Frequently Asked Questions
Is my data private?
Yes. All calculations happen entirely in your browser using JavaScript. We do not send your dimensions, costs, or any project data to any server. Saved inputs are stored only in your browser's localStorage on your own device.
Is this professional engineering advice?
No. ConcreteCalc provides estimates for informational purposes only. It is not a substitute for professional engineering advice. For load-bearing structures, always consult a licensed structural engineer.
How accurate is the calculator?
The calculator uses standard concrete density (2,400 kg/m³) and industry-standard geometric formulas. Results are mathematical estimates — actual concrete needed can vary due to ground conditions, formwork accuracy, and mixing waste. Always order 5-15% extra.
When was the calculator last updated?
The calculator formulas and pricing defaults were last reviewed in March 2025. Pricing is provided as a regional estimate only — concrete costs vary significantly by location and should be confirmed with local suppliers.
Can I export my results?
Yes. After calculating, a PDF Export button appears. The PDF includes all your inputs, calculated volumes, bag counts, cost comparison, and rebar details (if enabled). The PDF is print-friendly with no ads.
How do I report an error or bug?
Please use our contact form at /contact. Select 'Report a bug or error' as the subject and describe the issue. We appreciate all feedback and will investigate promptly.
How do I switch between metric and imperial?
Toggle the Metric/Imperial switch in the Dimensions section of the calculator. All inputs and results will update automatically. You can switch at any time without losing your entered values.
What waste factor should I use?
For well-prepared flat surfaces with sturdy formwork, use 5%. For typical residential projects with minor ground variation, use 10%. For uneven terrain, irregular shapes, or your first concrete project, use 15%.
How many bags of concrete are in a cubic yard?
It depends on bag size: approximately 45 bags of 80 lb, 60 bags of 60 lb, or 108 bags of 25 kg concrete per cubic yard. Our calculator computes the exact number based on your selected bag size.
When should I choose ready-mix over bags?
Ready-mix is generally more cost-effective for projects requiring more than 1 cubic yard of concrete. It also provides more consistent quality and requires far less labour. For projects under 0.75 cubic yards, bags are usually cheaper and more practical.
Does the calculator account for rebar?
Yes. Enable the rebar toggle in the Options section to calculate total rebar length, number of bars, and weight based on your chosen bar size and spacing. This is available for slab and footing shapes.
Can I save my calculations?
Your most recent inputs are automatically saved to your browser's localStorage when you click Calculate. When you return to the site, your previous inputs will be restored. This data never leaves your device.
Additional Resources
How ConcreteCalc Handles Different Slab Shapes
ConcreteCalc supports multiple slab geometries — including rectangular, square, circular, and irregular polygonal (via manual entry of side lengths and angles). For circular slabs, the calculator uses the standard area formula πr², then multiplies by depth to determine volume. Irregular shapes can be approximated by dividing the area into smaller rectangles or triangles and entering them sequentially using the 'Add Section' feature. Each shape type uses the same core volume formula (length × width × depth), but the interface adapts to input requirements: circular sections prompt for radius and depth, while polygonal entries require sequential vertex inputs. This flexibility ensures accurate estimation for patios, footings, columns, and custom foundations without requiring advanced drafting skills.
Understanding Concrete Mix Ratios and Their Impact
While ConcreteCalc focuses on volume estimation, it’s important to understand how mix ratios affect your project’s outcome. Standard concrete mixes (e.g., 1:2:3 — cement:sand:aggregate) influence strength, workability, and cost. A higher cement ratio increases compressive strength but also cost and heat of hydration, which can lead to cracking in large pours. For non-structural applications like pathways or driveways, a 1:3:6 mix may suffice; for foundations or retaining walls, consider 1:2:4 or stronger. Our calculator does not override local material standards, but you can input bag weights and unit prices to reflect regional mix options. Always refer to local building codes and consult suppliers for recommended mixes based on your climate and load requirements.
Preparing Your Site for Concrete Placement
Accurate volume calculation is only half the battle — proper site preparation ensures your concrete job succeeds. Begin by clearing vegetation, debris, and topsoil, then compact the subgrade to prevent settling. A well-compacted granular base (typically 4–6 inches of gravel or crushed stone) improves drainage and load-bearing capacity. Install formwork securely using wooden boards or metal edging, ensuring level alignment and adequate bracing to resist lateral pressure. Apply a curing compound or cover with plastic sheeting after finishing to retain moisture during the critical 7-day hydration period. Even minor deviations in preparation can increase waste by 10–20%, so invest time here to stay within your calculated estimates.
Have another question?
Contact Us