Fence Post Calculator: Spacing, Depth, and Concrete Guide
How to calculate fence post count, spacing, hole depth, and concrete volume for any fence project. IRC guidelines included.
Calculating fence posts, concrete, and panels before you start saves a trip back to the lumber yard — and prevents installing posts at the wrong spacing that won’t accept standard panel widths.
Quick Answer: Posts = (fence length ÷ post spacing) + 1. Standard spacing is 6–8 ft for wood privacy fences. Burial depth = 1/3 of total post length minimum, or to local frost line. Plan 2 bags of 80 lb concrete per post for a 10” hole at 30” deep.
How Many Posts Do You Need?
Posts = (Fence length ÷ Post spacing) + 1
For a 100-foot fence at 8-foot spacing: (100 ÷ 8) + 1 = 14 posts
Add extra posts for:
- Each corner: +1 post per corner
- Each gate opening: +2 posts per gate (gate posts are separate from standard posts)
- End posts are already included in the formula above
Use our Fence Post Calculator to calculate the full breakdown including panel count.
Standard Post Spacing by Fence Type
| Fence Type | Typical Spacing |
|---|---|
| 6-foot wood privacy | 6–8 feet OC |
| 4-foot wood picket | 6–8 feet OC |
| Split rail | 8–10 feet OC |
| Chain link | 10 feet OC |
| Vinyl privacy | 6–8 feet OC |
Closer spacing (6 feet) increases material cost but improves rigidity and is better for wind-exposed sites.
Post Depth Requirements
| Post Height | Recommended Burial | Post Length to Buy |
|---|---|---|
| 4 ft | 2 ft | 6 ft |
| 5 ft | 2 ft | 7 ft |
| 6 ft | 2–2.5 ft | 8 ft |
| 8 ft | 2.5–3 ft | 10 ft or 12 ft |
In freeze-thaw climates, extend depth to the local frost line — often 36–48 inches. Frost heave on a shallow post can destroy an entire fence run.
Concrete Per Post
A standard 10” diameter hole at 2.5 feet deep holds 1.23 cubic feet of concrete.
At 0.60 cu ft per 80 lb bag: 2 bags per post for this configuration.
| Hole Size | Depth | Concrete |
|---|---|---|
| 8” diameter | 24” | 0.70 ft³ (~1.2 bags) |
| 10” diameter | 30” | 1.23 ft³ (~2.1 bags) |
| 12” diameter | 36” | 2.36 ft³ (~4 bags) |
Use our Post Hole Calculator for any hole configuration.
Frequently Asked Questions
Do fence posts need concrete? For wood posts, concrete anchoring is strongly recommended — it prevents the post from rocking, which loosens the attachment over time. For metal T-posts (farm/ranch), no concrete — drive them directly.
How many bags of concrete for the whole fence? Multiply bags per post by total post count. For 14 posts at 2 bags each: 28 bags. Round up. Use our Concrete Bags Calculator for the total.
Should I use fast-setting concrete for fence posts? Fast-setting Quikrete is popular for fence posts — you pour the dry mix in the hole and add water. It sets in 20–40 minutes, so you can move post to post quickly without waiting for cure. It works well; just keep the mix away from the post below grade.
See also: Concrete Calculation Methodology — bag yield tables, mix ratio formulas, and ACI 318 references behind ConstructCalc concrete calculators.
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