How the fence post planner works
Setting posts in concrete is mostly a counting job: how many posts, how much space each one leaves in its hole, and how many bags fill all of those spaces. This planner counts the posts for you from your fence runs, takes the real size of the post out of every hole, and works out the bags for the whole job at once.
Two ways to count posts
If you already know the number of posts, choose “I know the post count” and type it in. Otherwise choose “Calculate from fence runs” and enter each straight run with its length and post spacing. Each run needs its length divided by the spacing, rounded up, plus one post to close the run. Add gate posts with “Extra posts”. Where two runs meet at a corner and share one post, that post is counted twice, so enter it under “Shared posts to subtract”.
Nominal and actual post sizes
Lumber is sold by nominal size, which is larger than the real size. A 4×4 post actually measures about 3.5 × 3.5 in, and a 6×6 about 5.5 × 5.5 in. The post fills part of the hole, so using the actual size gives a more realistic concrete volume. Round posts use their diameter. If your posts differ, choose a custom size and measure one.
Worked example
One 80 ft run with posts every 8 ft, plus one gate post, using 4×4 posts in holes 10 in wide and 24 in deep, with no gravel and 10% waste. Load it with the “Try the worked example” button.
- Posts: 80 ÷ 8 = 10, plus 1 to close the run = 11, plus the gate post = 12 posts.
- Space per hole: the hole covers 78.54 in², the post 12.25 in², leaving 66.29 in². Over 24 in of depth that is 1,591 in³, or 0.921 ft³ per post.
- Whole job: 12 × 0.921 = 11.05 ft³, plus 10% waste = 12.15 ft³.
- Bags: 21 bags of 80 lb (0.60 ft³ each), 28 bags of 60 lb (0.45 ft³) or 33 bags of 50 lb (0.375 ft³).
Gravel at the bottom of the hole
Some fence plans put a layer of gravel under each post. If yours does, enter its depth. The concrete is then calculated only above the gravel, and the gravel volume is the full hole area times the gravel depth, listed in bags. Whether to use gravel is a question for your plan or the post manufacturer; this tool only does the math.
Important assumptions
- Holes are straight-sided cylinders with the diameter and depth you enter.
- Concrete fills the space between the post and the hole wall, from the top of any gravel to the top of the hole.
- Waste starts at 10% because dug holes are rarely perfectly round or straight.
- Bag yields start at 0.375, 0.45 and 0.60 ft³ for 50, 60 and 80 lb bags. Metric yields use the same yield-per-weight ratio for 20, 25 and 30 kg bags. Yields vary by product, so check the bag.
- Posts and bags are always rounded up.
What this tool does not calculate
- Hole depth, post spacing, frost depth or post length.
- Permits, property lines, wind loads or whether a fence is structurally sound.
- Rails, panels, pickets, hardware or gates.
- Water for mixing, or how the concrete should be placed.
Fence post concrete FAQ
How many bags of concrete does one fence post need?
It depends on the hole size, the hole depth and the post. In the worked example below, a 4×4 post in a 10 in hole 24 in deep needs about 0.92 ft³ of concrete, which is roughly 1.7 bags of 80 lb once waste is added. Enter your own sizes to get your number.
Why calculate the whole job instead of one post at a time?
Rounding up once for the whole job wastes less. If you round each post up to 2 bags, 12 posts need 24 bags. Working out the total first and rounding once gives 21 bags of 80 lb for the same holes.
Should gate posts be included?
Yes. When you calculate from runs, every run already counts its two end posts. Add gate posts or extra corner posts with “Extra posts”, and use “Shared posts to subtract” where two runs meet at the same post.
Can I mix 50, 60 and 80 lb bags?
The concrete volume is what matters, so the planner shows all three bag sizes for the same job. The shopping list uses the size you pick and mentions the other two, so you can buy whichever your store has in stock.
Related planners
Concrete quantities are estimates. Hole shapes and bag yields vary, so check the bag label and confirm with your supplier. This tool does not recommend post depth, spacing or fence design. See how we calculate for the full method.