Insta-Footings Load Capacity: Sizing Guide (2026)

Choosing an undersized footing is the most common reason a post shifts or settles within a few seasons. This guide covers how Insta-Footings load capacity is determined by footing diameter, soil bearing conditions, and the structure it's supporting.

Why this matters

A footing's job is to spread a post's point load over enough soil area that the ground doesn't compress or shift under it. Insta-Footings size their bearing surface and depth to carry a rated load range — undersizing the footing for the actual point load is what leads to a post that settles, leans, or heaves within a season or two.

Most footing failures in 2026 aren't from a defective footing — they're from a footing sized to the structure's overall look rather than the actual load path. A deck post at a stair landing or a beam splice carries far more point load than a mid-span post on the same deck. Size to the highest point load in the structure, not the average.

What determines the load a footing can carry

1. Footing diameter and depth

A wider footing spreads the same point load over more soil surface area, reducing the pressure per square inch the ground has to resist; depth matters because it reaches past the shallow, less stable topsoil layer into firmer bearing soil. Insta-Footings come in sizes matched to common post loads — check the load rating for your specific footing against your calculated point load before installing. Common mistake: sizing the footing to match the post's width rather than the actual load it needs to carry.

2. Soil bearing capacity

The same footing performs very differently in firm, undisturbed clay versus loose sand, fill dirt, or soil with a high water table. If you're working in fill soil, near a downspout, or in an area with poor drainage, step up a footing size or improve drainage before installing. Common mistake: assuming the footing's rated capacity applies regardless of what's actually in the ground — always dig a small test hole to check soil consistency first.

3. The structure's point loads

Calculate load per post, not total structure weight divided evenly — posts at stair landings, beam splices, or corners typically carry more than posts at mid-span. A deck with six posts doesn't split its total load six equal ways; corner and landing posts often carry 40-60% more than field posts. Common mistake: using one footing size across every post in a structure regardless of where each one sits in the load path.

4. Frost depth and seasonal movement

In regions with freeze-thaw cycles, footing depth also has to reach below the local frost line or the footing itself can heave seasonally regardless of its load rating. Check your local frost depth requirement before finalizing footing depth, not just load capacity. Common mistake: sizing purely for load and skipping the frost-depth check, which causes seasonal movement even on a properly load-rated footing.

Matching footings to common structures

• Fence posts — lower point load, but frost depth and wind load on the fence panel still matter more than raw weight.
• Deck posts — size to the highest point load in the deck's load path (stair landings, beam splices), not the average post.
• Shed and pergola posts — Insta-Footings for shed and pergola foundations typically carry lighter, more evenly distributed loads than a deck, but roof snow load in some regions raises the requirement.
• Mailbox, sign, and light posts — lowest load case, but wind exposure on a sign face can exceed what the post's weight alone suggests.

Troubleshooting

• Post settles unevenly after a season — likely an undersized footing for the local soil, especially in fill dirt or an area with poor drainage; step up a size and check drainage.
• Post heaves seasonally — footing depth may not reach below the local frost line; check your jurisdiction's required frost depth.
• Unsure of actual point load — calculate load per post based on the structure's beam spans and post spacing rather than guessing from total structure weight.
• Working in sandy, loose, or previously disturbed soil — step up footing size or improve compaction before installing, since rated capacity assumes reasonably firm bearing soil.
• Structure includes stair landings or beam splices — these posts need a larger footing than field posts on the same structure.

Tools and resources

• A soil test (simple hand-auger check) to confirm bearing conditions before ordering footings
• Local frost depth requirement from your building department
• Beam span and post spacing plan to calculate point load per post
• Insta-Footings sized to your calculated load and soil condition

What to do next

If you're unsure how to calculate point load per post or your soil is fill, sandy, or poorly draining, check the load rating and sizing guidance for Insta-Footings before ordering — sizing to the structure's heaviest point load, not its average, is what prevents settling.

FAQ

One last thing

Size to your structure's worst-case point load — a stair landing or beam splice post — not its average, since that's where an undersized footing shows up first.