How the calculator works it out
List the fence in the order you would walk it: each straight section and each gate, one after the other. The calculator puts a post at the start, then walks along adding a post at the end of every bay. Because each section starts on the post where the last one ended, corner posts and gate posts are counted once. If the fence closes into a loop, the last post is the first one.
Bays in a section = section length ÷ post spacing, rounded up
Posts = 1 + all bays + 1 per gate (− 1 if the fence closes on itself)
Rails = bays × rails per bay · Pickets = section length (in) ÷ (picket width + gap), rounded up
Concrete per post = (π × (hole diameter ÷ 2)² − post area) × (hole depth − soil on top) ÷ 1,728
Worked example: a front fence with a walk gate and a corner
A 6 ft privacy fence runs 40 ft, then a 4 ft gate, then 30 ft more, then turns a corner for 50 ft. At 8 ft spacing that is 5 + 4 + 7 = 16 bays and 1 + 5 + 1 + 4 + 7 = 18 posts, two of them gate posts. Three rails a bay is 48 rails of 8 ft. Pickets 5½ inches wide: 88 + 66 + 110 = 264. Each 4×4 sits in a 10 inch hole 36 inches deep, filled to 3 inches below grade: (78.5 − 12.25) × 33 = 2,188 cubic inches, 1.27 cubic feet, or 3.4 bags of 50 lb fast-setting mix. For all 18 posts that is 22.8 cubic feet, 61 bags. Each post is 6 ft above ground plus 3 ft in the hole: buy 10 ft posts. This fence is loaded when the page opens.
Checked against Quikrete's own numbers
Quikrete's spec sheet says two 50 lb bags of Fast-Setting Concrete set a 4 inch post in a 10 inch hole 2 ft deep. With the mix stopped 4 inches below the top, the formula above gives (78.5 − 12.6) × 20 = 1,319 cubic inches, which is 0.76 cubic feet or 2.04 bags. The same sheet's slab yield, 0.375 cubic feet a bag, is the figure the calculator uses.
What the numbers are based on
- Concrete yield. Quikrete Fast-Setting Concrete spec data (#1004-50): about 0.375 cubic feet per 50 lb bag; a hole three times the post diameter and about half the above-ground post height deep; mix poured to 3 to 4 inches from the top. The 80 lb Concrete Mix option uses 0.6 cubic feet per bag, the yield printed on standard pre-mixed bags.
- Post sizes. Lumber is sold by nominal size: a 4×4 is actually 3½ × 3½ inches and a 6×6 is 5½ × 5½. The post area subtracted from the hole uses the actual size.
- Frost depth. Holes must reach below the local frost line, which is set by your jurisdiction (the International Residential Code leaves it to a local table). Call your building department or check your permit packet.
- Stock lengths. Posts are rounded up to 8, 10, 12, 14 or 16 ft and rails to 8, 10, 12 or 16 ft, the lengths lumber yards usually stock.
Before you dig
- Call 811 a few days before digging so buried utility lines are marked. It is free.
- Gate posts carry more load. Many builders use a 6×6 or set gate posts deeper; count them separately on the list.
- Check property lines and any HOA or city height limits before you buy.
Questions people ask
How many fence posts do I need?
One post at the start, then one more for every bay. A bay is the gap between posts, so a 40 ft run with posts 8 ft apart has 5 bays and needs 6 posts. Where two runs meet at a corner, or a run meets a gate, they share the post. A 40 ft run, a 4 ft gate, another 30 ft and a 50 ft side round the corner come to 1 + 5 + 1 + 4 + 7 = 18 posts, not the 21 you would get by working each piece out separately.
How many bags of concrete per fence post?
Work out the volume of the hole, take away the post, and divide by what one bag makes. Quikrete says a 50 lb bag of Fast-Setting Concrete makes about 0.375 cubic feet. A 4×4 post in a 10 inch hole 36 inches deep, filled to 3 inches below the ground, takes about 1.27 cubic feet, so 3.4 bags. Quikrete's own example is two bags for a 4 inch post in a 10 inch hole 2 ft deep, and the calculator agrees with it.
How deep should fence post holes be?
Quikrete's guidance is a hole about one-half the above-ground height of the post: 36 inches for a 6 ft fence. In cold climates the bottom of the hole should also be below the frost line, which your local building department sets. Make the hole about three times the width of the post, so 10 to 12 inches for a 4×4.
What post spacing should I use?
8 ft is the usual spacing for wood privacy fences, because rails are sold in 8 ft lengths and one rail spans one bay. Closer spacing, such as 6 ft, makes a stiffer fence for windy sites or heavy boards. Vinyl and metal panels come in fixed widths, so set the spacing to the panel width.
How many pickets for a privacy fence?
Divide the length of fence in inches by the actual width of a picket. A "1×6" picket is about 5½ inches wide, so 40 ft of privacy fence takes 480 ÷ 5.5 = 87.3, so 88 pickets. For a spaced picket fence, add the gap to the picket width first. Gates are left out because gate kits are usually built from their own boards.
Do I need three rails or two?
Two rails are common on fences up to about 4 ft; most 6 ft privacy fences use three so the boards do not warp between rails. The calculator starts at three. Change "Rails per bay" to match your design.