Concrete Mix Ratios

The Short Answer

By Gary Hodson · Ex-Army. I do the sums so you buy the right amount. · Ratios, product data and shop prices checked against the sources below on 8 October 2026 · Last reviewed 8 October 2026

For a shed base or a path, mix 1 part cement, 2 parts sharp sand and 3 parts 20mm gravel. If you buy ballast instead, that is 1 part cement to 4 parts ballast.¹ Fence posts can take a leaner 1:2:4, and steps, paving bases and drives take a richer 1:1.5:2.5.¹² Parts are by volume, so use one bucket for every ingredient.

Setting posts? Skip the ratios and use Postcrete, which goes into the hole dry on top of water.⁷ Laying slabs? That is a different mix again: mortar, with no gravel in it.⁵ Our concrete calculator tells you how much you need. This page tells you what to mix it from.

Which Mix for Which Job

Four suppliers and manufacturers publish ratios for small UK jobs. This table is the overlap between them.

Suppliers do not agree to the half part. B&Q gives 1:2.5:3.5 for posts and light wall foundations. Clarke Fencing gives the same 1:2.5:3.5 for garden slabs and 1:1.5:3 for drives. Marshalls gives 1:3:6 for foundations.¹²³ That spread tells you these are working rules for small jobs, not laws. A half part either way will not ruin a path. Changing recipe halfway through a pour will leave a visible join, so pick one and stick to it.

Measure by volume, not weight. A bucket of cement and a bucket of sand weigh very different amounts, and a shovel is a poor measure because no one shovels the same amount twice.¹

A Ratio Is Not a Strength Class

Hand-mixed concrete has no tested strength. Ready-mix does, and it is sold by grade. Heidelberg Materials lists footings for fence posts and small patio bases under its ST2 mix, shed and workshop bases under ST3 and ST4, and patio paving under ST3. It says a quality-assured supplier will offer GEN1, GEN2 or GEN3 in their place.⁸

Two grades matter outside the house. PAV1 is the common choice for house drives and other external paving. Its air bubbles protect the surface from freeze-thaw cycles, and its strength is 35N/mm² at 28 days. A garage used as a workshop needs at least RC25/30.⁸ If a job needs a known strength or frost resistance, order ready-mix by grade. Any C15, C20 or C25 label you see beside a hand-mix ratio online is a rule of thumb, not a result.

Foundations for a wall, an extension or anything holding up a building are a Building Regulations matter. Ask building control before you mix anything.

Ballast or Separate Sand and Gravel?

Ballast is a blend of sharp sand, small stones and gravel.⁴ It saves you buying and measuring two heaps, which is why B&Q gives a ballast version of every ratio.¹ For garden work, ballast is the easier route. Buy sand and gravel separately only if you want to tune the proportions.

Price is the other test. On 8 October 2026 Wickes sold a 22.5kg bag of ballast at £4.10 (about £0.18 a kilo) and an 800kg jumbo bag at £75 (about £0.09 a kilo), both excluding delivery.¹⁰ Past a few bags, the jumbo bag is half the price per kilo.

If you do buy sand, buy sharp sand. Marshalls says it is the sand for making concrete and for bedding paving. Building sand is finer, suits bricklaying mortar, and should not go into concrete unless it is mixed with a grittier sand.⁴

Water: Less Than You Think

Water starts the reaction that hardens cement, but extra water weakens the result and invites cracks.¹ Add it a little at a time. Clarke Fencing suggests starting at about half a bucket per 25kg of cement and adjusting for weather and materials.² Aim for thick porridge: wet enough to bind, stiff enough to hold a pile.

Marshalls gives a simple test. Press the edge of a shovel into the mix. A good mix keeps clean ridges. If the ridges slump quickly, there is too much water, so add dry material rather than waiting.³ In a mixer, run each batch for 3 to 5 minutes until the colour is even.¹¹ Use it within an hour or two, because it starts to set.²

Postcrete or Your Own Mix for Fence Posts?

Blue Circle's data sheet tells you to dig a hole about three times wider than the post, fill it a third full of water, then pour Postcrete in around the post until no standing water shows, then sprinkle water on top of the powder. It sets in 5 to 10 minutes. You must not mix it with water by hand or by machine.⁷ The minimum working temperature is 3°C.⁷ Postcrete is for domestic posts, decking posts, washing-line posts and gateposts under 1.0m wide. It is not for general concreting, screeds or mortar.⁷

The maker's table gives the bags for a 100mm square post:

Source: Blue Circle Postcrete data sheet.⁷ A bag cost £7.50 at Wickes on 8 October 2026.¹⁰

Worked sum: ten posts, two ways

Take ten 100mm posts in 30cm by 60cm holes. The concrete calculator gives about 0.037m³ of concrete per hole, so 0.37m³ in total.

