Concrete Footing Calculator
Enter the total footing length, width, and depth to calculate concrete volume in cubic yards and 80-lb bags. Includes waste factor and ready-mix cost estimate.
What is Concrete Footing?
A concrete footing calculator determines the volume of concrete needed to pour continuous strip footings for residential or commercial foundations. Footings are the wide, flat slabs of reinforced concrete that sit below the foundation walls and distribute the building's weight over a larger area of soil, preventing settling and structural failure. Continuous footings (also called strip footings) run the full perimeter of a building and under any interior load-bearing walls. The footing length input should include the entire linear footage — add up all sides of the perimeter plus any interior footings. For a 30x40-foot building, the perimeter footing length is 140 feet (30+40+30+40). Add additional footage for interior bearing wall footings. Standard residential footing dimensions are governed by building codes based on the number of stories and soil bearing capacity. For typical one-story construction on standard soil (2,000 psf bearing capacity), footings are 12 inches wide and 6 inches deep. Two-story buildings require 15-inch width and 7-inch depth. Three-story buildings need 18-inch width and 8-inch depth. In areas with poor soil (clay, fill, or high water table), engineers often specify wider footings — 20 to 24 inches — to reduce bearing pressure. The waste factor for footings is higher than for flat slabs (10% vs 5%) because footings are poured into earth trenches where the walls may be irregular. Soil can crumble into the trench, widening it in spots. The trench bottom may have soft areas that get dug deeper. And in rocky soil, the trench may be wider than intended where boulders were removed. A 10% waste factor covers these typical over-pours. Concrete for footings should be at least 2,500 PSI (most residential codes require 2,500-3,000 PSI). Reinforcement typically consists of two or three continuous number 4 or number 5 rebar running horizontally, with vertical dowels protruding up to tie into the foundation wall above. In cold climates, the bottom of the footing must be below the frost line (which ranges from 12 inches in the southern US to 60+ inches in northern states and Canada). For small projects under 1 cubic yard, hand-mixing 80-lb bags may be practical (each bag yields 0.6 cubic feet; 45 bags fill one cubic yard). For anything larger, ready-mix delivery is strongly recommended — it produces more consistent concrete, eliminates mixing labor, and allows continuous pours that avoid cold joints. Ready-mix costs approximately $125 to $175 per cubic yard delivered, with most plants requiring a minimum order of 3-4 cubic yards (plus short-load fees for orders under the minimum).
How to Calculate
- Calculate total footing length by adding all perimeter sides plus interior bearing wall footings
- Determine footing width from your building plans or local code requirements (12-24 inches typical)
- Determine footing depth from your plans or code (6-8 inches for residential)
- Set waste factor to 10% for typical trench conditions (15% for rocky or unstable soil)
- Review cubic yards for ready-mix ordering or bag count for small jobs
- Contact ready-mix suppliers for current pricing and minimum order requirements
- Ensure trench is dug, inspected, and rebar is placed before ordering concrete
Formula
Width (ft) = Footing Width (in) / 12 Depth (ft) = Footing Depth (in) / 12 Raw Volume = Footing Length (ft) x Width (ft) x Depth (ft) Volume (cu ft) = Raw Volume x (1 + Waste Factor / 100) Cubic Yards = Volume (cu ft) / 27 80-lb Bags = ceiling(Cubic Yards x 45) Estimated Cost = Cubic Yards x $150/yd Where 27 cu ft = 1 cu yd, 45 bags per yard is the standard yield for 80-lb bags (each bag = 0.6 cu ft), and $150/yd is a mid-range estimate for ready-mix delivery.
Example Calculation
A 40 ft perimeter footing, 20 inches wide, 8 inches deep, with 10% waste: Width = 20 / 12 = 1.6667 ft Depth = 8 / 12 = 0.6667 ft Raw Volume = 40 x 1.6667 x 0.6667 = 44.44 cu ft Volume = 44.44 x 1.10 = 48.89 cu ft Cubic Yards = 48.89 / 27 = 1.81 cu yd 80-lb Bags = ceil(1.81 x 45) = 82 bags Estimated Cost = 1.81 x $150 = $271.60
Frequently Asked Questions
How deep do footings need to be below ground?
The bottom of the footing must be below the local frost line to prevent frost heave. Frost depth varies from 12 inches in the southern US to 48-60 inches in northern states and Canada. Check with your local building department for the required depth. In non-freezing climates, footings still need a minimum of 12 inches of soil cover.
What size rebar goes in a residential footing?
Standard residential footings use two continuous number 4 (1/2-inch) rebar placed horizontally, positioned 3 inches from the bottom and 3 inches from each side. The rebar must be supported on chairs or bricks — never laid on the ground. Vertical number 4 dowels at 4-foot spacing tie the footing to the foundation wall above.
How long before I can build on new footings?
Foundation walls can be placed on footings after 2 to 3 days in warm weather (above 50 degrees F). The concrete reaches about 65% of its design strength in 3 days and full strength at 28 days. Avoid placing heavy loads or backfilling against foundation walls until the concrete has cured for at least 7 days.
Do I need a building permit for footings?
Yes. Virtually all jurisdictions require a building permit and footing inspection before the concrete pour. The inspector verifies the trench depth, width, soil conditions, rebar placement, and form work. Pouring without inspection may result in a stop-work order and potentially having to remove and redo the work.
What is a short-load fee for ready-mix concrete?
Ready-mix plants charge a short-load fee (also called a less-than-full-load fee) for orders under their minimum, which is typically 3 to 4 cubic yards. The fee ranges from $30 to $75 per cubic yard short of the minimum. For a 2 cubic yard order with a 4 yard minimum, expect $60 to $150 in additional fees.
Can I pour footings in sections or does it need to be continuous?
Footings can be poured in sections as long as proper construction joints are used. At each stopping point, create a square (keyed) joint by inserting a beveled 2x4 into the wet concrete to form a keyway. This mechanical interlock transfers loads across the joint. Remove the form after the first pour hardens, then pour the adjacent section against it.