic footage based on your dimensions and converts it into pre-mixed bag counts for 60lb and 80lb sizes. It works for any brand — Quikrete, Sakrete, or store brands — since standard bags all use the same Portland cement formula with identical yields per bag weight.

A standard gravel base under a concrete slab should be 4–6 inches deep using compacted crushed stone (not pea gravel). To calculate the volume, use our slab calculator with your slab's length and width but set thickness to 4–6 inches. For a 10×10 area with a 4-inch base, you need about 1.23 cubic yards of gravel. Always compact in 2-inch lifts with a plate compactor before pouring concrete on top.

Select "Round Post Hole" mode and enter your sonotube diameter (common sizes: 8", 10", 12", 16") and depth. For a standard 12-inch sonotube at 4 feet deep, you need about 3.14 cubic feet — roughly 6 bags of 80lb concrete. The calculator uses the cylinder volume formula (π × r² × h) automatically and shows results with waste factor included.

Use the 3-4-5 triangle method: measure 3 feet along one form side, 4 feet along the adjacent side, and the diagonal between those points should be exactly 5 feet. For larger slabs, use multiples like 6-8-10 or 9-12-15 for better accuracy. Also verify that opposite diagonal measurements are equal — if both diagonals match, your slab is perfectly square. Our calculator handles the volume math so you can focus on layout.

For rectangular shapes (slabs, footings, walls): Volume = Length (ft) × Width (ft) × Thickness (in) ÷ 12. For cylinders (sonotubes, round post holes): Volume = π × (diameter in inches ÷ 24)² × depth (in) ÷ 12. Divide cubic feet by 27 to get cubic yards, then add 10% for waste. Our calculator handles all unit conversions and formulas automatically for 8 different project types.

Sakrete and Quikrete 80lb bags both yield 0.6 cubic feet of concrete; their 60lb bags yield 0.45 cubic feet. Enter your project dimensions in our calculator and it shows exact bag counts for both sizes. The results are brand-neutral — they work for Sakrete, Quikrete, or any store-brand concrete since standard bags all have the same yield per weight.

The amount depends on three measurements: length, width, and thickness. A 10×10 patio at 4 inches thick needs about 1.23 cubic yards (62 bags of 80lb). A 10×12 at the same thickness requires 1.48 yards (74 bags). A 20×20 slab needs 4.94 yards. Use our slab calculator for exact results with waste factor included.

Our concrete cost calculator estimates total materials expense. Enter your local bag price (typically $5–$10 per 80lb bag at Home Depot or Lowe's) and the calculator multiplies by the number of bags needed. For ready-mix delivery, expect $125–$175 per cubic yard. The tool shows cost for both options so you can choose the most economical route for your project size.

FreeConcreteCalc supports 8 project types (Quikrete has 3), includes adjustable waste factor, cost estimation, rebar calculation, and multi-zone projects. It's brand-neutral — results work whether you buy Quikrete, Sakrete, or order ready-mix. Brand calculators only recommend their own products, while ours helps you find the best option for your budget and project size.