Concrete Footing Calculator — Yards, Bags and Rebar

A footing is the buried pour that spreads a load into the ground. Four shapes cover nearly every job. A strip footing is a continuous band under a wall, and it is the usual choice for house foundations and block walls. A pad footing is a square or rectangular block under a single column or post. A pier is a round shaft, formed with a cardboard tube or an auger hole, and it carries deck posts and porch columns. A trench fill foundation skips formwork and fills a narrow trench with concrete to near ground level. If you were looking for a foundation concrete calculator or a concrete base calculator, the four tabs above cover those jobs too.

The core formula is length × width × depth in feet, divided by 27 to get cubic yards. A 40 ft run at 16 × 8 in is 40 × 1.333 × 0.667 = 35.6 cubic feet, and 35.6 ÷ 27 = 1.32 cubic yards. Add 10% for an uneven trench bottom and spillage and the order is 1.45 cubic yards. Convert inches to feet first, which is where most hand calculations go wrong: 16 in is 1.333 ft, not 1.6.

Piers are cylinders, so the volume is 0.785 × diameter² × depth. A 12 in tube 36 in deep is 0.785 × 1 ft² × 3 ft = 2.36 cubic feet. Six of them come to 14.1 cubic feet, which is still only 0.52 cubic yards, or 24 bags of 80 lb mix. That is why piers are nearly always bagged rather than delivered. The round column shape in the main concrete calculator runs the same cylinder maths for a single tube, and a belled base here adds a 12 in flare at the bottom to spread the load over weaker soil. For the buying side, the form tube calculator counts how many 12 ft tubes the piers need and how to cut them with the least waste.

Footing width is driven by the wall it carries, not the other way round. A common starting point is twice the wall thickness, so an 8 in block wall sits on a footing about 16 in wide, with the wall centred so there is roughly 4 in of footing either side. Depth is rarely less than the wall width or 8 in, whichever is greater, so the concrete does not behave like a thin slab under load. A heavy wall needs more: the weight of a block wall tells you how many pounds per linear foot the soil has to resist, and wider footings lower that pressure. Weak, wet or clay soils need more width again, which is why code tables are organised by soil bearing capacity.

Frost depth is the number that catches people out. Water in the soil expands by about 9% as it freezes, and ice lenses grow and push upwards with enough force to lift a house corner. What matters is the bottom of the footing, not its top: the base has to sit below the depth the ground freezes, so the soil underneath stays put. A 6 in footing set 36 in down clears a 36 in frost line, while a 12 in footing whose base is only 24 in down does not. The figure is local, from 12 in on the Gulf Coast to 60 in or more in Alaska, and your building department can give you the exact depth for your site.

Rebar keeps a footing from cracking when the soil under it settles unevenly. A common residential pattern is two continuous bars with 3 in of cover, held near the bottom where the tension is, because a footing bends like a beam spanning soft spots. Bars are lapped roughly 30 bar diameters where they join, which is 15 in for #4 bar, and this calculator allows a more generous 24 in. Stirrups are optional ties around the cross-section for deep or heavily loaded footings. Local code and an engineer decide what a structural footing actually needs.

Trench fill exists because forming a footing, pouring it, then building a stem wall is slow and labour-heavy. If the soil is firm and the trench sides stand vertical, you can pour the concrete straight into the trench and start the wall on top. It uses noticeably more concrete than a formed footing and wall, but it saves forms, time and a second pour, so it often wins on small buildings in stable clay or stiff ground. It is a poor fit for loose sand or wet ground where the sides collapse.

For the total foundation cost, concrete is only part of it. A 16 × 8 in strip footing costs roughly $11 per linear foot in materials once rebar and form board are added, before excavation, labour and delivery. If a slab sits inside the footed perimeter, pricing the slab line by line gives the other half of the budget. Always confirm sizes with your building department before you dig.