Deck Footing Concrete Volume Calculator
Use this deck footing concrete calculator to find out how much concrete you need for your post holes. Enter the footing diameter, depth, and number of footings, and the tool will estimate total volume in cubic feet and cubic yards, plus an approximate number of 80 lb premix bags. This helps you buy enough concrete, avoid extra trips, and plan whether to mix by hand, use a portable mixer, or order ready-mix.
How to use this deck footing concrete calculator
- Measure the footing diameter in inches (for example, 8", 10", 12", or 18"). This is the width of the hole you will drill or dig.
- Measure or choose the footing depth in inches. Make sure the depth meets or exceeds your local frost-depth and building code requirements.
- Enter the number of footings (post holes) you plan to pour.
- Click the Calculate button.
The calculator then outputs:
- Concrete volume per footing (cubic feet)
- Total concrete volume for all footings (cubic feet)
- Total concrete volume (cubic yards), useful when comparing to ready-mix truck minimums
- Approximate 80 lb bag count, assuming each 80 lb premix bag yields about 0.6 cubic feet of concrete
Concrete volume formula for cylindrical deck footings
Each deck footing is treated as a simple cylinder. The volume of a cylinder depends on its radius and its depth (height). The basic formula is:
V = π × r² × h
Where:
- V = volume of one footing
- r = radius of the footing (half the diameter)
- h = footing depth
Because you enter diameter and depth in inches, the calculator first works in cubic inches, then converts to cubic feet and cubic yards.
The cylinder volume in cubic inches is:
Vin³ = π × (diameter / 2)² × depth
There are 12 inches in a foot, so one cubic foot is 12 × 12 × 12 = 1,728 cubic inches. To convert from cubic inches to cubic feet, the calculator divides by 1,728:
Vft³ = Vin³ / 1,728
Then, to convert to cubic yards, it divides by 27 (because one cubic yard is 27 cubic feet):
Vyd³ = Vft³ / 27
In a more compact form, with diameter and depth in inches and volume in cubic feet, the calculator effectively uses:
Vft³ = [π × (diameter / 2)² × depth] / 1,728
Formula in MathML
The core cylinder volume formula can also be written in MathML as:
Here, V is the volume of one footing, r is half of the footing diameter, and h is the footing depth.
Interpreting your results
After you click Calculate, the results panel shows several values. Here is how to use each one in planning your project:
- Volume per footing (cubic feet) – This tells you how much concrete is needed for a single post hole. It is useful if you are pouring some footings on one day and the rest later, or if some footings are a different size.
- Total volume (cubic feet) – This is the main figure for purchasing bagged concrete. Compare this number against the yield of each bag size you plan to use. The calculator assumes 0.6 cubic feet per 80 lb bag as a standard rule of thumb.
- Total volume (cubic yards) – Use this value if you are considering ordering ready-mix concrete from a truck. Suppliers often quote minimum deliveries in cubic yards.
- Estimated 80 lb bag count – This divides the total cubic feet by 0.6 ft³ per bag. You should always round this number up to the next whole bag and consider adding an extra 5–10% for spillage, over-excavation, and minor errors.
If your calculated volume or bag count seems unexpectedly high, double-check that:
- Diameter and depth are entered in inches, not feet
- You entered the correct number of footings
- You did not accidentally double a dimension or misread a tape measure
Worked example: 6 footings, 12 in diameter, 36 in deep
The following example illustrates how the calculator estimates concrete volume for a typical medium-size deck.
- Footing diameter = 12 in
- Footing depth = 36 in
- Number of footings = 6
Step 1: Volume of one footing
Radius r = 12 in ÷ 2 = 6 in.