• Postcrete: 2 bags a hole is 20 bags, and 20 x £7.50 = £150. That is the maker’s fixing quantity for the post, not a full fill of the hole.
• Own 1:6 mix: dry ingredients take up more room than the wet concrete they make, so our working figure is 1.54 times the wet volume. That is our own estimating assumption, not manufacturer data.¹² That gives 0.37 x 1.54 = 0.57m³ dry. One part in seven is cement: 0.081m³, which is 81 litres. A 25kg bag of cement takes up about 17 litres (25kg ÷ 1,440kg/m³ = 0.0174m³, using 1,440kg/m³ as our working bulk density for loose cement),¹² so you need 5 bags, 5 x £7.50 = £37.50. Six parts in seven is ballast: 0.49m³. A Wickes jumbo bag covers 9.7m² at 50mm,¹⁰ which is 0.485m³, so buy one jumbo bag (£75) and one small bag to be safe (£4.10).

Own mix comes to £37.50 + £75 + £4.10 = £116.60 against £150, a gap of £33.40 on paper before delivery. The two prices buy different amounts: the own mix fills all ten holes to the top, the Postcrete figure does not. It is not a like-for-like comparison, so do not read the gap as a saving. The maker’s instructions finish by covering the top of the hole with soil, so its two bags fill less than the 0.37m³ used for the own mix. The data sheet does not give enough to work out the like-for-like difference, so treat the gap as a rough guide only. These are GardenCalc's own sums from dated shop prices, not a quote. The own mix buys you ten batches of mixing and a wait before the posts are firm. For two or three posts, Postcrete wins on time and costs little more.

Worked Sum: Mixing a Shed Base

Our concrete calculator sizes a base for an 8ft by 6ft shed at 2.6m by 2.0m and 100mm thick: 2.6 x 2.0 x 0.1 = 0.52m³. For a shed base the ratio is 1:2:3, which is 1:4 in ballast.¹

1. Dry volume: 0.52 x 1.54 = 0.80m³.¹²
2. Cement is 1 part in 5: 0.16m³, or 160 litres. At about 17 litres a bag, that is 9.2 bags, so buy 10 (£75 at £7.50).
3. Ballast is 4 parts in 5: 0.64m³. A jumbo bag holds about 0.485m³, so you need two (£135 for the pair at Wickes).¹⁰ The spare two-thirds of a bag will not go to waste on a path.

Materials come to £75 + £135 = £210, before delivery, the sub-base under the slab and the timber for the edges. That is our own sum from 8 October 2026 shop prices. Compare it with a ready-mix quote before you commit, because a lorry load changes the sum. The concrete calculator covers the point where ready-mix starts to win, and the sub-base calculator sizes the layer underneath.

Mortar for Laying Slabs

Slabs do not sit on concrete mix. Marshalls says to support flags on a full, workable mortar bed of 1 part cement to 3 or 4 parts sharp sand, laid 30mm to 40mm thick. It settles to about 25mm once you tamp the slab down.⁵ Check the slab does not rock, and adjust the bed if it does. For pointing, Marshalls says to use 4 parts sand to 1 part cement, mixed to a firm consistency.⁶

Mortar does not need gravel. The mix is cement and sand only, and it is weaker than concrete. It works as glue and a levelling bed.³ Use sharp sand under flags. Building sand is too fine for bedding.⁴

A 3m by 3m patio on a 35mm bed needs 9 x 0.035 = 0.315m³ of mortar. At 1:4 and the same 1.54 allowance, that is about 6 bags of cement and 0.4m³ of sharp sand. Treat that as an estimate and add a tenth. Blue Circle also sells a pre-mixed slab-bedding product, Slablayer, which you rake out dry and wet with a watering can.¹³ The paving calculator counts the slabs.

Curing: Keep It Covered, Keep It Warm

Concrete hardens by reacting with water, and it stops if it dries out. Heidelberg Materials says insufficiently cured slabs can lose up to half their surface durability, and show cracking and dusting soon after.⁹ Most specifications ask for 3 to 7 days of curing. The first 24 hours matter most. Four days is the recommendation for ordinary Portland cement, and 7 days in hot or windy weather.⁹

In dry weather, cover the surface with polythene, laid in close contact with the concrete.⁹ In frost, use insulation blankets, or straw over polythene, and keep it dry. Heidelberg says never to use water for curing in cold weather.⁹ Clarke Fencing allows light use after 24 hours and says to avoid heavy loads for at least 7 days.² Strength is rated at 28 days.⁸

Common Mistakes

• Too much water. It is the commonest way to weaken a mix.¹²
• Extra cement for extra strength. More cement will not always help and can cause cracking.²
• Building sand in concrete. It is too fine unless blended with a grittier sand.⁴
• Postcrete for a base. The maker rules it out for general concreting.⁷
• Pouring onto dry ground. Dry ground pulls water out of the mix. Dampen the base first in hot weather.²
• Skipping the cure. Uncovered concrete in wind or sun loses water too fast.⁹
• Hand-mixing a big job. Price a ready-mix delivery once the batches stack up. The concrete calculator shows where the line sits.

Frequently Asked Questions

Work Out Your Quantity

Pricing the surface as well? Read tarmac cost per tonne or pea gravel cost per tonne for the alternatives to a concrete drive or path.