Ballast and Cement Calculator

To work out ballast and cement, find how much concrete the trench, hole or steps hold, then allow about 1,906kg of ballast and 229kg of cement for every cubic metre at a 1:6 mix. Outdoor Aggregates' calculator does it for footings, fence posts, steps and edging, and tells you which bags to order.

How Do I Calculate How Much Ballast and Cement I Need?

The amount of ballast and cement you need comes from the volume of concrete: length times width times depth in metres, plus an allowance, times 1,906kg of ballast and 229kg of cement per cubic metre. Here is a 5m wall footing, 400mm wide and 200mm deep, worked through:

1. Volume: 5 x 0.4 x 0.2 = 0.40m³ of concrete.
2. Allowance: add 10% for spills and an uneven dig, so 0.44m³.
3. Ballast: 0.44 x 1,906 = 839kg, so one 850kg bulk bag.
4. Cement: 0.44 x 229 = 101kg, so five 25kg bags.
5. Water: about half the weight of the cement, so around 50L, added a little at a time.

Those per-metre figures come from what a cubic metre of finished concrete weighs, about 2,250kg, split between cement, ballast and water at a 1:6 mix by the bucket. I always kept the 10% on top. Footing trenches are never quite as tidy as the tape measure says, and a cold joint (where fresh concrete meets a batch that has already started to set) is a weak line you cannot undo.

How Many Bags of Cement to a Bulk Bag of Ballast?

An 850kg bulk bag of ballast needs four to five 25kg bags of cement at a 1:6 mix, and makes about 0.45m³ of concrete.

Round the cement up, never down. A spare bag of cement keeps if it stays dry and sealed; running one short means a trip out mid-pour. Outdoor Aggregates sells ballast in 850kg bulk bags and in packs of 20kg mini bags (40 bags or 20 bags), and the calculator picks the cheapest mix of them for your job. The mini bags are handy where a lorry cannot get close, because one person can carry a 20kg bag through a house.

What Is the Mix Ratio for Ballast and Cement?

The mix ratio for ballast and cement is 1 part cement to 6 parts ballast by volume, measured with the same bucket for both. Ballast is a blend of sharp sand and stone, graded from dust up to 20mm, so it already holds the sand that a three-part recipe (cement, sand and stone) adds separately. The 1:6 ballast mix is the same idea as the familiar 1:2:4: one part cement to six parts of everything else.

I kept to the one mix for footings, fence posts and steps. You will see other ratios quoted, 1:4 and 1:5 for stronger work, but those belong to slabs and driveways, which are jobs ballast is not meant for (see the floors and bases section further down). For the jobs it is meant for, 1:6 does it. A few things that matter more than the ratio once you are mixing:

1. Use the same bucket for the cement and the ballast, filled to the same level each time.
2. Add water gradually. A wet, sloppy mix is weaker than a stiff one that holds its shape when squeezed.
3. Wear gloves and cover your arms. Wet cement is alkaline and burns skin.

Why Does Concrete Need More Ballast Than the Hole Holds?

Concrete needs more ballast than the hole holds because the cement and the sand fill the gaps between the stones, so the loose heap shrinks as it is mixed. A cubic metre of finished concrete takes about 1.06m³ of loose ballast plus the cement and water. The ballast alone is a little more than the hole you are filling, before the cement goes in. It is why a trench seems to eat bags, and why ordering exactly the hole's volume in ballast leaves you short. The weight side catches people too. Concrete is heavy: about 2,250kg in a cubic metre, so even a modest footing is well over a tonne once it is mixed.

How Much Concrete Do I Need for Fence Posts?

The concrete for a fence post is the volume of the hole less the space the post takes up. A 300mm round hole, 600mm deep, around a 100mm square post holds about 36L of concrete. The post's own space is worth taking off. On a 100mm post in a 600mm hole it is 6L, about a seventh of the concrete, and across a run of fence posts that is a bag or two of cement you would otherwise buy.

Hole width makes a bigger difference than most people expect. Going from 250mm to 350mm more than doubles the concrete in each hole, which is why the bags-per-post figures on the back of a postcrete bag rarely match what a real, slightly ragged hole takes. Measure a couple of your holes once they are dug and use those figures in the calculator.

Postcrete is quicker for a few posts because it sets fast and needs no mixer. For a full run, ballast and cement mixed at 1:6 is the cheaper way to fill the same holes; just brace each post while it sets.

How Much Concrete for a Wall Footing, Steps or Edging?

