What Concrete Do You Need for Retaining Wall Post Footings?

Getting the concrete wrong at the post footing stage is one of the few mistakes you can't easily fix later. Too weak, and the wall leans within a year or two. Too little, and the posts rock under load. Here's what actually matters before you mix a bag.

Post footings don't get a second chance. Once concrete is poured and cured around a post, digging it back out to fix an under-sized or under-strength footing means starting the job again — hole, post, panels, the lot. That's why it pays to get the concrete choice right the first time, rather than treating it as an afterthought once the posts are already standing in their holes.

Why the Right Concrete Matters For Post Footings

A retaining wall's posts carry the full lateral load of the soil behind it. That load transfers straight down into the footing. If the concrete is under-strength, under-sized, or hasn't cured before the wall is loaded, the posts can shift — and once a post moves, the whole wall is compromised.

This matters more for retaining walls than for a simple fence line. A fence post mostly resists wind load, which is intermittent and relatively light. A retaining wall post is holding back soil that's pushing on it constantly, and that pressure increases after rain, when the soil behind the wall becomes saturated and heavier. The footing has to resist that load day in, day out, for decades. A weak or undersized footing won't necessarily fail immediately — it's more likely to show up as a slow lean over the first one to three years, as the post rocks fractionally with each wet season until the concrete cracks and the post loosens in the ground.

Three things determine whether a footing holds:

• Concrete strength (grade) — how much load it can bear once cured.
• Footing size — hole diameter and depth relative to post length.
• Cure time before loading — backfilling or fixing panels too early undoes the strength you paid for.

Get any one of these wrong and the other two won't fully compensate. A correctly-sized footing poured with too weak a mix will still crack under load. A strong mix poured into an undersized hole won't have enough concrete mass to resist the soil pressure. And even the right mix at the right size will underperform if it's loaded before it's had time to cure. Treat all three as a single decision, not three separate ones.

What "Wrong" Looks Like in Practice

The most common failure pattern on retaining wall posts isn't a dramatic collapse — it's a gradual lean. Signs to watch for on an existing wall include:

• A visible gap opening up between the post and the concrete collar around it.
• Hairline cracks radiating out from the top of the footing.
• The post rocking slightly when you push on the top of the wall.
• Panels or sleepers starting to bow between posts that were previously plumb.

Any of these usually trace back to one of the three factors above — under-strength concrete, an undersized footing, or the wall being loaded before the concrete had cured.

Minimum Concrete Grade (N20)

For most residential retaining wall post footings, N20 (20MPa) is the practical minimum. It's a widely used benchmark for structural post work — enough strength to hold a loaded post without over-specifying for a job that doesn't need commercial-grade concrete.

 

The "N" grading refers to normal-class concrete, and the number is the characteristic compressive strength in megapascals (MPa) — essentially how much load per unit area the concrete can bear once it's fully cured, usually measured at 28 days. Higher numbers mean stronger, denser concrete, but also a mix that's more expensive and typically slower to work with. For a domestic retaining wall post footing, N20 sits at a sensible middle ground: strong enough to resist the lateral soil load on a typical residential wall, without paying for structural-grade concrete a footing doesn't need.

Both our Concrete for Retaining Wall Post Footings and Rapid Set Bags 20kg (25MPa) meet or exceed this. If you're working on a taller wall, deeper embedment, or a council-permitted structure, check your engineer's spec first — some jobs call for higher-strength mixes.

A few situations where it's worth checking rather than assuming N20 is enough:

• Walls over 600mm–1m in height, where soil pressure on the posts increases significantly with wall height.
• Council-permitted or engineered walls, which will usually specify a minimum grade in the approved drawings.
• Sloping or unstable sites, where the soil load isn't purely lateral and posts may need to resist some additional movement.
• Sites with poor drainage, where saturated soil behind the wall adds extra sustained pressure through wetter months.

For a standard suburban retaining wall under those heights, on a reasonably stable site with normal drainage, N20 from either the footing concrete or the rapid set bags is the sensible default. It's the grade most Australian trade suppliers stock as their standard post-setting mix, precisely because it covers the vast majority of residential jobs without over-engineering the footing.

Rapid Set vs Standard Concrete

The main trade-off is time versus flexibility:

Product	Cure time	Best for
Rapid Set Bags 20kg (25MPa)	~1 hour to set, load same day	Single-day jobs, no bracing time
Post Mix Bags 20kg	48–72 hours to full cure	Multi-day builds, standard budget jobs
Concrete for Retaining Wall Post Footings	Standard cure schedule	Purpose-sized footing mix for post holes

Want the full breakdown, including cost and when each one actually makes sense? See Rapid Set vs Post Mix vs Standard Concrete: Which for Your Posts?

How Rapid Set Actually Works

Rapid set concrete uses a different cement chemistry to standard mixes, designed to hydrate and gain strength much faster. In practice, that means you can often skip the bracing step altogether — mix it dry-ish, pour it around the post, add water, and the post is typically load-bearing within about an hour. This is genuinely useful when you're working solo and can't hold a post plumb with props for two or three days, or when you need to finish an entire run of posts in a single visit.

The trade-off is workability. Rapid set has a shorter working window before it starts to go off, so you need to have the post positioned, levelled, and braced (even if only briefly) before you mix it. It's not the mix to use if you're still figuring out post spacing on the fly.

