Concrete Footing Calculator
Size check (IRC R403.1.1)
Estimates concrete only, not a structural design. Footing size and depth come from your local code, soil, and load. Check with your building department.
A round footing holds π × radius² × height. A 10 in Sonotube 4 ft deep takes 2.18 cubic feet of concrete, about 4 bags of 80 lb mix. Six of them with 10% extra come to 14.4 cubic feet, or 0.53 cubic yards and 24 bags. Pick your footing type below for cubic yards, bags per footing, ready-mix order, weight, and cost.
How much concrete does a Sonotube take?
| Tube diameter | ft³ per foot of height | ft³ in a 4 ft tube | 80 lb bags (4 ft) | 60 lb bags (4 ft) |
|---|---|---|---|---|
| 6 in | 0.20 | 0.79 | 2 | 2 |
| 8 in | 0.35 | 1.40 | 3 | 4 |
| 10 in | 0.55 | 2.18 | 4 | 5 |
| 12 in | 0.79 | 3.14 | 6 | 7 |
| 14 in | 1.07 | 4.28 | 8 | 10 |
| 16 in | 1.40 | 5.59 | 10 | 13 |
| 18 in | 1.77 | 7.07 | 12 | 16 |
| 24 in | 3.14 | 12.57 | 21 | 28 |
How to calculate concrete for a footing
A footing is a simple solid, so the volume comes straight from its shape. Work in feet, multiply, then divide by 27 to get cubic yards.
- Wall or strip footing: run × width × thickness.
- Square pad: length × width × thickness, times the number of pads.
- Round pier or Sonotube: 3.1416 × (diameter ÷ 2)² × height, times the number of piers.
- Tube on a pad: the pad plus the tube, per footing.
For example, a 40 ft wall footing 16 in wide and 8 in thick is 40 × 1.33 × 0.67 = 35.6 cubic feet, or 1.32 cubic yards. Inches are the usual trap: divide them by 12 before you multiply. The calculator lets you enter each dimension in whichever unit you measured it in, so it does this for you.
Sonotube math
Volume grows with the square of the diameter, so tube size matters more than depth. Going from an 8 in tube to a 12 in tube at the same depth more than doubles the concrete. The table above lists concrete per foot of height and for a full 4 ft tube. Tube forms come in a range of standard diameters; Sonoco's brochure lists standard sizes up to 36 in, and the FinishFree line in 6, 8, 10, and 12 in. Check what your supplier stocks, since not every store carries every size.
How big and how deep should a footing be?
This calculator tells you how much concrete a footing takes, not how large it needs to be. Size depends on the load, the soil, and your local code. Three points cover most residential questions.
Minimum size
The International Residential Code, section R403.1.1, is summarized as a floor of 12 in wide and 6 in thick for concrete footings. These are minimums, and the required size can be larger depending on stories, wall type, and soil bearing value. The size check in the calculator tests your numbers against that floor only. Passing it doesn't mean the footing is big enough for your load.
Minimum depth
Under R403.1.4, exterior footings go at least 12 in below undisturbed ground and below the frost depth your jurisdiction sets. Frost depth varies a lot by region, and that local number is usually what drives the length of a pier. Your building department can give it to you, usually over the phone.
A rule of thumb for wall footings
A common starting point is a footing twice as wide as the wall it carries and as thick as the wall is wide, so an 8 in wall sits on a footing about 16 × 8 in. That is a rule of thumb for estimating, not a code table. The code's own tables and an engineer's design take priority over it.
Choosing a footing type
- Wall (strip) footings run continuously under foundation and retaining walls. Enter the total run, adding up every side.
- Round piers carry point loads such as deck posts and porch columns. A cardboard tube forms the part above ground and gives a smooth, round column.
- Square pads spread a post load over a larger area of soil. They use more concrete than a narrow pier of the same depth.
- A tube on a pad combines both: a wide base for bearing and a narrower column up to grade. It takes less concrete than a full-width pier, and it is the shape to enter for a tube set on a spread footing or a poured base.
For deck footings specifically, the deck calculator counts the posts for your layout, which gives you the number to enter here.
Bags or ready-mix?
Bag yields are smaller than most people expect. An 80 lb bag of QUIKRETE Concrete Mix makes 0.60 cubic feet, a 60 lb bag 0.45, a 50 lb bag 0.375, and a 40 lb bag 0.30. That means one cubic yard takes 45 of the 80 lb bags, which is 3,600 lb of dry mix to carry and mix by hand. Other brands differ slightly, so confirm the yield printed on the bag.
