# Concrete Calculator > Multi-shape volume + bag counts + mix ratio breakdown — no signup, 100% private Free concrete calculator. 5 shapes (slab, circular, footing, column, stairs), imperial + metric, bag counts (40/60/80 lb or 25/40/50 kg), mix ratio. URL: https://tools.scoreroute.com/tools/concrete-calculator/ Markdown: https://tools.scoreroute.com/tools/concrete-calculator/.md ## Estimate Concrete Volume, Bags & Materials ## How This Concrete Calculator Works The calculator picks one of five volume formulas based on the shape you choose: rectangular slab = length x width x thickness, circular slab = pi x radius squared x thickness, footing = length x width x depth, column = pi x radius squared x height, stairs = sum of tread x riser x width for every step. The result is the wet volume in cubic feet, then converted to cubic yards and cubic meters. A waste factor (default 10%, adjustable 0 to 15%) covers spillage, over-excavation, and short loads so you do not run short on pour day. From the total volume the calculator computes how many 40 lb, 60 lb, and 80 lb pre-mixed bags (or 25 kg, 40 kg, 50 kg in metric) you need, rounded up to whole bags. Standard yields per bag: 40 lb = 0.30 cu ft, 60 lb = 0.45 cu ft, 80 lb = 0.60 cu ft. For pours over one cubic yard, ready-mix delivery is almost always cheaper per cubic yard than bags; the cost estimator lets you compare both. ## Why This Beats Asking an AI Chatbot AI chatbots are unreliable for concrete math. They confuse wet volume with dry volume (dry volume is 1.54x wet after bulking), forget the waste factor, do not apply mix ratio correctly, and cannot convert between cubic yards and bag yields. Ask ChatGPT how many 80 lb bags make a yard and you may get 40, 45, or 50. They also cannot visualize the shape or break out the cement, sand, and aggregate components. This tool applies the standard engineering formulas, multiplies by your waste factor, and outputs both volume and bag counts in one click. The mix-ratio breakdown converts your chosen ratio into per-bag cement, sand, and aggregate weights so you can mix from bulk materials instead of buying pre-mixed bags. ## Frequently Asked Questions **How much concrete do I need for a 10x10 slab?** A 10 ft by 10 ft slab 4 inches thick is 100 sq ft x 0.333 ft = 33.3 cu ft = 1.23 cu yd. With the 10% waste factor the calculator returns about 1.36 cu yd, or roughly 113 of the 40 lb bags, 75 of the 60 lb bags, or 56 of the 80 lb bags. Change the inputs to match your slab and the results update instantly. **What is a 1:2:4 concrete mix ratio?** 1:2:4 means 1 part cement, 2 parts sand, 4 parts gravel (coarse aggregate) by volume. It is the most common general-purpose mix, roughly M15 grade (~2200 psi at 28 days), and is fine for patios, walkways, fence posts, and residential slabs. For driveways or higher load use 1:2:3 (~2900 psi). For columns and structural beams use 1:1:2 (~3600 psi). **How many 80 lb bags make a yard?** One 80 lb bag yields about 0.60 cu ft. There are 27 cu ft in a cubic yard, so 1 cu yd takes 45 of the 80 lb bags, 60 of the 60 lb bags, or 90 of the 40 lb bags. The calculator applies the same yields automatically and rounds up to whole bags. **Should I order by cubic yard or buy bags?** For pours over 1 cubic yard, ready-mix delivery is almost always cheaper per cubic yard and saves hours of manual mixing. For small jobs under 1 cubic yard (post holes, fence footings, small pads), pre-mixed bags are practical because you avoid the minimum delivery fee that most concrete trucks charge. The calculator shows both side by side. **How thick should my slab be?** Patios and walkways: 4 inches. Residential driveways: 5 inches. Garage floors and RV pads: 6 inches with rebar or wire mesh. Industrial or commercial slabs: 6 to 8 inches. Always check your local building code for the required thickness and reinforcement in your area. **How deep should a footing be?** Footings must extend below the frost line for your region (typically 12 to 48 inches in cold climates, 0 to 12 inches in warm climates). Width is usually 1.5 to 2 times the wall thickness. Check local code; the calculator gives you the cubic yardage once you enter your dimensions. **Does this work in metric?** Yes. Toggle the unit switch at the top to Metric and inputs change to meters and centimeters, bag sizes switch to 25 kg, 40 kg, and 50 kg, and outputs switch to cubic meters and liters. The math is the same; only the units and bag yields change. **Is my project data private?** 100% private. Everything runs in your browser using JavaScript. No dimensions, mix ratios, or cost values are sent to any server. Close the tab and your last calculation is gone. The only data persisted is the last calculation in localStorage so the form reloads when you come back. ## References - [NIST — Handbook of Mathematical Functions](https://dlmf.nist.gov/) - [Wikipedia — Mathematics](https://en.wikipedia.org/wiki/Mathematics)