How deep should fence posts be?
The standard rule is to bury at least one third of the post's above-ground height, and never less than 2 ft. Gate, end and corner posts take more strain, so set them about 6″ deeper. Here's what that works out to with 4×4 posts and no frost to worry about:
| Fence height | Line post depth | Line post length | Gate/end post depth | Gate/end post length |
|---|---|---|---|---|
| 3 ft | 2 ft | 8 ft | 2.5 ft | 8 ft |
| 4 ft | 2 ft | 8 ft | 2.5 ft | 8 ft |
| 5 ft | 2 ft | 8 ft | 2.5 ft | 8 ft |
| 6 ft | 2 ft | 8 ft | 2.5 ft | 10 ft |
| 7 ft | 2.5 ft | 10 ft | 3 ft | 10 ft |
| 8 ft | 3 ft | 12 ft | 3.5 ft | 12 ft |
Post length is the shortest standard lumber length that covers the height plus the buried depth. A 6 ft privacy fence uses 8 ft line posts, but its gate posts need 10 ft.
Go below the frost line
In cold climates the ground freezes and expands every winter. A post that stops above the frost line gets pushed up a little each year (called frost heave) until the fence leans or the gate won't close. If your frost depth is deeper than the rule of thumb, set the post at least that deep. The calculator always uses whichever is deeper.
How wide should the hole be?
Dig the hole about three times the width of the post: roughly 10–11″ across for a 4×4 (which is actually 3½″ wide) and 16–17″ for a 6×6. A wider hole means more concrete around the post, which resists leaning better than simply going deeper.
Why put gravel at the bottom?
Dig 6″ deeper than the post depth and put in a layer of gravel before setting the post. It lets water drain away from the end of the post, which is where wooden posts rot first. The calculator includes the gravel layer in the total hole depth.
Concrete or gravel only?
Concrete is the most common choice and the right one for gate posts and windy spots. Setting line posts in tamped gravel instead drains better and makes a post easier to replace later, but it needs firm soil and holds a little less well. Either way, slope the top of the fill away from the post so rain runs off. To count bags for all your posts, use the fence post concrete calculator.
Soil makes a difference
Sandy or loose soil holds posts less firmly, so go 6″ deeper or dig a wider hole. Heavy clay holds well but drains poorly, so the gravel layer matters more. In rocky ground, a post hole digger or auger rental will save you a lot of time.
Planning the whole fence?
The Fence Calculator works out every post, rail, picket and bag of concrete for your fence length, with a material cost estimate.
Open the Fence Calculator →