A continuous footing is a rectangular run of concrete under a foundation wall. Enter its width and depth in inches and its total length in feet. The calculator returns cubic feet, cubic yards, and a 10% waste line.
For a 16-by-8-inch footing running 100 feet, the correct raw volume is 3.29 cubic yards, not 1.3 cubic yards. Adding 10% produces 3.62 cubic yards. Before ordering, measure the completed excavation and ask the ready-mix supplier what order increments and minimums it accepts.
Concrete Footing Calculator
Enter footing width and depth in inches and the total run in feet. Returns cubic feet, cubic yards, and a 10% waste line to order to.
Volume88.9 cu ft
Concrete3.3 cu yd
With 10% waste3.6 cu ydorder to this
Estimates for planning only. Check the result against your plans and a supplier quote before you order.
How to use the calculator
Read the specified footing width and depth from the approved plans.
Add every run with that same cross-section, including returns and interior bearing lines.
Enter width and depth in inches and total run in feet.
Run different footing sizes separately, then add their cubic-yard totals.
Compare the raw result with the waste-adjusted line and round up to an increment your supplier accepts.
Do not use the calculator output to choose footing dimensions. It estimates material from dimensions already established for the project.
The footing volume formula
Convert the rectangular section to feet, multiply by the run, then convert cubic feet to cubic yards. Most concrete purchases are made in cubic yards, and one cubic yard equals 27 cubic feet. (Cemex ready-mix calculator)
Formula
(width in inches ÷ 12) × (depth in inches ÷ 12) × length in feet ÷ 27 = cubic yards
Keep full precision until the final step. Rounding 16 inches to 1.3 feet too early understates the order.
Choose the order margin
Cemex recommends adding 10% to 15% to cover spillage, uneven subgrades, and other variables. (Cemex ready-mix calculator)
The built-in result adds 10%. Treat that as a planning line, not an automatic order. A formed footing with dependable measurements may justify the low end. An irregular trench, sloughed sides, step-downs, or uncertain measurements needs a larger allowance. Ask the supplier about minimum loads, short-load charges, and accepted increments before rounding.
Footings are not slabs or block walls
This calculator handles continuous rectangular footings: cross-sectional area multiplied by total run. A slab uses plan area multiplied by thickness, while an isolated pier may be circular or rectangular and should be calculated separately.
If the footing will support a CMU wall, use the concrete block calculator for block count. Keep footing concrete, wall units, grout, and mortar as separate estimate lines so a change in one scope does not hide another.
Measure the excavation before ordering
Build to the approved dimensions, then measure the completed excavation before calling the plant. Concrete fills the actual void. Check several widths and depths rather than one convenient spot, and calculate step-downs or changed sections separately.
Small dimensional drift matters. On a nominal 16-by-8-inch section, one extra inch of depth increases volume by 12.5%; one extra inch of width adds 6.25%. Volume is only one budget input, so use the concrete foundation cost guide when the estimate also needs labor, reinforcement, forms, excavation, and pump access.
Footing concrete reference
Check
Value
Cubic-yard formula
W(in) ÷ 12 × D(in) ÷ 12 × L(ft) ÷ 27
16 × 8 in × 100 ft
3.29 yd³ raw
Same example + 10%
3.62 yd³
Same example + 15%
3.79 yd³
One extra inch of depth on 16 × 8
12.5% more volume
One extra inch of width on 16 × 8
6.25% more volume
Concrete footing calculator FAQ
How much concrete does a 16-by-8-inch footing need for 100 feet?
The rectangular volume is 88.89 cubic feet, or 3.29 cubic yards. Adding 10% produces 3.62 cubic yards; round up to an increment your ready-mix supplier accepts.
How do I calculate concrete for a continuous footing?
Convert width and depth from inches to feet, multiply both by total run in feet, then divide the cubic-foot result by 27 to get cubic yards.
How much extra concrete should I order?
Cemex recommends a 10% to 15% margin for spillage, uneven subgrades, and other variables. Use the lower end only when measurements and forms are dependable.
Should I use plan dimensions or actual trench dimensions?
Use the approved design to build the footing, then measure the completed excavation before ordering. If the trench is over width or over depth, the actual void will require more concrete.