How many fence posts do I need?
Take the gate widths off the fence length. Divide what is left by the post spacing and round up: that is the number of sections. You need one post more than the sections, because the run has a post at each end, plus one more post for each gate, because a gate needs a post on both sides.
A 100 ft fence with one 4 ft gate and posts 8 ft apart has 96 ft of fencing. 96 ÷ 8 = 12 sections, so 12 + 1 + 1 = 14 posts.
The formula
- Fenced length: fence length minus (gates x gate width).
- Sections: fenced length ÷ post spacing, rounded up.
- Posts: sections + 1, plus 1 for each gate.
- Rails: sections x rails per section.
- Pickets: fenced length in inches ÷ (picket width + gap), rounded up. The waste allowance is added and the total is rounded up again.
- Concrete: one hole per post. Each hole is a cylinder, π x (diameter ÷ 2)² x depth, less the post standing in it. Bags = total volume ÷ the bag's yield, rounded up.
Mixed units are fine. The calculator converts every measurement to feet first, using 1 in = 2.54 cm exactly.
Worked example: 100 ft with one gate
The fence is 100 ft long with one 4 ft gate. Posts are 8 ft apart with 2 rails per section. Pickets are 5.5 in wide, butted tight, with a 10 percent allowance. Each post is a 4x4 (3.5 in square) in a hole 10 in across and 24 in deep.
- Fenced length: 100 - 4 = 96 ft.
- Sections: 96 ÷ 8 = 12.
- Posts: 12 + 1 + 1 gate = 14 posts.
- Rails: 12 x 2 = 24 rails.
- Pickets: 96 ft is 1,152 in, and 1,152 ÷ 5.5 = 209.5, rounded up to 210. With 10 percent, 210 x 1.10 = 231 pickets.
- Concrete: one hole is π x 5² = 78.54 sq in, less the post's 12.25 sq in, = 66.29 sq in, x 24 in deep = 1,591 cu in, or 0.92 cu ft. 14 holes take 12.89 cu ft. At 0.60 cu ft per 80 lb bag, that is 21.5, rounded up to 22 bags of 80 lb, or 29 bags of 60 lb, or 35 bags of 50 lb.
With a 1/2 in gap between the same pickets, each picket and its gap take 6 in, so 1,152 ÷ 6 = 192 pickets, or 212 with 10 percent.
Fences with corners
The calculator counts one straight run. For a fence with corners, run it once for each side. Neighboring sides share their corner post, so take one post off the total for each corner. Use the "Add to project list" button to keep the sides together.
Post spacing and rails
The calculator starts at posts 8 ft apart and 2 rails per section. Those are starting values you can change, not recommendations. If you build with ready-made panels, use the panel width plus the post as your spacing. The calculator counts one rail per section for each row of rails, so each rail spans one section.
Picket width and gap
The calculator starts at a 5.5 in picket with no gap. Measure your picket across its face. For a spaced picket fence, enter the gap you want between pickets. The pickets are counted on the fenced length only: gates are bought or built separately.
Post hole concrete
Enter the hole diameter and depth to count concrete for every post hole. The post size is taken out of each hole, because the post fills part of it. Leave the post size blank for a solid hole. Bag counts use the yields in the table below and add no allowance, so holes dug wider than planned take more.
How deep should fence posts be?
This calculator does not set a post depth. The depth depends on the fence height, the soil and, in cold areas, the frost line. Use the depth in your post or fence maker's installation guide and your local building code, then enter it as the hole depth.
Waste allowance
The calculator starts at 10 percent extra pickets. That is a planning estimate, not a published figure. It covers split, warped and rejected pickets. Posts, rails and concrete get no allowance.
What the calculator leaves out
- Gates: the gate leaves, hinges and latches.
- Fasteners and brackets: nails, screws and rail brackets.
- Gravel under the posts: if your plan puts gravel in the bottom of each hole, use the gravel calculator.
- Post caps, kickboards and trim.
Common mistakes
- Forgetting the last post. A run of 12 sections needs 13 posts, not 12.
- Counting the gate as fence. Take the gate width off before counting sections and pickets.
- Dividing by the bare picket width. Each picket takes up its width plus its gap.
- Rounding down. Posts, rails, pickets and bags are sold whole, so always round up.
Values this calculator uses
Concrete bags are counted from the maker's stated yield per bag. Yields are approximate. Your bag's label wins if it differs.
| Item | Value | Source |
|---|---|---|
| 80 lb bag | 0.6 cu ft | Quikrete Concrete Mix (No. 1101) data sheet yield |
| 60 lb bag | 0.45 cu ft | Quikrete Concrete Mix (No. 1101) data sheet yield |
| 50 lb bag | 0.375 cu ft | Quikrete Concrete Mix (No. 1101) data sheet yield |