Footing Calculator

The footprint goes in, not a length you worked out first

Of the footing calculators we could read on the live first page, none works a continuous footing out from the building itself. They ask for a length — a total you are expected to have added up somewhere else — and multiply it by a width and a depth. Here you type the two footprint dimensions off the plan, and the sheet takes the ring out of them.

The second thing it does is add up. Most of these pages take one shape at a time; the one that totals several sections hands back a single number with no breakdown, so you cannot see which part of the pour is the strip and which is the piers. This sheet keeps the continuous footing, the pads and the piers on their own takeoff lines, then adds them into one truck. It also switches between US customary and metric, which no page in the measured set does.

The four corners, and the 3.96 percent they cost

Run a tape around the outside of a house, multiply by the footing width, and you have just bought the four corners twice: the corner square belongs to the long wall and the short wall at once, and a perimeter measurement gives it to both.

A footing is therefore measured on its centreline: 2 × (length + width) − 4 × footing width. On the default job above — 30 by 40 feet, a 16 inch footing (1.3333 ft) poured 8 inches deep (0.6667 ft):

1. Outside perimeter: 2 × (40 + 30) = 140 ft.
2. Centreline: 140 − 4 × 1.3333 = 134.6667 ft.
3. Ring: 134.6667 × 1.3333 × 0.6667 = 119.7037 cu ft.
4. The perimeter method: 140 × 1.3333 × 0.6667 = 124.4444 cu ft.
5. Difference: 4 × 1.3333² × 0.6667 = 4 × 1.7778 × 0.6667 = 4.7407 cu ft.

4.7407 cubic feet on 119.7037 is 3.96 percent of the pour, and it is not an approximation: the error is exactly four corner squares, 4 × b² × depth. It grows with the square of the footing width, so a wide footing on a small building loses more, and it never depends on how big the house is.

That is why Corners not double-counted is a takeoff line of its own rather than a buried correction: widen the footing in the form and watch it move. If the footing is wide enough to meet itself across the building, the ring has closed and the sheet warns you instead of quietly returning nonsense.

Three shapes, one order

A foundation is rarely one shape. The continuous footing is the ring under the walls, computed from the footprint. Pads are the isolated footings under a post, a column or a beam pocket: a count, a length, a width and a depth, all the same size in one run. Piers are round — a tube or an augered hole — and are poured as full cylinders, so a belled bottom holds more.

All three groups are optional and independent. A deck with nothing but piers gives one line, not three zeros. What they share is the bottom of the sheet: one volume, one order, one bag count — because waste and rounding belong to the whole pour, applied once, not shape by shape. Every figure written out in this prose is US customary; the calculator is not, and switching it returns the form, the takeoff and the drawings in metric.

Bags against a truck

Below roughly a cubic yard, bagged mix is usually the cheaper way to place a footing; above it the truck wins. The sheet gives you both from the same takeoff, counted differently on purpose.

Ready-mix is rounded up to the next 0.25 cubic yard. That step is this site’s convention, not a rule published by any plant — yours may batch in tenths, in half yards, or to a minimum load with a short-load charge on top. An exact quantity never jumps a step: 27 cubic feet is one cubic yard and orders 1, not 1.25.

Bags are counted in cubic feet, on the volume with your waste allowance on it, never off the rounded truck figure — rounding twice buys concrete you will not place. The yields are read off the QUIKRETE Concrete Mix No. 1101 data sheet: 0.6 cubic feet of mixed concrete from an 80 lb bag, 0.45 from a 60 lb, 0.3 from a 40 lb. Those four deck piers, 19.5477 cubic feet, are 33 bags at 80 lb or 44 at 60 lb. The 90 lb bag is not stocked everywhere, and the option says so.

Waste is your figure and it opens at 10 percent, because a trench wall that sloughs in is not something anyone publishes an allowance for. At zero the default job orders 4.5 cubic yards instead of 5. For the delivered price, with the extras a yardage figure does not carry, go to G-201 concrete cost: S-103 gives the quantity, G-201 gives the price.

Ring     C  =  max( 0, 2 x (L + W) - 4 x b )    centreline
         Vs =  C x b x d                        cu ft
Corners  Sv =  4 x b^2 x d                      not poured twice
Pads     Vp =  n x l x w x d
Piers    Vr =  n x pi / 4 x dia^2 x h
Net      V  =  ( Vs + Vp + Vr ) / 27            cu yd
Order    Vo =  roundUp( V x (1 + waste), 0.25 )
Bags     Bg =  ceil( (Vs + Vp + Vr) x (1 + waste) / yield )

How big should the footing be? This sheet does not know

Two of the four questions people actually ask around this search are about size and load, and this page answers neither. Footing width, thickness, projection beyond the wall and the bearing value assumed under it are prescribed work: they come from IRC chapter 4 (R403, Footings) as amended by your jurisdiction, and from the engineer or designer who stamped your drawings. Frost depth is local, and your building department is the only authority on it.

We do not reproduce those tables. A footing size copied off a calculator onto a job with different soil, a different load path or a deeper frost line is worth nothing, and the correct figures are one click away in the code itself. Type what your plans say; the sheet measures it, orders it, and stops there.

What the sheet can do once the size is settled: the steel in that footing is S-102 rebar, laps and stock lengths included, and the slab that lands inside the ring is S-101.

Worked example: a ring, six pads and nine piers

A 30 by 40 foot house: 16 inch continuous footing 8 inches deep, six 2 by 2 foot pads a foot thick under a centre beam, nine 12 inch piers 4 feet down for the porch. Waste at zero.

1. Ring: 119.7037 cu ft on 134.6667 feet of centreline.
2. Pads: 6 × 2 × 2 × 1 = 24 cu ft.
3. Piers: 9 × π/4 × 1² × 4 = 28.2743 cu ft.
4. Total 171.978 cu ft ÷ 27 = 6.3696 cu yd, ordered as 6.5 cu yd.

One delivery, three shapes, the corner correction of 4.7407 cubic feet still on its own line.

Frequently asked

How do I calculate concrete for footings?

Measure the continuous footing on its centreline, not around the outside: 2 × (length + width) − 4 × footing width, times the width, times the depth. Add each pad as count × l × w × d and each pier as count × π/4 × diameter² × depth, then divide the total by 27 for cubic yards. This sheet does all three at once.

How do I calculate my footing size?

This sheet will not tell you, and no calculator should. Footing width, thickness and the bearing value under it are prescribed by IRC chapter 4, section R403 and set on your drawings by your engineer or designer. Bring the size back here and the sheet measures it.

How do I calculate the load on a footing?

Again, not here. Tributary area, dead and live load, snow and soil bearing are a structural calculation, not a takeoff. Take the loads to an engineer; this sheet turns the size they give you into concrete.

Will a 4 inch thick concrete footer hold up a house?

Same question, and the answer is on your plans and in your local amendments to the IRC, not on a calculator: a footing is sized for what it carries and for the soil under it. Enter what the drawings say.

Limits of this sheet

S-103 is a quantity takeoff for concrete in footings: a rectangular ring from your footprint, equal pads in one group, cylindrical piers in another. It does not size anything, step a footing down a slope, model an L-shaped footprint, or price excavation, forms, steel, labour or spoil. Keyways, thickened edges and grade beams are not shapes it carries. Material only, and confirm with your plans, your supplier and your building department.

Sheet S-103 prepared by TakeoffMetric Editorial; no author, credential or field measurement is claimed.