How Much Concrete for Fence Posts?
Setting fence posts in concrete is one of the most common garden DIY jobs in the UK. Getting a good estimate of the right amount of concrete helps avoid waste and ensures the posts stay firm. For 2 fence posts with 300mm diameter holes dug to 600mm deep, you may need approximately 0.085m³ of concrete, which works out to around 11 x 20kg bags of ready-mix. This guide covers everything from digging the holes to setting the posts plumb and working towards a strong, long-lasting result that should withstand British weather.
Planning and Preparation
Before digging any holes, check your boundary line and confirm there are no underground services such as gas, water, or electricity cables. Use a cable avoidance tool (CAT scanner) if in doubt, or contact your local authority for plans. Mark out the post positions with wooden pegs and string line, spacing posts evenly at 1.8m to 2.4m apart depending on the panel width. Each hole should be 300mm in diameter and at least 600mm deep. Use a post hole borer or a narrow spade for clean, consistent holes. Remove any large stones from the base and add 50mm of gravel for drainage before inserting the post.
Mixing Concrete
For this project our estimate suggests around 0.085m³ of concrete. You have two options: use approximately 11 x 20kg bags of ready-mix concrete (such as Blue Circle, Postcrete, or Hanson Multicem), or mix your own using a C20 mix ratio of 1:2:3 by volume. To mix your own, you'll likely need around 23.3kg of Portland cement (1 x 25kg bag), 49.8kg of sharp sand, and 84kg of 20mm gravel aggregate — a total of roughly 157kg of materials. For small batches, mix by hand on a clean board or in a wheelbarrow: combine the dry materials first until the colour is uniform, then gradually add water until the mix holds its shape when squeezed but is still workable. For larger volumes, hire a cement mixer — a half-bag (electric) mixer handles most DIY jobs and costs around £25-35 per day from tool hire shops. Avoid adding too much water, as this weakens the finished concrete significantly.
Pouring Technique
Insert the fence post into the hole and check it is upright using a spirit level on two adjacent faces. Have a helper hold the post plumb while you pour the concrete around it, or use temporary braces nailed to stakes in the ground. Pour the concrete in stages, tamping it down with a length of timber or a stick to remove air pockets. Fill to about 50mm below ground level so you can cover the concrete with soil for a neater appearance. Shape the top of the concrete into a slight dome or slope away from the post so that rainwater sheds off rather than sitting against the timber, which would cause premature rot. Double-check the post is vertical before the concrete begins to set.
Weather Considerations
The ideal temperature for pouring concrete is between 5°C and 25°C. Avoid concreting in frosty conditions, as ice crystals within the mix will weaken the set concrete. If you must work in cold weather, use a rapid-setting concrete like Postcrete which sets in about 10 minutes, minimising exposure to low temperatures. In hot, dry weather, dampen the inside of the holes before pouring and keep the posts supported for longer, as rapid drying can cause the surface to crack before the concrete has fully cured. Rain shortly after pouring can wash the cement paste from the surface, so cover fresh concrete with polythene sheeting if rain is forecast. Light rain after the first few hours is usually not a problem for fence posts.
Curing Times
Ready-mix and rapid-set products like Postcrete achieve their initial set within 5-10 minutes, but the concrete continues to gain strength over the following days. Allow at least 24 hours before attaching fence panels or applying any lateral force to the posts. For standard concrete mix, the initial set takes 4-6 hours and you should leave the post braces in place for at least 24 hours. Full strength (the design strength of the mix) is reached at 28 days, though for domestic fence posts the concrete will be more than strong enough after 48 hours. During the curing period, avoid knocking or leaning anything against the posts. If possible, leave all panel fitting until at least 48 hours after the last post was concreted in.
Estimated Cost
Prices for concrete materials in the UK vary by region and supplier. Using ready-mix bags, approximately 11 x 20kg bags may cost around £44-£66 at typical retail prices of £4-6 per bag. If you mix your own, the individual materials may cost roughly £80-£123 (cement £5-£8 for 1 x 25kg bag, sharp sand £35-£55 for 1 bulk bag, aggregate £40-£60 for 1 bulk bag). For a job this size, mixing your own concrete from bulk materials may be significantly cheaper than buying individual ready-mix bags. Consider ordering a bulk bag (dumpy bag) of sand and aggregate from a local builders’ merchant for the best price. If you need to hire a cement mixer, expect to pay £25-35 per day from most tool hire companies. A wheelbarrow, shovel, and spirit level are also essential tools you may need to buy or borrow.