Concrete for Fence Posts: Bag Math That Adds Up

The short answer on bags per post

A standard 254 mm diameter hole, 762 mm deep, holds about 0.0386 m³ of space. An 80 lb bag of concrete yields roughly 0.017 m³, so the hole alone would take about 2.3 bags. Subtract the volume the post occupies and you land at roughly two bags per post for a 4x4, which is the number most US suppliers will quote for this hole size. Buy a little extra rather than shorting the last hole.

That figure assumes the hole is actually 762 mm deep and 254 mm wide. If your auger drifts wider, or you dig deeper for frost, the bag count climbs. The fence calculator does this arithmetic for every post at once so your order list matches the ground you are digging, not a rule of thumb from a forum.

Measure the run before you touch concrete

Walk the fence line and record total length in feet, then note gate openings and any corners. With a 2.44 m (8.00 ft) post spacing and 2.44 m panel width, a 24.4 m (80 ft) straight run gives you 11 posts and 10 panels, plus one gate if you are cutting one in. Corners and ends each need their own post, so count them explicitly rather than dividing length by spacing alone.

Height matters too. The default is 1.83 m (6.00 ft), and changing it changes rail count and wind load but not hole volume, because hole size is set by post dimension and local frost depth. Write down post size, hole diameter and hole depth for each section of the run. Those three numbers, not the fence height, drive your concrete order.

Hole volume minus post volume

Treat the hole as a cylinder: volume equals pi times radius squared times depth. For a 254 mm diameter hole at 762 mm deep, radius is 127 mm, giving about 0.0386 m³. Now subtract the post. A nominal 4x4 actually measures about 89 mm by 89 mm, so over 762 mm it displaces roughly 0.0060 m³. Net concrete volume is about 0.0326 m³ per post.

Divide by the bag yield of 0.017 m³ and you get 1.92 bags, so two bags per post. A 6x6 post displaces more, about 0.0136 m³, leaving 0.0250 m³ and about 1.5 bags, though most people still round to two for a margin. Keep units consistent: the calculator works in metric internally and displays imperial equivalents, so do not mix inches into a metric formula.

Frost depth changes the hole, not the bag

In cold US states, footing depth is often set below the local frost line, which can push holes past 762 mm. Frost depth is set locally, so confirm the required depth with your county or city building department before digging. A 1067 mm deep hole at the same 254 mm diameter holds about 0.0541 m³; minus a 4x4 post, that is about 0.0457 m³, or roughly 2.7 bags, so round to three.

This is where a fixed bags-per-post rule fails. Two fences in different counties can need two and three bags per post for identical panels. Enter your actual depth into the fence calculator and it recalculates the total. If you are unsure of the frost depth requirement, ask the authority that issues your permit, not the lumber yard.

Worked example: a 24.4 m run

Take a 24.4 m (80 ft) straight run at 2.44 m spacing: 11 posts and 10 panels. At 1.83 m height with two rails per bay, that is 20 rails. Pickets at 89 mm wide with a 13 mm gap cover about 102 mm each, so a 2.44 m bay needs roughly 24 pickets, or about 240 across 10 panels. Add one gate if required.

Concrete: 11 posts at two bags each is 22 bags, and at three bags for a deeper frost hole it is 33. The calculator totals this from hole volume minus post volume and rounds up per post, then converts bags to a material cost using your local price. Confirm current bag prices with your supplier, since they vary by region and season. Order the higher figure if your dig runs wide.

Turning bags into an order list

Your final list should read: posts, panels or pickets, rails, bags of concrete, gates and an estimated cost. Check that post count equals panels plus one on a straight run, and that rail count equals bays times two. Concrete bags should be the rounded-up sum of per-post needs, not a single division of total volume, because partial bags cannot be split across holes neatly.

Mix to the bag instructions rather than adding water to stretch the count, and set posts plumb before the mix stiffens. If your soil is rocky and holes end up oversized, add a bag per post to the order. The fence calculator gives you the baseline; the ground gives you the final number.

Cross-checking before you buy

Recount posts and holes on site, then compare with the calculator output. A mismatch usually means a missed corner post, a gate post, or a spacing assumption that does not match your panels. Fix the count before converting to bags, because every extra post adds two or three bags to the order.

Finally, confirm bag yield on the product label, as yields can differ slightly between brands and mixes. The 0.017 m³ figure is typical for an 80 lb bag, but the label is authoritative. With counts, depth and yield confirmed, the bag math holds and you can order with confidence.

Why the calculator rounds per post

Concrete is bought in whole bags, and each hole is poured separately, so rounding happens at the hole, not at the total. Ten holes at 1.92 bags each need 20 bags, not 19.2 rounded to 20 by luck. The calculator applies that logic per post and then sums, which is why its total can exceed a simple total-volume division.

It also keeps hole geometry separate from fence layout. Post spacing sets the post count; post size and hole depth set the concrete. Changing panel width moves the post count and therefore the bags, while changing fence height does not. Understanding that split is the whole point of the bag math.

Common mistakes that skew the order

Digging shallow to save concrete weakens the post and may fail inspection where frost depth rules apply. Using the post's nominal size instead of its actual dimension overstates displacement and understates bags. Mixing imperial and metric mid-calculation produces errors of a factor of 25 or more, so stay in one system.

Buying exactly the calculated count leaves nothing for a collapsed hole or a spilled mix. Add one or two spare bags for a typical residential run. Confirm permit and inspection requirements with your local building department, and confirm bag yield and price with your supplier before ordering.

From measurement to checkout

Measure the run, count posts including corners and gates, set hole diameter and depth from post size and local frost requirements, subtract post volume, divide by bag yield, round up per post. That sequence is the entire method, and it scales from a garden gate to a full property line.

Run the numbers through the fence calculator once, then verify the post and hole counts against the ground. The output is only as good as the depth you enter, so check frost depth with your local authority first. Everything else is arithmetic you can now do by hand.

Frequently asked questions