Foundation Calculator
Calculate concrete volume, premix bags, and rebar count for slab, footing, or pier foundations.
Written by Golam Rabbani, Founder & Lead Engineer
How to use this foundation calculator
- Pick imperial (ft) or metric (m) at the top.
- Choose the foundation shape: slab, footing, or pier.
- Enter the length, width, and depth (or diameter, depth, and count for piers).
- Optional: set a rebar grid spacing for slabs/footings, and a waste percentage.
- Press Calculate to see total concrete volume, 80 lb and 60 lb premix bag counts, and rebar.
- Use Copy to share the result or Reset to start over.
About this foundation calculator
The foundation calculator converts the dimensions of your slab, footing, or pier into a concrete volume and translates that volume into the number of premixed bags and length of rebar you need to buy. It handles both rectangular pours (volume = L × W × D) and circular piers (π × r² × depth), multiplies by your selected count, and then adds the waste percentage you specify so the order survives spillage and over-excavation.
For example, a slab 20 ft × 10 ft × 4 in (0.33 ft) with a 1 ft rebar grid and 10% waste comes to 20 × 10 × 0.33 = 66 ft³ ≈ 2.44 yd³ of concrete. Adding 10% waste gives ≈ 2.69 yd³ — about 121 × 80 lb bags or 162 × 60 lb bags. The rebar grid produces 11 long bars + 21 short bars (32 bars) for a total length of 320 ft, again with waste added on top.
Bag yields come from manufacturer specs (80 lb premix ≈ 0.6 ft³; 60 lb premix ≈ 0.45 ft³). Use the result as a shopping target, not as engineered structural advice — for code-bearing walls and load-bearing footings, follow your local building official’s drawings.
FAQ
- How does the foundation calculator decide how many bags to buy?
- It divides the total cubic-meter (or cubic-foot) volume of your pour by the yield of one bag — 0.017 m³ for an 80 lb bag and 0.013 m³ for a 60 lb bag — then rounds up. Waste percentage is applied to the volume before the bag math.
- What rebar spacing should I use?
- For a standard residential slab on grade, 12-in (0.3 m) on-center grid is common. The tool counts bars in both directions plus end stubs, then sums the total linear length, so you can order rebar by the foot or meter.
- Does this tool replace a structural engineer?
- No. It estimates material quantities so you can order concrete and rebar. Load-bearing footings, slab thickness, and rebar size for critical structures must be specified by a licensed engineer per local code.
- Why a 10% waste factor by default?
- Concrete is sold in fixed bag or truck quantities, and over-excavation, formwork bulge, and spillage typically eat 5–15% of a pour. 10% is a common contractor default — bump it up for irregular ground.
- Can it handle piers?
- Yes — choose “pier,” enter the diameter, depth, and number of piers. Volume is π × r² × depth × count, then waste is added on top.