Round vs. Square Concrete Footings: Which Should You Use for Deck Posts?
Both shapes carry a deck post just fine structurally — the real differences are in concrete usage, forming method, and what your local code actually requires.
On this page13 sections
- Round vs. Square at a Glance
- Why the Shape Matters
- Shape-Related Terms
- Concrete Usage Comparison
- How to Decide Which Shape Fits Your Project
- Concrete Volume Difference by Size
- Alternative: Octagonal or Chamfered Forms
- Common Mistakes
- Which Should You Use: Decision Table
- Other Factors to Weigh
- Choosing Your Shape: Step by Step
- Frequently Asked Questions
- Related Calculators
Whichever shape you choose, the concrete pier calculator can work out your concrete volume — this guide covers which shape actually fits your situation and why the difference matters.
Round vs. Square at a Glance
| Round (Sonotube) | Square (Formed Pad or Box) | |
|---|---|---|
| Concrete usage for same bearing area | Lower — a circle has the smallest perimeter for a given area | Higher — corners add area beyond what’s structurally needed |
| Typical use case | Standalone pier, most common residential choice | Wider base pad under a pier, or code-specified footing shape |
| Forming method | Cardboard sonotube or unformed auger hole | Plywood or dimensional-lumber form box |
| Ease of digging | Matches a round auger bit directly | Requires hand-squared corners even with an auger-started hole |
| Combining with a wider footing pad | Less common — usually paired with a round pad if used | Common — square pad under a round or square pier |
Why the Shape Matters
For a given cross-sectional area, a circle has less perimeter than a square, which is why round piers consistently use less concrete than square ones covering the same bearing footprint. That’s the main practical reason round dominates standalone deck pier construction — it’s both cheaper and matches the shape a power auger naturally digs.
Square footings earn their place in a different scenario: when the design calls for a wider base pad beneath a narrower pier shaft — a stepped or “footing-and-pier” design — a square pad is often easier to form flat and level than a round one, especially when hand-dug rather than machine-formed.
Shape-Related Terms
| Term | What It Means |
|---|---|
| Pier | The vertical concrete shaft supporting the post, round or square in cross-section. |
| Footing pad | A wider, flatter base poured beneath a pier to spread load over more bearing area. |
| Footing-and-pier (stepped) design | A two-part pour: a wide square or round pad at the bottom, with a narrower pier shaft rising from its center. |
| Monolithic pier | A single-diameter pier with no separate wider pad — the most common simple residential design. |
Concrete Usage Comparison
Round pier volume = π × radius² × depth
Square pier volume = side² × depth
For the same bearing area (cross-sectional area), a square footing uses roughly 27% more concrete than a round one, since a circle inscribes the maximum area for the minimum perimeter of any shape.
How to Decide Which Shape Fits Your Project
- Check your local code for a specified footing shape. Some jurisdictions specify a minimum square footing area rather than a diameter, which can make square the simpler compliance path.
- Check whether your design needs a stepped footing-and-pier. If your soil bearing capacity requires a wider base than the pier shaft itself, a square pad is often easier to hand-form than a round one.
- Check what forming equipment or materials you have access to. A sonotube is a simple, inexpensive purchase; a square form box requires plywood, stakes, and more setup labor per footing.
- Check whether you’re using a power auger. Auger-dug holes are inherently round, making an unformed round pier the path of least resistance if soil holds its shape.
Example 1: Standard Standalone Pier
- Situation: Simple residential deck, auger-dug holes, firm soil
- Shape chosen: Round, 12 in. diameter
- Reasoning: Matches auger shape, uses less concrete, no wider pad needed
This is the default case for most residential deck piers — round is both the cheaper and easier option with no offsetting benefit to going square.
Example 2: Weak Bearing Soil, Wider Pad Needed
- Situation: Soil bearing capacity requires a wider base than the pier shaft
- Shape chosen: Square 24 in. x 24 in. pad, with a 12 in. round pier shaft above
- Reasoning: Square pad forms flat and level more easily by hand than a wide round pad
The stepped design spreads load over a wider footprint without pouring the entire pier shaft at that larger diameter, saving concrete versus a full-width round pier the whole way up.
Example 3: Local Code Specifies Square Footing Area
- Situation: Local code table lists minimum footing size in square inches, based on a square footing assumption
- Shape chosen: Square, sized to meet the code table directly
- Reasoning: Avoids converting a square-based code requirement into an equivalent round diameter
Some code tables are written assuming a square footing, and building to that shape directly avoids any ambiguity in translating the required bearing area into a round diameter.
Round or square, get your exact concrete volume with the concrete pier calculator before you order — enter your dimensions and it handles the math either way.