The concrete for a wall footing, a set of garden steps or a run of edging comes from its size, and the table below shows four common jobs worked out with a 10% allowance.

Steps are worked as solid concrete, each step a block the full width. The bottom step is one rise high, the next two rises, and so on, which is why the concrete climbs fast as you add steps. Edging and kerbs sit on a concrete bed with more concrete packed behind them, called the haunch, to hold them in place. The calculator has twelve edging and kerb types, from 100mm blocks on a 50mm bed to large kerbs on a full 100mm bed and haunch, worked per metre of run. For laying them, my guide to installing garden edging covers each type.

How Do I Measure the Tread and the Rise?

The tread is the flat part of a step that you stand on. The rise is the height of one step. Both go into the calculator in millimetres.

• Tread: measure from the front edge of the step straight back to the face of the next step up.
• Rise: measure from the top of one step straight up to the top of the next.
• Width: measure across the step, from one side to the other.

If the steps are not built yet, measure the total height of the slope from the bottom to the top and divide it by the rise you want. A 450mm slope at a 150mm rise is three steps.

Can I Use Ballast for Critical Applications Such as Floors and Bases?

Ballast is not the right material for critical concrete such as floors, shed bases and other slabs, because you cannot control its blend of sand and stone closely enough. Ballast stone tends to be smooth, water-worn gravel from rivers or the sea, and the share of sand shifts from one quarry to the next. That is fine for a footing, a post or a step, where the concrete is chunky and boxed in. A slab is a different job: it is thin, it spans soft spots, and it has to take load across its whole surface.

For a floor or a base I would use concrete mixed from separate sharp sand and stone, so the proportions are under your control, or ready-mix ordered to a stated strength. Ballast also has no place under a slab. The layer beneath a base is compacted MOT Type 1, and our sub-base calculator works out how much of that you need.

Should I Mix It Myself or Order Ready-Mix?

Mixing ballast and cement yourself makes sense up to about 1m³ of concrete; beyond that, ready-mix delivered to the site is usually quicker and often cheaper. Under a cubic metre, which covers most footings, every fence and nearly any set of garden steps, ballast and cement win on flexibility. You mix as you go, at your own pace, and you can stop for the day.

Over a cubic metre the sums change. That is about a dozen wheelbarrow loads of mixed concrete, every one of which has to be batched, wheeled and placed before the last one starts to stiffen. The calculator flags it when your job goes over 1m³.

Next Steps

Put your sizes into the calculator above and it gives you the ballast, the cement and the water, and picks the cheapest combination of bags. We stock Ballast in 850kg bulk bags from £136 and in packs of forty 20kg mini bags from £166. Cement we do not sell yet, so the calculator shows a typical UK price for it. For a big footing, or anything over a few tonnes, call us on 0330 043 7585 and we will price a loose tipped load.

Have you ever dug in a run of fence posts or poured a footing with ballast and cement? Which mix did you use, and how many bags did the job really take? We would love to hear how it went in the comments.

FAQs

How many 20kg bags of cement make 1 cubic metre of concrete?

A cubic metre of concrete at a 1:6 ballast mix takes about 229kg of cement, which is twelve 20kg bags or ten 25kg bags. Round up in both cases.

Is ballast the same as MOT Type 1?

Ballast and MOT Type 1 are two different products. Ballast is sand and rounded stone blended for mixing concrete, while MOT Type 1 is crushed rock with fines that compacts into a firm sub-base and is never mixed with cement.

Why is my bag of ballast mostly sand?

All-in ballast is around half sand by design, because the sand fills the gaps between the stones in the finished concrete. The ballast Outdoor Aggregates sells is graded from dust up to 20mm, so a bag that looks sandy on top usually has its stone further down.

Should I put rubble in a fence post hole to save concrete?

Rubble in a fence post hole saves a little concrete but makes the old post far harder to dig out when the fence is replaced. I would rather size the hole sensibly and fill it with concrete.

Should the concrete come up to ground level round a fence post?

Concrete round a fence post works best stopped just below ground level and sloped away from the post, so rain runs off instead of sitting against the timber. Soil or gravel over the top finishes it.

How much water goes in a ballast and cement mix?

A ballast and cement mix needs about half the cement's weight in water, so roughly 12L to each 25kg bag of cement. Add it gradually and stop as soon as the mix holds its shape.

What can I do with leftover ballast?

Leftover ballast keeps indefinitely if it stays clean, so store it on a sheet under a tarp for the next set of posts or a step repair. It is not a substitute for MOT Type 1 under a path or base.