Fence Post Concrete Calculator

1. Your fence post holes

Measure the base you intend to build, which may be larger than the shed footprint. All measurements are editable.

Base length

Base width

Concrete thickness

Enter a thickness suitable for your design; this is not a recommendation.

Extra allowance

Optional allowance for variation and wastage.

Number of post holes

holes

Hole diameter

Concrete fill depth

Post width (square)

Assumes cylindrical holes and a centred square post. The entered depth is the concrete-filled depth. Confirm the required hole size and installation method for your fence.

2. Base options

Choose which estimates you want to see. Turn either option off to hide its inputs and results.

Concrete basePaving slab base

3. Concrete buying assumptions

Premixed bag size

Concrete yield per bag

The 10 L yield is an illustrative starting assumption, not a universal specification. Check the exact yield stated by your chosen product.

Price per bag

Ready-mix price per m³

Both prices are placeholders. Ready-mix delivery, minimum order, pump charges and VAT treatment may change the quote.

4. Shared groundwork assumptions

Enter suitable depths based on the manufacturer's instructions and site conditions. The calculator does not determine what construction is safe.

Sub-base depth

Sub-base density

t/m³

Sub-base bulk bag content

Price per sub-base bulk bag

5. Paving slab assumptions

Slab length

Slab width

Price per slab

Bedding layer depth

Bedding material density

t/m³

Bedding bulk bag content

Price per bedding bulk bag

All default prices, densities, yields and bag sizes are illustrative. Check actual product specifications, including whether bedding requires a particular mortar mix rather than loose sand.

Joint width between slabs

Used for a simple straight rectangular grid. Cuts and patterns are not modelled.

Your materials estimate

Base area—Both options use the same base footprint

Concrete base

—

Estimated bagged concrete + sub-base

Concrete—

Premixed bags—

Ready-mix alternative*—

Paving slab base

—

Estimated slabs + bedding + sub-base

Slabs—

Bedding—

Sub-base—

*Ready-mix figure is concrete-only; it excludes the sub-base, delivery, minimum order and other fees. This is not a direct comparison with the paving total.

Printable shopping lists

Concrete option

Paving slab option

All totals are estimates of selected materials only, not supplier quotations. Labour, plant hire, delivery, VAT differences, edge restraints, drainage, membranes, reinforcement, fixings and site preparation are excluded.

Before you buy

Important: Concrete thickness, base preparation, reinforcement, drainage and ground suitability depend on the shed, its loads and local conditions. Follow the shed manufacturer's requirements and seek qualified advice where appropriate. Do not rely on this calculator to decide structural specifications.

The two shed-base options are not structurally interchangeable in every situation. Patio layouts and joint widths can change slab counts. Fence-hole dimensions depend on the ground, fence design and exposure. Verify product yields, material suitability and installation methods before ordering. Affiliate links, if added, will be clearly disclosed.

How to use this calculator

Enter the number of post holes, hole diameter, concrete-filled depth and square post width. The calculator subtracts the approximate volume occupied by the posts, then estimates bag quantities using your product’s declared yield.

This is a quantity estimator, not a fence design tool. Hole size, post installation, drainage and concrete requirements vary with fence height, wind exposure, soil and manufacturer instructions.

Prices and material properties are example starting values, not live supplier quotes. Always check bag yields, dimensions, delivery costs and suitability before ordering.

How to estimate concrete for fence posts

The amount of concrete around a fence post depends on the hole shape, post dimensions and fill depth. The calculator estimates volume; it does not design a safe fence foundation.

Step-by-step overview

Plan the fence line.Check boundaries, permission, access and the location of underground utilities before digging.

Choose posts and spacing.Follow the fence system manufacturer’s guidance for post size, spacing and support. Wind exposure, soil and fence height all affect the design.

Confirm hole dimensions.Use the hole diameter and concrete-filled depth specified for your installation, not just the calculator defaults.

Enter the post size.The calculator assumes a square post centred in a cylindrical hole and subtracts the post’s volume from the concrete-filled space.

Check the concrete product.Use the actual litres of finished concrete per bag stated by the manufacturer. Different bagged products have different yields and installation methods.

Order and install safely.Round up to whole bags and follow the product’s instructions for mixing, placement, setting and curing.

Why post size matters

A 100 mm square post takes up space inside the hole. Our estimate subtracts that space; a different shape or an uneven hole will change the result.

Depth is not a recommendation

The 600 mm example is only a test input. Actual post depth and footing requirements depend on fence height, ground conditions, wind loading and the chosen system.

Common questions

How much concrete do I need per post?

Enter your actual hole diameter, fill depth and post width. The calculator works out the remaining hole volume and rounds the bag count up.

Does this work for rectangular holes?

No. The calculation assumes cylindrical holes. Rectangular or irregular holes need a different volume estimate.

Can I use postcrete or fast-setting concrete?

Use only products suitable for your installation and follow their instructions. Check the manufacturer’s finished yield per bag before using the estimate.

Does it account for gravel under the post?

Only indirectly: enter the actual depth that will be filled with concrete, excluding any separate drainage layer where your design specifies one.