Bags suit a few post holes. Once you reach roughly a yard, price a ready-mix delivery, and ask about the minimum order and short-load fee, because both vary by supplier. The calculator rounds ready-mix up to the next quarter yard, the way it is commonly ordered. The total bag count can be lower than bags per footing times the number of footings, because leftovers from one bag carry over to the next hole.
Common mistakes
- Measuring the tube, not the hole. In a hole dug without a form, concrete fills whatever you dug. A hole that bells out at the bottom takes more than the tube formula says, so measure it or raise the waste percentage.
- Forgetting the part above grade. Tube height is the full length of the concrete: buried depth plus anything above ground.
- Ordering exactly the calculated amount. Running short mid-pour leaves a cold joint. A 10% allowance is a sensible default for footings, because trench sides are rarely straight.
- Skipping the rebar question. Footing reinforcement barely changes the concrete volume, but it does matter. Use the rebar calculator for quantities and follow the plans for placement.
- Pouring on frozen or disturbed soil. Footings should bear on firm, undisturbed ground. Soil that was loosened by digging, or that is frozen, can settle later.
For a slab or anything else that isn't a footing, use the general concrete calculator.
Frequently asked questions
How do I calculate concrete for a footing?
Multiply length × width × thickness in feet and divide by 27 for cubic yards. A 40 ft footing 16 in wide and 8 in thick is 35.6 cubic feet, or 1.32 cubic yards. For a round footing, use 3.1416 × radius² × height.
How many bags of concrete do I need for a Sonotube?
Divide the tube volume by the bag yield. A 10 in tube 4 ft tall holds 2.18 cubic feet, about 4 bags of 80 lb mix or 5 bags of 60 lb. A 12 in tube 4 ft tall holds 3.14 cubic feet, about 6 bags of 80 lb or 7 of 60 lb.
How much concrete goes in an 8 inch Sonotube?
About 0.35 cubic feet per foot of height. A 4 ft tube takes 1.40 cubic feet, about 3 bags of 80 lb mix or 4 bags of 60 lb, before any extra for waste.
What is the minimum size for a concrete footing?
IRC R403.1.1 sets a floor of 12 in wide and 6 in thick for residential concrete footings. Your load, soil, and local code can require more, so treat those as minimums rather than a design.
How deep should a footing be?
Exterior footings go at least 12 in below undisturbed ground and below the local frost depth, under IRC R403.1.4. Your building department sets the frost depth, and in cold regions it is much deeper than 12 in.
How wide should a footing be for a wall?
A common rule of thumb is twice the wall width and as thick as the wall, so about 16 × 8 in for an 8 in wall. The required width really depends on the load and soil, so check the code tables or your plans.
How many bags of concrete are in a cubic yard?
45 bags of 80 lb, 60 bags of 60 lb, 72 bags of 50 lb, or 90 bags of 40 lb, using QUIKRETE published yields. Other brands differ slightly.
Should I use bags or ready-mix for footings?
Bags suit a few post holes. At around a yard, get a ready-mix price, since 45 bags means about 3,600 lb of dry mix to handle. Ask about the supplier’s minimum order and short-load fee.
How much extra concrete should I order for footings?
A 10% allowance is a sensible default, because trench sides and hole walls are rarely straight. Unformed holes that bell out at the bottom may need more.
Does the size check mean my footing is safe?
No. It only compares your width and thickness with the IRC minimums. It does not check load, soil, frost depth, or reinforcement. Those need your local code, your plans, or an engineer.
Related calculators
Sources
- Concrete Mix — product lineQUIKRETEPublished bag yields: 80 lb 0.60 ft³, 60 lb 0.45 ft³, 50 lb 0.375 ft³, 40 lb 0.30 ft³.
- Concrete Footing Size for a House (IRC 2021)JaspectorSummary of IRC R403.1.1: footings no less than 12 in wide and 6 in thick, with larger sizes by load and soil. A secondary summary; check the edition your jurisdiction adopted.
- Footing Depth and Frost Line Requirements (IRC 2018)JaspectorSummary of IRC R403.1.4: exterior footings at least 12 in below undisturbed ground and below the local frost depth.
- Sonotube Concrete Forming Products brochureSonocoTube form diameters: standard sizes up to 36 in; FinishFree in 6, 8, 10, and 12 in.
- NIST Guide to the SI, Appendix B: Conversion FactorsNational Institute of Standards and Technology (NIST)Exact factors for cubic feet, cubic yards, and cubic meters.