How do you calculate concrete volume for a slab?

Concrete volume = Length × Width × Thickness. The key is consistent units. In imperial: - Convert thickness from inches to feet (divide by 12) - Multiply Length (ft) × Width (ft) × Thickness (ft) = Cubic Feet - Divide by 27 to convert to Cubic Yards Example: 10 ft × 10 ft × 4 inches thick = 10 × 10 × (4/12) = 10 × 10 × 0.333 = 33.33 cubic feet = 33.33 / 27 = 1.23 cubic yards Always add 10% for waste: 1.23 × 1.10 = 1.36 CY to order.

Why do I need to add 10% extra concrete?

Always order 10% more concrete than your calculated volume for these reasons: 1. Uneven sub-base — ground is rarely perfectly level, so some areas are thicker than planned. 2. Spillage and formwork leakage during the pour. 3. Over-excavation — if you dig slightly too deep, the slab uses more concrete. 4. Calculation rounding — small measurement errors compound. Running short of concrete mid-pour is a serious problem — the joint between pours (a 'cold joint') is structurally weak. It is always better to have leftover concrete than to run short.

How many 80-pound bags of concrete do I need?

One 80 lb bag of pre-mixed concrete (like Quikrete or Sakrete) yields approximately 0.6 cubic feet when mixed. Bags needed = Volume in cubic feet ÷ 0.6 Example: 33.33 cubic feet ÷ 0.6 = 55.6 → order 56 bags. Note: Pre-mixed bags are only economical for small pours (under 1 cubic yard). For larger slabs, ready-mix concrete delivered by truck is far cheaper and more practical. The threshold is roughly 50–60 bags — beyond that, call a ready-mix supplier.

What thickness should a concrete slab be?

Standard thicknesses by use: 3–4 inches (75–100 mm): Walkways, garden paths, light foot traffic 4 inches (100 mm): Patios, residential floors, basement floors — the most common slab 5 inches (125 mm): Residential garage floors — supports passenger vehicles 6 inches (150 mm): Driveways, RV pads, areas with heavier vehicle traffic 8 inches (200 mm): Commercial/industrial floors, areas with forklift or heavy equipment For structural slabs (above grade), consult a structural engineer. These calculations are for ground-supported slabs only.

How much does a concrete slab cost?

In the US (2025 averages): - Ready-mix concrete: $125–$165 per cubic yard, depending on region and mix strength - Labour for pouring and finishing: $2–$7 per square foot - Reinforcing steel (rebar or mesh): $0.50–$1.00 per square foot - Forming and prep: $1–$3 per square foot Total installed cost: $4–$12 per square foot for a basic 4-inch residential slab, or roughly $5–$10 per sq ft all-in for a standard patio. This calculator estimates material cost only (concrete). Always get at least 3 quotes from local contractors for a full project.

What is a cubic yard of concrete?

A cubic yard (CY) is the standard unit for ordering ready-mix concrete in the US. 1 cubic yard = 3 ft × 3 ft × 3 ft = 27 cubic feet ≈ 0.765 cubic metres A standard concrete truck (transit mixer) carries 8–10 cubic yards. Minimum orders are typically 1 CY. Short-load fees apply for orders under the truck's full capacity — usually $15–$25 per CY under the minimum. In metric countries, concrete is ordered in cubic metres (m³). 1 m³ ≈ 1.308 cubic yards.

Concrete Slab Volume Calculator — Cubic Yards, Bags & Cost

The Concrete Slab Volume Calculator tells you exactly how much concrete to order for any rectangular slab. Enter your slab length, width, and thickness in either imperial (feet and inches) or metric (metres and millimetres) — the calculator instantly outputs volume in cubic yards and cubic feet (US) or cubic metres (me...

SLAB DIMENSIONS

LENGTH (feet)

WIDTH (feet)

THICKNESS (inches)

CONCRETE PRICE ($/CY, optional)

US average: $125–$165/CY ready-mix (2025)

RESULTS

VOLUME (exact)

1.23 CY

= 33.33 cubic feet

WITH 10% WASTE — order this amount

1.36 CY

80 LB BAGS

60 LB BAGS

ESTIMATED MATERIAL COST

$170

at $125/CY ready-mix + 10% waste

SLAB VISUALISER (not to scale)

SLAB SUMMARY

Surface area100.00 sq ft

Thickness4" (0.3333 ft)

Volume exact33.333 CF

Volume exact1.235 CY

Volume +10% waste1.358 CY

80 lb bags56 bags

60 lb bags75 bags

THICKNESS GUIDE

3–4"

Walkways, pathways

Foot traffic only

Patios, residential slabs

Standard residential

Garage floors

Passenger vehicles

Driveways

Heavy vehicles, RVs

Commercial floors

Forklifts, heavy loads

UNIT CONVERSIONS

1 cubic yard = 27 cubic feet

1 cubic yard ≈ 0.765 m³

80 lb bag ≈ 0.6 cubic feet

60 lb bag ≈ 0.45 cubic feet

25 kg bag ≈ 0.01 m³

Always order 10% extra for waste

Created with❤️byeaglecalculator.com

HOW TO USE

1
Choose your unit system: Imperial (feet and inches, for US projects) or Metric (metres and millimetres). The calculator remembers your choice as you adjust inputs.
2
Enter the slab Length and Width. For irregular shapes, break the slab into rectangular sections and calculate each separately, then add the volumes.
3
Enter the Thickness. Use the quick preset buttons — 4 inches for a standard patio or residential floor, 6 inches for a driveway, 8 inches for commercial. Thickness is the most critical input: doubling thickness doubles the concrete needed.
4
Optionally enter your concrete price per cubic yard (US) or per cubic metre (metric) to get a cost estimate. US ready-mix averages $125–$165/CY in 2025; check with your local supplier for exact pricing.
5
Read the results: Volume without waste, Volume with 10% waste (always order this amount), number of pre-mixed bags if hand-mixing, and total estimated material cost. Click COPY to save the full result.

WORKED EXAMPLE

Calculating concrete for a 20 ft × 20 ft garage floor, 5 inches thick: Step 1: Convert thickness to feet 5 inches ÷ 12 = 0.4167 ft Step 2: Calculate volume in cubic feet 20 ft × 20 ft × 0.4167 ft = 166.67 cubic feet Step 3: Convert to cubic yards 166.67 ÷ 27 = 6.17 cubic yards Step 4: Add 10% waste 6.17 × 1.10 = 6.79 CY → order 7 CY Step 5: Bag count (if hand-mixing — not recommended at this scale) 166.67 CF ÷ 0.6 CF/bag = 278 × 80 lb bags Step 6: Cost estimate at $140/CY 6.79 CY × $140 = $951 for concrete material only

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Last updated: April 29, 2026 · Formula verified by EagleCalculator team · Eagle-eyed accuracy for every calculation.