Concrete Calculator
Calculate concrete volume for slabs, foundations, and cylindrical post holes. Accurately determines ready-mix cubic yards and pre-mix bag yields.
Job-site cuts, irregular contours, compaction, and spillage typically require 5–15% extra material.
Estimate & BOQ Generator
Turn calculated quantities into an itemized customer bid with overhead and profit markup.
How to Calculate Concrete Volume
Concrete is measured in cubic yards (27 cubic feet) for truck orders, and in pre-mixed dry bags for small DIY pours. Because uneven soil subgrades and formwork deflection cause dimensional variations, always add 10% waste for slabs and 15% for post holes.
Concrete Calculation Formulas
Volume (cu ft) = Length (ft) × Width (ft) × (Thickness (in) ÷ 12)Volume = π × (Diameter ÷ 24)² × (Depth (in) ÷ 12) × CountPre-Mix Concrete Bag Yield & Quantity Guide
| Bag Size | Cubic Foot Yield | Bags per 1 Cubic Yard | Best Used For |
|---|---|---|---|
| 80 lb Bag (Standard) | 0.60 cu ft | 45 bags | Slabs, footings, commercial post anchors |
| 60 lb Bag | 0.45 cu ft | 60 bags | Fence posts, deck footings, steps |
| 40 lb Bag | 0.30 cu ft | 90 bags | Mailbox posts, concrete repairs, small curbs |
Frequently Asked Questions
How many 80 lb bags of concrete do I need for a 10×10 slab?▼
A 10 ft × 10 ft slab that is 4 inches thick equals 33.3 cubic feet (1.23 cubic yards). Adding 10% waste equals 36.7 cubic feet (1.36 cu yds). Since an 80 lb bag yields 0.60 cubic feet, you will need 62 bags (80 lb) or approx. 1.5 cubic yards from a ready-mix truck.
How much concrete does one 80 lb, 60 lb, and 40 lb bag yield?▼
An 80 lb bag yields 0.60 cu ft (0.022 cu yds). A 60 lb bag yields 0.45 cu ft (0.017 cu yds). A 40 lb bag yields 0.30 cu ft (0.011 cu yds). It takes 45 bags of 80 lb concrete to make 1 cubic yard.
When should I order ready-mix concrete instead of bags?▼
As a rule of thumb, if your project requires more than 1.0 to 1.5 cubic yards (approx. 45 to 65 eighty-pound bags), ordering a ready-mix truck is significantly faster, easier, and yields a stronger, more consistent cure than hand-mixing dozens of individual bags.
How thick should a concrete patio or driveway slab be?▼
Sidewalks, walkways, and residential patios are typically poured 4 inches thick. Residential vehicle driveways should be poured 5 to 6 inches thick with steel rebar or wire mesh reinforcement.
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