Concrete Calculator for Fence Posts

How many bags of concrete per fence post?

For the standard setup, 4x4 post, 10-inch hole, 30 inches deep, you need about 1.6 cubic feet of concrete, or two 60 lb bags per post (the second bag won't be empty). A 6x6 in a 16-inch hole takes four to five. This calculator does the actual cylinder-minus-post volume for your dimensions and converts to 50, 60, or 80 lb bags, because guessing by rule of thumb across 20 posts leaves you either two trips short or ten bags heavy.

Hole size: the 3× and 1/3 rules

Dig the hole three times the post width (a 3.5"-actual 4x4 wants a 10–12" hole) and one-third the post's above-ground height deep: a 6-foot fence wants 2 feet minimum, always below the local frost line so heave doesn't lift your fence the first winter. Bell the bottom slightly wider than the top and the footing locks against pull-out.

Fast-setting vs. standard mix

Fast-setting mix pours dry into the hole, takes water on top, and sets in 20–40 minutes: no mixing, no bracing overnight, ideal for fence lines. Standard mix is cheaper per bag and stronger fully cured, worth it for gate posts and 6x6 structural posts that see real load. Many pros run fast-set for line posts and standard for gates.

Related tools

Pricing the whole fence job? Use the job costing calculator with the contractor markup calculator. Pouring a slab or piers instead? The construction cost estimator covers bigger pours. Gravel base for the gate path: gravel calculator.

How the concrete volume is calculated

IRC code requirements for fence post footings

IRC Section R403.1.4 requires footings to extend below the frost depth for the local jurisdiction. In Minneapolis (frost depth 42 in), a post hole must reach at least 42 inches before concrete is poured. In Atlanta (frost depth 12 in), 12 inches is the minimum. Ignoring frost depth causes post heaving - the freeze-thaw cycle generates uplift forces exceeding 2,000 lb per post in cold climates. Check ASCE 7-22 Table C301.2(1) for county-level frost depths or your local building department's adopted depth map.

Many jurisdictions require a permit for fences exceeding 6 ft in height. Permit fees typically run $50–$150 for residential fence work. Setback requirements commonly place fences no closer than 6 inches from the property line, though some municipalities enforce 12 inches. A concrete footing that encroaches a property line can trigger removal orders regardless of fence height.

Soil bearing capacity and post spacing limits

Standard residential soil has a bearing capacity of 1,500–2,000 psf. Expansive clay soils drop to as low as 1,000 psf and require wider footing diameters - typically 30–36 inches for 8-ft panels in high-wind zones. ASTM D1586 standard penetration test values below N=4 indicate soft soil; increase hole diameter by 25% and add a concrete bell-bottom rather than straight-sided holes. Sandy or loose fill soil requires the same treatment plus gravel compaction at the base.

Post spacing directly controls concrete volume per linear foot of fence. At 8-ft spacing with 12-in diameter holes at 42-in depth, each footing consumes approximately 0.24 cubic yards. Reducing spacing to 6 ft increases footing concrete cost by 33% per linear foot of fence run. Use the construction cost estimator to roll concrete volume into a full material budget.

What this calculator does not cover

This tool calculates concrete volume only. It does not account for post embedment ratios for moment-resisting applications (typically 1:3 depth-to-above-grade ratio per AISC Design Guide 1). It does not size footings for gate posts, which carry dynamic load from swinging weight and require 50–100% larger diameter holes than line posts. Corner posts in high-wind zones (ASCE 7 Exposure Category C or D) require engineering review - standard calculators cannot substitute for a licensed engineer's footing design in those conditions. To price the full job including labor and markup, pair this output with the job costing calculator.