Concrete Footing Calculator
Size a strip footing — enter its length, width, and depth to get the concrete you need in cubic yards and in 40, 60, and 80-lb bags. Set a quantity for multiple footings.
Free · no sign-up ·
You need · incl. +10% extra
- 40-lb bags
- 66
- Cubic feet
- 19.6 ft³
- Cubic meters
- 0.55 m³
- Exact volume
- 0.66 yd³
Use cubic yards to order ready-mix by the truck; bag counts for a smaller hand-mixed pour. Bag yields: 80-lb ≈ 0.6 ft³, 60-lb ≈ 0.45 ft³, 40-lb ≈ 0.3 ft³.
Order about 10% extra — you can’t pause a pour to mix more, and the subgrade is never perfectly level. Past ~½ yard, ready-mix is usually cheaper and far less work than bags.
The formula
Volume = Length × Width × Depth (× quantity). Convert to cubic yards (1 yd³ = 27 ft³). Bags = volume in ft³ ÷ bag yield (80-lb ≈ 0.6 ft³).
How to estimate concrete for a concrete footing
A footing is the wide concrete base that spreads a wall or post load onto the soil. For a continuous strip footing — under a foundation wall or a block wall — you size it by its run length, its width, and its depth. A common residential footing is 16–20 inches wide and 8 inches deep, but the right size depends on the load and the soil, and is often set by code or an engineer. Enter the dimensions and the calculator returns the concrete in cubic yards and bags.
For multiple separate footings — a row of deck or pier footings, say — set the quantity to the number of footings and enter the size of one. The calculator multiplies it out. For round pier footings or columns, use the column calculator instead, which handles the circular cross-section.
Footings are structural, so do not guess on size or reinforcement. Depth must reach below the frost line in cold climates, the bottom must bear on firm, undisturbed (or compacted) soil, and most footings carry rebar. Order about 10% extra concrete, and check your local code or have an engineer confirm the footing size before you dig.
Frequently asked questions
How much concrete do I need for a footing?+
Multiply the footing length × width × depth (in feet) for cubic feet, then divide by 27 for cubic yards. A 20-foot run at 16 inches wide and 8 inches deep is about 17.8 cubic feet, or 0.66 cubic yards — roughly 0.73 with overage. Enter your dimensions above for an exact figure.
How deep should a footing be?+
Deep enough to bear on firm soil and, in cold climates, below the frost line — often 12 inches or more of depth, with at least 8 inches of concrete. The exact depth is set by your local code, the load, and the soil, so confirm before you dig.
How wide should a footing be?+
A common residential strip footing is 16–20 inches wide, typically about twice the width of the wall it supports. Heavier loads or weaker soil need wider footings. Width is a structural decision — check code or an engineer.
How many bags of concrete for a footing?+
An 80-lb bag yields about 0.6 cubic feet. A 20-foot footing at 16 × 8 inches (≈18 ft³) needs about 33 eighty-pound bags with waste. For footings much bigger than that, ready-mix by the yard is usually cheaper.
How do I calculate concrete for multiple footings?+
Enter the size of one footing and set the quantity to the number of identical footings — the calculator multiplies the volume. For round pier footings, use the concrete column calculator instead.
Do footings need rebar?+
Most structural footings do — typically horizontal rebar run continuously, sized and placed per code or an engineer. The calculator estimates concrete volume, not reinforcement; confirm the rebar schedule for your footing.
What is the difference between a footing and a slab?+
A footing is a narrow, deep concrete base that spreads a concentrated load (a wall or post) onto the soil; a slab is a wide, thin surface you walk or drive on. They are often poured together but sized differently — use the slab calculator for the flat surface.
How much extra concrete should I order for footings?+
About 10%. Trenches are rarely cut perfectly, so the actual volume runs over the nominal size, and you cannot pause a pour to mix more. The calculator includes an adjustable overage.
Should there be gravel under a footing?+
Often a few inches of compacted crushed stone goes under a footing for drainage and a clean, level bearing surface, though some footings bear directly on firm undisturbed soil. Follow your code or engineer; size any base with the Crushed Stone calculator.
Is this concrete footing calculator free?+
Yes — free, no sign-up, and it runs entirely in your browser, so nothing you enter is stored or sent anywhere.