Concrete Calculator

What This Tool Does

This calculator estimates the volume of concrete needed for three common project types: rectangular slabs (patios, driveways, floors), rectangular footings (foundation supports), and round columns (deck posts, fence posts, pillars). Enter your dimensions and the calculator returns cubic feet, cubic yards, the number of 80-pound premixed concrete bags required, and an estimated material cost. It handles unit conversions automatically — you enter depth in inches while length and width are in feet, which matches how most builders measure.

How It Works: Volume Formulas

For slabs and footings, the volume formula is length × width × depth. Because depth is typically measured in inches while length and width are in feet, the calculator converts depth to feet by dividing by 12. For example, a 10-foot by 8-foot patio slab 4 inches thick equals 10 × 8 × (4/12) = 26.67 cubic feet. Dividing by 27 converts to cubic yards: 0.988 cubic yards.

For round columns, the formula is π × radius² × depth. A column with 12-inch diameter (6-inch radius = 0.5 feet) and 48-inch depth (4 feet) equals π × 0.5² × 4 = 3.14 cubic feet. An 80-pound bag of premixed concrete covers approximately 0.6 cubic feet, so this column needs 6 bags (rounded up from 5.23). The calculator always rounds up because partial bags cannot be purchased, and the 10% extra rule means you should buy 7 bags for this project. For related construction calculations, see our Square Footage Calculator.

Worked Example: Backyard Patio

You plan to pour a concrete patio measuring 12 feet by 10 feet with a 4-inch thickness. Step 1 — Convert depth to feet: 4 inches ÷ 12 = 0.333 feet. Step 2 — Calculate volume: 12 × 10 × 0.333 = 40 cubic feet. Step 3 — Convert to cubic yards: 40 ÷ 27 = 1.48 cubic yards. Step 4 — Calculate bags: 40 ÷ 0.6 = 66.67 bags, rounded up to 67 bags. Step 5 — Add 10% extra: 67 × 1.10 = 73.7, so buy 74 bags. At $5.50 per bag, material cost is approximately $407. If ready-mix concrete costs $140 per yard delivered, a truck order of 1.5 yards would cost $210 plus delivery — potentially cheaper and definitely less labor than mixing 74 bags by hand.

Concrete Coverage Reference

Bag counts include 10% extra for spillage and over-excavation.

When to Use This Calculator

DIY home projects: Before starting a patio, walkway, or shed foundation, use this calculator to determine material needs and budget. Knowing exactly how many bags to buy prevents multiple trips to the hardware store and ensures you have enough concrete to complete the pour in one session.

Contractor bidding: Contractors preparing estimates for small residential jobs can use this tool to quickly calculate material quantities. For larger projects, the cubic yard output helps compare ready-mix quotes from different suppliers on an equal basis.

Deck and fence construction: Round column calculations are essential for deck posts, fence posts, and mailbox pillars. Each post hole requires precise volume calculation because the concrete must fill the hole completely while supporting the post plumb. Use this alongside our Volume Calculator for complex irregular shapes.

Frequently Asked Questions

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