When Standard Post Mix Makes More Sense

If you're setting a full run of posts over a weekend and can leave them propped while the concrete cures, standard post mix is usually the more economical choice. It costs less per bag than rapid set, and the slower cure isn't a downside if your schedule already allows for it. It's also generally more forgiving to work with — a longer window to adjust the post before it goes off, which matters if you're setting posts solo without a second pair of hands to hold things plumb.

Cost Considerations

Rapid set concrete typically costs more per bag than standard post mix or footing concrete, largely because of the faster-curing cement additives. For a small job — a handful of posts — the extra cost is often worth it for the time saved and the ability to finish in a day. For a longer wall with a lot of posts, the cost difference across many bags can add up, and a standard mix with a planned multi-day pour schedule may work out more cost-effective overall.

How Much You Need

As a rough guide, one 20kg bag fills roughly 10 litres of set concrete once mixed — enough for a small post hole, but most footings need two to four bags depending on hole size and post embedment depth.

The two measurements that decide bag count are hole diameter and embedment depth:

• Hole diameter — typically 250–350mm for a residential post, depending on post size and soil type.
• Embedment depth — as a general rule of thumb, around a third of the post's overall length should be in the ground, though this varies with soil conditions and wall height.

A wider or deeper hole obviously takes more concrete, and it's a common mistake to under-order because the volume increases faster than people expect as diameter grows — doubling the hole diameter roughly quadruples the concrete volume needed, not doubles it. Under-ordering means a trip back to the supplier mid-pour, which is worth avoiding by rounding your bag order up rather than down.

For exact quantities by post size and hole depth, see How Much Concrete Do You Need Per Post Footing? — it includes a worked example and bag-count table. If you'd rather not do the maths by hand, our retaining wall calculator works out quantities for your exact wall dimensions.

Curing Time Before Loading the Wall

Don't backfill, attach panels, or apply any lateral load until the footing has cured:

• Rapid set: safe to load after about 1 hour, but give it a few hours if the weather's cold or wet.
• Post mix / standard footing concrete: wait a minimum of 48 hours, ideally 72, before loading.

Loading a footing too early is one of the most common reasons posts lean or crack at the base within the first year.

Weather and Cure Time

Cure times assume reasonable weather. Cold temperatures slow the curing reaction down, sometimes significantly — below about 5°C, standard concrete can take considerably longer than the usual 48–72 hours to reach a safe load-bearing strength. Hot, dry conditions bring a different risk: the surface can dry out faster than the concrete underneath cures, leading to surface cracking. If you're pouring in hot weather, keeping the surface lightly damp for the first day or covering it can help it cure more evenly.

Rain shortly after pouring is generally less of a problem than extreme heat or cold, since the concrete has already begun to set, but heavy rain in the first hour or two can wash cement paste off the surface. If rain's forecast right after your pour, a simple cover over the footing while it starts to set is cheap insurance.

Common Mistakes to Avoid

• Backfilling too early. The concrete might look solid on the surface well before it's actually cured through.
• Mixing too wet. Extra water makes the mix easier to pour but weakens the cured strength — follow the bag's water ratio rather than eyeballing it.
• Skipping the plumb check. Once concrete sets, an out-of-plumb post is a genuinely difficult fix. Check level immediately after pouring and again a few minutes later.
• Under-sizing the hole to save a bag. The concrete cost saved is small compared to the risk of an undersized footing failing early.
• Not accounting for soil type. Sandy or loose soil may need a wider hole than clay to achieve the same holding strength.

Setting new posts as part of the build? Check our steel retaining wall posts range to match posts and concrete in the one order.

Tools and Materials Checklist

Before you start mixing, it's worth having everything on hand — running back and forth for a missing tool while rapid set concrete is going off is not the time to discover you're short a spirit level. A typical post footing job needs:

• Post hole digger or auger — hand auger for a few posts, powered auger for a longer run.
• Spirit level — ideally one long enough to check plumb on two faces of the post at once.
• Mixing tub or wheelbarrow — for hand-mixing bagged concrete on site.
• Bracing timber or stakes — to hold the post plumb while the concrete sets, particularly important with standard post mix given its longer cure time.
• Tape measure and string line — to keep post spacing and height consistent along the wall.
• Gloves and eye protection — wet concrete is caustic and irritates skin on contact.
• Water source — measured out per the bag instructions rather than guessed.

For rapid set jobs, have the post already positioned, plumbed and lightly braced before you start mixing — the working time is short enough that fumbling with tools mid-pour can mean the difference between a smooth job and a rushed one.

Choosing Between The Three Products at a Glance

If you're still weighing up which of the three to order, this is the short version:

• Need it load-bearing today, or working solo without props? Rapid Set Bags 20kg.
• Setting a full run of posts over a weekend, budget-conscious? Post Mix Bags 20kg.
• Want a mix purpose-built for retaining wall footings specifically? Concrete for Retaining Wall Post Footings.

All three are available in 20kg trade bags, which is manageable for one person to carry and pour without machinery — useful if you're working on a residential site without vehicle access to the back or side yard, or where a concrete truck simply isn't practical for a handful of post holes.

FAQs