Post Calculator UK: Price & Size Guide

This calculator works out the volume of concrete or post mix you need for a fence or deck post hole in the UK, based on the hole’s length, width, and depth. Enter the hole dimensions in feet and inches, and you get the volume in cubic yards and cubic feet, which you can then convert to the number of 25kg bags of post mix required. The result is a starting point for buying materials, but always check the coverage stated on the bag you choose, as it varies by brand.

Formula

The calculator uses the standard formula for the volume of a rectangular prism (a box):

Volume (ft³) = Length (ft) × Width (ft) × Depth (in) / 12

To convert to cubic yards, divide by 27 (since 1 yd³ = 27 ft³):

Volume (yd³) = Length (ft) × Width (ft) × Depth (in) / 324

Where:

• Length – the length of the hole or area in feet (ft)
• Width – the width of the hole or area in feet (ft)
• Depth – the depth of the hole in inches (in)

The division by 12 converts inches to feet, and the division by 27 converts cubic feet to cubic yards.

Worked Example

Using the default values from the calculator: length = 40 ft, width = 25 ft, depth = 3 in.

First, convert depth to feet: 3 in ÷ 12 = 0.25 ft

Now compute volume in cubic feet: 40 ft × 25 ft × 0.25 ft = 250 ft³

The calculator shows 250.0 ft³ (rounded to 1 decimal place).

For cubic yards: 250 ft³ ÷ 27 = 9.259 yd³, which rounds to 9.26 yd³.

So for a 40 ft by 25 ft area filled to 3 inches deep, you need about 9.26 cubic yards of material.

Why Use a Post Hole Calculator?

When setting fence posts, deck posts, or mailboxes, you need to know how much concrete or post mix to buy. The hole is usually wider than the post – typically three times the post width – and the depth depends on frost lines and the height of the fence. A common rule in the UK is to dig the hole one-third of the post height, but local conditions vary. This calculator helps you avoid overbuying (which wastes money) or underbuying (which leaves you short mid-job).

For a standard 8 ft (2.4 m) fence post, a typical hole might be 1 ft (0.3 m) wide, 1 ft long, and 2 ft (0.6 m) deep. That is a volume of 2 ft³, which is about 0.074 yd³. Most 25 kg bags of post mix cover roughly 0.5 ft³ when mixed, so you would need about 4 bags per hole. However, always check the bag’s coverage – it can vary from 0.4 to 0.6 ft³ per bag.

How to Choose the Right Post Mix

Post mix is a ready-to-use blend of cement, sand, and aggregate that you pour dry into the hole and add water. It is convenient because you don’t need to mix a whole batch. When choosing, consider:

• Setting time – most post mix sets in 5–10 minutes, but some are faster. Check the label for how long you must hold the post upright.
• Coverage per bag – typically 0.5 ft³ per 25 kg bag, but it varies. Use the bag’s stated coverage to calculate the number of bags.
• Weather conditions – in cold weather, setting may be slower; in hot weather, you may need to add more water.
• Type of post – for heavier gates or structural posts, use a stronger mix or add rebar.

To calculate bags needed: take the hole volume in cubic feet (from the calculator) and divide by the coverage per bag (in ft³). Round up to the nearest whole bag.

Common Mistakes and How to Avoid Them

One frequent error is mixing up units. Depth is often given in inches, but length and width in feet – the calculator handles that, but if you do it manually, remember to convert. Another mistake is forgetting that the hole is not a perfect rectangle – if you dig a round hole, the volume is less than a square hole of the same width. For a round hole, use the formula for a cylinder: π × (radius)² × depth. For a 1 ft diameter hole, the radius is 0.5 ft, so volume = 3.14 × 0.25 × 2 = 1.57 ft³, not 2 ft³.

Also, people often underestimate how much concrete is needed for multiple posts. Always add a little extra (10% is a good rule) for spillage and variations in hole shape. It’s better to have a bag left over than to run out mid-pour.

Cost Considerations

The cost of post mix varies by brand and retailer, but as a rough guide, a 25 kg bag typically costs between £3 and £6. For a standard fence with 10 posts, each needing 4 bags, that’s 40 bags – a total cost of £120 to £240. However, prices change, so always check current prices at your local builders’ merchant or garden centre. Buying in bulk (pallets) often reduces the cost per bag.

If you use ready-mix concrete delivered by a truck, the cost is higher but saves labour. For most domestic fencing jobs, post mix bags are the most practical choice.

Alternatives to Post Mix

If you prefer not to use post mix, there are other ways to set posts:

• Traditional concrete mix – buy cement, sand, and aggregate separately and mix on site. This is cheaper but more labour-intensive.
• Gravel or crushed stone – for temporary or lightweight posts, compacted gravel can hold a post in place without concrete.
• Foam post supports – expanding foam that sets hard, often used for lightweight structures like birdhouses or small fences.
• Mechanical post anchors – metal spikes or brackets that are driven into the ground, avoiding concrete entirely.

Each has pros and cons – concrete is strongest, gravel is removable, foam is quick but less strong. Choose based on the load and how permanent the installation is.

Frequently Asked Questions

How many bags of post mix do I need per post?

For a typical 1 ft × 1 ft × 2 ft hole (2 ft³), you need about 4 bags of 25 kg post mix, assuming each bag covers 0.5 ft³. Always check the bag’s coverage, as it can range from 0.4 to 0.6 ft³.

What is the difference between post mix and concrete mix?

Post mix is a pre-blended dry mix that you pour directly into the hole and add water – no separate mixing needed. Concrete mix is the raw ingredients (cement, sand, aggregate) that you must mix yourself. Post mix is more convenient but usually more expensive per volume.

Can I use this calculator for round holes?

No, this calculator assumes a rectangular hole. For a round hole, use the cylinder volume formula: π × radius² × depth. For example, a 12-inch diameter hole (radius 6 inches) that is 24 inches deep has a volume of about 1.57 ft³.

How deep should a fence post hole be?

A common rule is to bury one-third of the post’s height. For an 8 ft post, that’s about 2.7 ft (32 inches). Check local building codes, as frost depth and soil type affect the required depth.

Do I need to add gravel at the bottom of the hole?

Adding 4–6 inches of gravel at the bottom of the hole improves drainage and helps prevent the post from rotting. This gravel takes up space, so you may need to dig slightly deeper to maintain the same post depth.

How long does post mix take to set?

Most post mixes set in 5–10 minutes, but you should allow at least 24 hours before attaching anything heavy to the post. The mix continues to harden over several days.

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in l | Lawn length | ft | 40 | 0 | 100000
in w | Lawn width | ft | 25 | 0 | 100000
in d | Depth | in | 3 | 0 | 120
out yd | Volume | yd³ | l * w * d / 324 | 2
out ft3 | Volume | ft³ | l * w * d / 12 | 1