Foundation Calculator
Estimate concrete volume, rebar weight, excavation volume and total cost for slabs, strip footings, isolated footings, raft foundations, stem walls and custom foundations — with engineering drawings and a printable PDF report.
Material cost estimate (optional)
How to estimate concrete for any foundation
Three quick steps from rough dimensions to concrete volume, rebar, excavation & cost.
Slab, strip footing, isolated footing, raft / mat, stem wall or custom — choose the type so the right engineering drawing is shown and the correct volume formula is applied, whether it's a simple slab pour or a perimeter stem wall.
Length, width and thickness/depth for slabs and footings (plus a footing count for isolated pads, or perimeter & wall height for stem walls), then pick your concrete strength from 2500–4000 PSI, a rebar size from #3 to #6, and a wastage allowance.
Concrete volume in cubic feet, yards and metres, estimated rebar weight, excavation volume with over-dig, waste added, and a four-part cost breakdown (concrete, rebar, excavation, labor) in your local currency — ready to export as a PDF.
Foundation Calculator — Free & Accurate
Our foundation calculator helps builders, concrete contractors, structural designers and DIYers work out exactly how much concrete a foundation needs, how much rebar to order, and how much soil to excavate. Pick a foundation type — slab, strip footing, isolated footing, raft / mat, stem wall or custom — enter the dimensions, and get a complete concrete volume, rebar weight, excavation estimate and cost in seconds.
Concrete volume is calculated directly from the geometry: L × W × T for slabs and rafts, L × W × D for strip footings, L × W × D × N for a set of identical isolated footings, and perimeter × height × thickness for stem walls. A 20 × 20 ft slab at 4″ thick works out to 20 × 20 × 0.333 = 133.3 ft³, which is about 4.94 cubic yards before wastage. Results are shown in cubic feet, cubic yards (the usual US ordering unit) and cubic metres so they match whatever your supplier quotes in.
Rebar weight is estimated from a two-way grid at 12″ on-center (or longitudinal runs for strip footings), multiplied by the unit weight of the bar size you choose: #3 = 0.376 lb/ft, #4 = 0.668 lb/ft, #5 = 1.043 lb/ft, #6 = 1.502 lb/ft. Excavation volume adds roughly 15% over the concrete volume for over-dig and working room. An optional cost section covers concrete, rebar, excavation and labor in USD, GBP, EUR, AUD, CAD, INR, BDT or AED. Everything runs in your browser — no signup, no data sent anywhere.
Why use this calculator?
Built for the slab, the footing and the stem wall — simple, fast, and accurate.
No spreadsheets, no estimating tables. Type your dimensions, hit calculate — concrete volume in cubic feet, yards and metres on screen in an instant.
Slab, strip footing, isolated footing, raft / mat, stem wall and a flexible custom mode — each with its own engineering drawing and the right volume formula.
Estimates rebar weight from a standard grid and bar size, plus excavation volume with a 15% over-dig allowance — the three quantities every foundation pour needs.
Add concrete, rebar, excavation and labor prices for a four-part estimate in 8 currencies — per cubic yard, per metre, per pound, per ton, hourly or flat-fee.
Everything runs in your browser. Your dimensions, prices and cost data never leave your device.
Mix feet, inches, yards, metres and centimetres on the same job. Fully responsive — open it on your phone right on the job site, no app needed.
Related construction tools
The foundation calculator sits right between digging the hole and pouring the concrete — pair it with these.
Frequently asked questions
The short, practical answers to the questions builders and DIYers ask most.
Concrete volume is length × width × thickness, all in the same unit. For a slab or raft it's L × W × T; for a strip or isolated footing it's L × W × D (depth); for a stem wall it's perimeter × height × thickness. A 20 × 20 ft slab at 4″ thick = 20 × 20 × (4÷12) = 133.3 cubic feet, or about 4.94 cubic yards. Since concrete is ordered by the cubic yard in the US and cubic metre elsewhere, the calculator shows both, plus a wastage allowance on top so you don't come up short on the pour.
The calculator assumes a two-way reinforcing grid at 12″ on-center (the most common residential spacing) for slabs, rafts and pad footings, and longitudinal bars plus ties for strip footings. It works out the total linear feet of bar, then multiplies by the unit weight of the size you choose: #3 (3⁄8″) = 0.376 lb/ft, #4 (1⁄2″) = 0.668 lb/ft, #5 (5⁄8″) = 1.043 lb/ft, #6 (3⁄4″) = 1.502 lb/ft. This gives a solid order-quantity estimate — your structural engineer's drawings are the final authority on bar size, spacing and lap lengths.
Concrete strength is specified as the 28-day compressive strength in PSI. 2500 PSI is a minimum for non-structural work; 3000 PSI is the typical choice for residential footings and slabs; 3500–4000 PSI is used for structural slabs, heavily loaded footings, garage floors and cold-climate foundations exposed to freeze-thaw. The strength you pick doesn't change the volume you need — a cubic yard is a cubic yard — but it does change the mix price and is shown on your estimate. Always follow the strength specified on your engineered drawings or local code.
You always dig a bit larger than the finished concrete. The calculator adds about 15% over the concrete volume to account for over-dig (you can't excavate to an exact knife-edge), working room around the forms, and the base material (gravel or sand) under footings and slabs. On real sites the figure varies with soil type, shoring and how the forms are set, so treat it as a planning estimate for spoil removal and truck loads rather than an exact dig quantity.
Some loss is unavoidable on every pour from spillage, uneven subgrade, over-excavated spots and the concrete left in the truck and pump. 5% is reasonable for a clean, well-formed slab on a level base; 10% is the recommended default for most residential footings and slabs; 15% is wise for complex layouts, hand-dug footings with rough sides, or difficult-access pours. Running short mid-pour means a cold joint or an expensive second truck, so it's always better to round up. Order to the nearest quarter or half cubic yard your supplier sells in.