How should a post hole be dug in terms of bottom vs top dimensions?

How should a post hole be dug in terms of bottom vs top dimensions?

• A. About one third larger on the bottom than the top
• B. Same size on the top and bottom
• C. About one third larger on the top than the bottom
• D. 3 times wider on the top than the bottom

Answer: (A)

The key idea is that a post hole should have a wider bottom than top to create a stable base that resists tipping and allows the concrete or soil around the post to lock it securely in place. Making the bottom about one third larger than the top gives a slight “bell” shape: the post sits snugly up near the surface, while the broader bottom provides a solid anchor and plenty of surrounding material for the concrete to grip. This shape helps counteract frost heave and lateral movement, improving long-term stability. If the hole were the same size throughout, there wouldn’t be that extra base to resist tilt or provide a good anchor for the surrounding material. If the top were larger than the bottom, you’d have a widening near the surface that can invite water pooling and moisture issues at the post, weakening the hold. A hole that is three times wider at the top would severely destabilize the post and waste material, making it difficult to properly seat and anchor the post.

The key idea is that a post hole should have a wider bottom than top to create a stable base that resists tipping and allows the concrete or soil around the post to lock it securely in place. Making the bottom about one third larger than the top gives a slight “bell” shape: the post sits snugly up near the surface, while the broader bottom provides a solid anchor and plenty of surrounding material for the concrete to grip. This shape helps counteract frost heave and lateral movement, improving long-term stability.

If the hole were the same size throughout, there wouldn’t be that extra base to resist tilt or provide a good anchor for the surrounding material. If the top were larger than the bottom, you’d have a widening near the surface that can invite water pooling and moisture issues at the post, weakening the hold. A hole that is three times wider at the top would severely destabilize the post and waste material, making it difficult to properly seat and anchor the post.