Calculate concrete for continuous footings and pier pads under decks, fences, porches and small additions.
Quick answer: A continuous footing is length × width × depth. A 40 ft footing, 16 in wide and 8 in deep, is 40 × 1.33 × 0.67 = about 36 cubic feet, or 1.3 cubic yards. A 12 in diameter pier hole 3 ft deep holds roughly 2.4 cubic feet, which is about 4 bags of 80 lb mix.
What are you pouring?
ft
in
Rule of thumb: footing width is twice the wall thickness.
in
6–8 in is typical for a one-storey footing; check your local code.
Concrete needed
0.66 cu yd
17.8 cu ft · 0.50 m³
Order this
0.75 cu yd
Rounded up with 5% waste — ready-mix is sold in 1/4 yd steps
Bag equivalent
80 lb bags (0.60 cu ft each)30 bags
60 lb bags (0.45 cu ft each)40 bags
Weight of the pour1.3 tons (about 4,000 lb per cu yd)
Ready-mix trucks usually carry 8–10 cu yd. A short load under the minimum often carries a fee, and some suppliers charge for the return of unused concrete — say yes to 5% extra rather than a second trip.
Concrete is ordered in cubic yards and almost always rounded up. Add 5–10% for spillage, uneven subgrade and washout. Bag counts assume 80 lb bags at 0.60 cu ft each.
Core facts
Footing width rule
usually twice the wall thickness
Frost depth
12–48 in, set by local code
Deck pier size
10–12 in diameter, typical
80 lb bag fills
about 0.6 cu ft
What is Concrete Footing Calculator?
Footings spread the weight of a structure over enough soil that the ground does not settle unevenly. They come in two shapes: a continuous strip under a wall or foundation line, and isolated pads under posts, piers or columns. Both are estimated the same way, cross-sectional area times length, so a footing 16 inches wide and 8 inches deep uses 1.33 times 0.67 cubic feet for every foot of run. The complication is depth. Footings must sit below the local frost line, which runs from about 12 inches in the warmest states to 48 inches or more in the north, and that depth changes pier volume dramatically.
Common uses for Concrete Footing Calculator
Size the concrete for a deck's continuous perimeter footing
Count bagged mix for a row of fence post piers
Estimate footing volume for a porch or small addition
Work out pier depth below the frost line for your area
Compare a continuous footing with individual pier pads
Continuous footings versus pads
A continuous footing works where a load is spread along a line: foundation walls, stem walls, long deck beams. A pad works where the load is concentrated: posts, piers and columns. Pads use less concrete but need enough area to avoid punching into the soil, and they must be connected to the structure so frost movement does not lift a post out of place.
Frost depth is the number to look up first
Frost depth is the deepest your ground freezes in a typical winter, and it is published in local building codes. Almost everything about footing design follows from it. A shallow footing in a cold state is the most common cause of heaved decks and cracked steps, and there is no cost-effective fix after the fact. Look it up before you dig.
Forming, pouring and finishing a footing
Most footings are poured against the sides of an excavated trench, which makes volume estimates a little generous because a trench is rarely perfectly rectangular. Add 10 percent. Pour in one pass where you can, rod the concrete to remove air pockets, and keep the top roughly level so the wall or post base sits flat. Backfill only after the concrete has cured for several days.
Frequently Asked Questions
How deep do footings need to be?
Below the frost line, because freezing soil expands and lifts. That depth is set by local building code and ranges from roughly 12 inches in the far south to 48 inches or more across the northern states. A footing poured above it will heave and crack the structure above within a few winters.
How wide should a footing be?
A common rule is twice the thickness of the wall it carries, so an 8 inch foundation wall sits on a 16 inch footing. Heavier loads, weak soil or a two-story structure need an engineer's bearing calculation. Wider is safer up to a point, but the concrete cost rises quickly.
What size footing do deck posts need?
Most decks use concrete piers 10 to 12 inches in diameter, poured to below frost depth, with a metal post base on top. A 12 inch pier hole 3 feet deep takes about 2.4 cubic feet, or four bags of 80 lb mix. Check your deck plan or an inspector for the required diameter.
Should there be rebar in a footing?
Yes for anything structural. Two continuous bars running the length of a strip footing, and a single bar or a small cage in a pier, keep the concrete together if the ground shifts. Small decorative pads may not need it. Rebar must sit surrounded by concrete, not pushed against the bottom of the trench.
Can I pour footings and slab together?
Monolithic pours, where footing and slab go in together with a thickened edge, are common for garages and sheds and save formwork. Separate pours give you more control over level and are easier on large buildings. Either way the footing volume must be calculated separately from the slab and added to the order.