Concrete Volume Difference by Size
| Bearing Dimension | Round Pier Volume (24 in. deep) | Square Pier Volume (24 in. deep) |
|---|---|---|
| 10 in. diameter / 10 in. side | ~1.09 cu ft | ~1.39 cu ft |
| 12 in. diameter / 12 in. side | ~1.57 cu ft | ~2.00 cu ft |
| 16 in. diameter / 16 in. side | ~2.79 cu ft | ~3.56 cu ft |
The roughly 27% concrete difference holds consistently across sizes, since it’s a function of the shapes themselves rather than the specific dimension.
Alternative: Octagonal or Chamfered Forms
Some manufactured concrete form systems offer an octagonal or chamfered-corner option as a middle ground — closer to round’s material efficiency while still forming with flat panel sections rather than a curved tube. These are less common on standard residential decks and usually show up on architectural or exposed-finish applications where the pier itself is visible and a faceted look is intentional, rather than as a practical alternative for a buried structural footing.
Common Mistakes
- Assuming shape affects structural strength. For the same bearing area, round and square footings perform comparably — the difference is material efficiency and forming method, not strength.
- Converting a code’s square footing spec into an undersized round diameter. A round footing needs a larger diameter than a square footing’s side length to match the same bearing area — don’t use the two numbers interchangeably.
- Hand-digging a round hole for a square form. Squaring off an auger-started round hole adds labor that a straight square form box from the surface avoids.
- Skipping a wider pad where soil bearing capacity requires one. A narrow pier shaft with no wider base can settle unevenly in weaker soil regardless of shape.
- Not checking local code before defaulting to round. Some jurisdictions specify footing dimensions in a way that assumes a particular shape — confirm before assuming round is always acceptable.
Which Should You Use: Decision Table
| Situation | Recommended Shape |
|---|---|
| Standard standalone pier, no wider pad needed | Round (sonotube or auger hole) |
| Auger-dug holes, firm soil | Round — matches the hole shape naturally |
| Weak bearing soil requiring a wider base pad | Square pad, round or square pier shaft above |
| Local code specifies footing area assuming a square shape | Square, sized directly to the code table |
| Minimizing concrete cost is the priority and code allows either | Round — uses meaningfully less concrete for the same bearing area |
Other Factors to Weigh
- Available forming materials. Sonotubes are a simple retail purchase; square forms need plywood, stakes, and more assembly time per footing.
- Number of piers on the job. Concrete savings from round shape scale up meaningfully across a deck with many footings.
- Soil bearing capacity. Weak soil may push toward a stepped footing-and-pier design regardless of which shape you’d otherwise prefer.
- Local code table format. Whether the code specifies diameter or square footage directly can make one shape simpler to verify compliance with.
- Labor and equipment on hand. A power auger naturally favors round; hand-digging with square forming lumber on hand may favor square.
Choosing Your Shape: Step by Step
- Check your local code for any specified footing shape or bearing area format.
- Check soil bearing capacity to see if a wider pad beyond a standard pier diameter is needed.
- Choose round for a standard standalone pier unless code or soil conditions point elsewhere.
- Choose square (or a stepped footing-and-pier) if a wider base pad is required or code specifies square dimensions.
- Confirm your chosen dimension meets the required bearing area either way.
- Calculate concrete volume for your final shape and size with the concrete pier calculator.
Frequently Asked Questions
Is a round or square footing stronger?
Neither shape is inherently stronger for the same bearing area — strength depends on concrete quality, depth, reinforcement, and soil bearing capacity, not the cross-sectional shape itself.
Why does round use less concrete than square?
A circle encloses the maximum area for a given perimeter of any shape, so a round footing needs less material to achieve the same bearing area as a square one — roughly 27% less for an equivalent footprint.
Can I combine a square pad with a round pier?
Yes, this is a common stepped footing-and-pier design — a wider square pad spreads load over more bearing area at the bottom, with a narrower round pier shaft rising from its center to grade.
Does my local code specify a shape?
It varies — some codes specify a minimum diameter (implying round), others a minimum square footage (implying square), and some just require a minimum bearing area you can meet with either shape. Check your specific local code table.
Is a square footing easier to dig by hand?
It can be, since squaring off corners with a shovel is often more intuitive by hand than maintaining a perfectly round shape without a form — though a sonotube form solves this for round footings just as easily.
Do I need a form for a square footing, or can I dig it unformed?
An unformed square hole is possible in very firm soil but is harder to keep square and vertical than a round unformed hole, since corners tend to slump inward — a form box is the more reliable approach for a clean square shape.
More deck pier & footing guides
Shape decided? Use the concrete pier calculator to get your exact concrete volume and bag count before you order.