Volume in cubic inches:
Vin³ = π × 6² × 36 ≈ 3.1416 × 36 × 36 ≈ 4,071.5 in³
Convert to cubic feet:
Vft³ = 4,071.5 ÷ 1,728 ≈ 2.36 ft³ per footing
Step 2: Total volume for all footings
Total ft³ = 2.36 × 6 ≈ 14.16 ft³
Convert to cubic yards:
Total yd³ = 14.16 ÷ 27 ≈ 0.52 yd³
Step 3: Estimate 80 lb bag count
Assuming each 80 lb bag yields about 0.6 ft³ of concrete:
Bags = 14.16 ÷ 0.6 ≈ 23.6 bags
You would round up to 24 bags at a minimum and likely buy one or two extra to cover waste and any slightly deeper or wider holes.
Quick reference: volume per foot of depth for common diameters
For rough planning, it is useful to know how much concrete each additional foot of depth adds for common footing diameters. The values below assume straight-sided cylindrical holes.
| Footing diameter (in) | Volume per 1 ft depth (ft³) | Approx. 80 lb bags per ft |
|---|---|---|
| 8 | 0.35 | 0.6 |
| 10 | 0.55 | 0.9 |
| 12 | 0.79 | 1.3 |
| 18 | 1.77 | 3.0 |
You can use this table for quick mental estimates. For example, if you are planning four 10 in footings that are 3 ft deep, each footing needs about 0.55 × 3 ≈ 1.65 ft³, and the total is about 6.6 ft³. Dividing 6.6 by 0.6 ft³ per bag gives roughly 11 bags of 80 lb premix.
Comparison of calculation approaches
You can estimate concrete for deck footings using several methods. The calculator automates the precise cylindrical volume formula, but the table below compares this approach with other common rules of thumb.
| Method | How it works | Typical accuracy | Best use case |
|---|---|---|---|
| Exact cylinder formula (this calculator) | Uses π × r² × h with your actual diameter, depth, and footing count; converts to ft³, yd³, and 80 lb bag count. | High, assuming holes match design size. | Most decks where you want a reliable estimate before purchasing materials. |
| Rule-of-thumb table | Uses precomputed volume per foot of depth for standard diameters (e.g., 8", 10", 12"). Multiplies by depth. | Moderate; fine for quick planning, less exact for non-standard sizes. | Early project planning or rough budgeting when final footing sizes are not yet fixed. |
| Visual guesswork | Contractor or DIYer estimates volume by experience or by comparing to bucket or tub sizes. | Variable; can be off significantly, especially for deeper or wider holes. | Small, informal projects where minor over- or under-buying of bags is acceptable. |
| Ready-mix supplier minimums | Orders a standard truck minimum (e.g., 3 yd³) and accepts leftover concrete as waste. | Depends on project; tends to overshoot true need. | Large decks or combined pours where meeting the supplier minimum is not an issue. |
Assumptions, limitations, and practical tips
The calculator is designed as a material estimating tool. It does not check structural capacity or building code compliance. Keep the following assumptions and limitations in mind when using the results:
- Simple cylindrical shape only – Footings are assumed to be straight-sided cylinders with a constant diameter from top to bottom. Bell-shaped bases, flared tops, or pier blocks are not modeled.
- No allowance for over-excavation – Real holes are often slightly wider or deeper than planned. Soil sloughing, rocks, and uneven bottoms can all increase the actual volume beyond the calculated value.
- No waste factor built in – The calculator does not automatically add a cushion for spillage, losses in the mixer, or leftover concrete in bags and tools. Many builders add 5–10% to the calculated volume or bag count.
- Standard 80 lb bag yield – A yield of about 0.6 ft³ per 80 lb bag is a commonly used approximation. Actual yield can vary slightly by manufacturer, mix design, and how much water is added. Always check the printed yield on the bag.
- Units are critical – Inputs are in inches. Entering feet by mistake (for example, 3 instead of 36) will drastically under-estimate concrete needs.
- No structural design – The calculator does not evaluate whether your footing diameter, depth, or spacing is adequate for the loads from your deck, nor does it address uplift, lateral loads, or soil bearing capacity.
- Local codes and inspections – Building codes for footing size, depth below frost, and reinforcement vary by location. Your local building department or a structural engineer should confirm compliance before you build.
Because of these factors, treat the output as a solid starting point for ordering materials, not as an engineering stamp of approval. When in doubt, consult with a qualified professional.