Example Data Table
| Fence job |
Posts |
Hole size |
Depth |
Post size |
Waste |
| Backyard wood fence |
12 |
Round, 10 in |
24 in |
Round, 4 in |
10% |
| Gate and corner section |
6 |
Round, 12 in |
30 in |
Square, 6 in by 6 in |
15% |
| Metric garden fence |
10 |
Round, 25 cm |
60 cm |
Round, 10 cm |
10% |
Formula Used
Round hole volume: π × radius² × concrete height.
Square hole volume: width × length × concrete height.
Post displacement: post area × embedded depth inside concrete.
Concrete per post: hole concrete volume − post displacement.
Total concrete: concrete per post × post count × (1 + waste percent ÷ 100).
Bags needed: ceiling of total concrete volume ÷ yield per bag.
Estimated cost: bags needed × cost per bag + extra material cost.
How to Use This Calculator
Choose imperial or metric units first. Enter a manual post count, or use fence length and spacing. Select the hole shape. Add the hole size and depth. Enter the post shape and the part of the post inside concrete. Add gravel depth if used. Set bag yield from the concrete label. Add waste, cost, water, and density. Press calculate. The result appears above the form.
Fence Post Concrete Estimating Guide
Why Concrete Volume Matters
A fence post looks simple, but the concrete below it does most of the work. Good estimating helps the post resist wind, gate movement, frost, and daily fence pressure. It also helps you avoid wasted bags on a long run. This calculator compares hole volume with the part of the post that fills the hole. The remaining space is the concrete volume.
Planning Strong Post Holes
Post holes are not always the same. Corner posts, gate posts, and end posts may need wider holes. Line posts often use smaller holes. Deep holes usually give more support, but soil type matters. Loose soil, wet clay, and high wind sites need extra care. Always check local fence depth rules and utility lines before digging.
How the Estimate Works
Round holes use a cylinder formula. Square holes use a rectangular prism formula. The tool subtracts the post volume because the post displaces concrete. It can also subtract a gravel base by reducing the concrete height. Waste percentage then adds a reserve for spillage, uneven holes, and overdigging.
Bag Yield and Buying
Bag count is based on yield, not only bag weight. Different mixes have different yields. An eighty pound bag often covers about six tenths of a cubic foot. A custom yield field helps when your product label uses another value. This makes the estimate more useful for store buying and crew planning.
Cost and Job Control
Cost matters on larger jobs. Enter price per bag, extra posts, and waste. The result shows total bags, unused yield, estimated water, weight, and cost. These totals help you schedule delivery, loading, mixing, and cleanup.
Field Measurement Tips
For best results, measure actual holes after digging. Augers can make wider holes than planned. Depth can also change across sloped ground. Recheck gate and corner holes because they carry more force. Use the calculator again when measurements change. A careful estimate saves time, reduces shortage risk, and gives each post a stronger start. Concrete choice also affects the estimate. Fast-setting mix is common for small fence projects. Standard mix may be preferred when alignment time is important. Do not pour into standing water unless the product allows it. Brace each post until it is firm. Keep records of actual bag use for the next fence section. This improves future estimates and reduces surprise material trips too.
FAQs
How much concrete do I need per fence post?
It depends on hole width, hole depth, post size, and waste. The calculator subtracts the post volume from the hole volume. This gives a more realistic concrete amount for each post.
Should I subtract the post volume?
Yes. The post takes up space inside the hole. Subtracting it avoids overestimating concrete, especially when using large posts or many holes.
What bag yield should I enter?
Use the yield printed on the concrete bag. Common eighty pound bags often yield about 0.60 cubic feet, but products can differ. The custom yield field handles different mixes.
Why add a waste percentage?
Waste covers rough digging, spilled mix, uneven hole walls, and small measurement errors. A 10% allowance is common. Larger or rocky holes may need more.
Can this calculator handle gravel below posts?
Yes. Enter the gravel base depth. The calculator reduces the concrete height and also estimates the gravel volume for the selected hole size and post count.
Does deeper always mean stronger?
Not always. Depth helps stability, but soil, post type, fence height, wind, and local rules also matter. Gate and corner posts often need more support.
Can I use metric measurements?
Yes. Select metric units. Enter dimensions in centimeters and fence length in meters. The result shows cubic meters, liters, kilograms, and metric water volume.
Why is my bag count rounded up?
You cannot buy part of a bag for most jobs. The calculator rounds up to the next full bag so the estimate is practical for purchasing concrete.