Footing Calculators
Written by Shakeel Alvi · Technically reviewed by Muhammad Qasim, PEC Reg. No. 63430 · Last reviewed: 2026-07-11
One calculator covers every footing shape in residential work — continuous strip, isolated pad, or combined — because the geometry behind all three is the same box-of-concrete math. What isn't the same is how far down that box needs to go, and that number comes from outside the calculator entirely.
What you're deciding in this category
Start by naming what you're actually pouring. A continuous footing runs under a wall as a long trench, measured as length times width times depth; an isolated footing sits under a single column or post as a square or rectangular pad; a combined footing ties two columns together when they sit too close for separate pads. Width is a code ratio, not a guess — most residential footings need to be at least twice the width of the wall or member they carry. Depth is the harder decision, and it isn't structural math at all: it's whichever number is deeper, the frost line for your jurisdiction or the undisturbed load-bearing soil beneath any topsoil and fill. Waste allowance runs higher here than almost any other category, 10 to 15%, because a footing gets poured directly against an earthen trench that's rarely cut straight or level.
Which calculator fits which job
One tool, three footing types selected as a shape input — it prices the concrete correctly no matter which type you pick, but it has no way of telling you how deep to dig.
- Footing Concrete Calculator
- Use it for any of the three residential footing types — enter length, width, and depth for a continuous or combined footing, or length and width alone for an isolated pad — and lean on the higher end of the waste range, since trench-poured footings rarely come out perfectly uniform.
Standards that govern Footing
IRC Table R403.1 — Sets minimum footing width and thickness by the number of stories a foundation carries and the soil's load-bearing capacity — the source of the roughly 2-to-1 width ratio between a footing and the wall it supports, and the reason an engineer, not a rule of thumb, should size anything outside typical single-story residential loads.
ACI 318 — Chapter 13 governs footing depth relative to bearing capacity and reinforcement development length once a load moves beyond a light residential case — the provisions a stamped foundation design actually runs on, not the volume figure the calculator returns.
Digging to a depth that feels right instead of the one that's required
The volume arithmetic for a footing is never the problem — length times width times depth divided by 27 doesn't care what the number means. What actually fails inspection, or fails the footing itself after a few winters, is a depth chosen by eye instead of by the local frost line. In a climate with meaningful ground freeze, a footing bottom sitting above that line heaves every winter as the soil beneath it freezes and expands, cracking slabs and racking door frames above it slowly enough that nobody connects the two. Look up the jurisdiction's frost depth before entering a number, not after the trench is already dug.
Frequently Asked Questions
- Why isn't footing depth an input the calculator can suggest?
- Because depth depends on your local frost line and soil-bearing capacity, both jurisdiction-specific — a number the calculator would have to guess at. Check your building department's frost depth table before entering a depth.
- How wide should a footing be relative to the wall above it?
- Most residential codes call for the footing to be at least twice the width of the wall or column it supports — an 8-inch wall typically sits on a 16-inch footing, though local soil conditions can push that wider.
- What waste factor should I use for a footing pour?
- 10 to 15% is standard, higher than most other categories, because trench footings are poured directly against excavated earth that's rarely cut to a clean, uniform width.
- Can this calculator handle a stepped footing on a sloped lot?
- Calculate each horizontal segment separately, at its own depth, then add the volumes together — the tool isn't built for a single stepped input, but running it once per step handles the total correctly.
- Do isolated pad footings need the same width ratio as continuous footings?
- The governing number changes — an isolated pad is sized by the load from the column above it and the soil's bearing capacity, not a fixed ratio to a wall, so pad footings vary more in size than continuous ones.